Skip to main content

Development of New 950Pd Investment Casting Alloys with Superior Properties

Development of New 950Pd Investment Casting Alloys with Superior Properties

State of the Art

Palladium alloys were a topic at the Santa Fe Symposium mainly during 2006 and 2009 when the Pd demand in the jewelry section increased rapidly. Several papers on the palladium market [1], the investment casting process [2-4] and on the fabrication and manufacturing [5, 6] were published. The first investment
casting study on 950Pd alloys was presented by Fryé [4]. Due to the fact that limited information was available, the core purpose of this paper was to gain a better understanding of the casting characteristics of 950Pd alloys used in the jewelry industry. The work of Battaini [2] aimed at presenting
the main physical and chemical properties of palladium-based dental alloys and transferring the experience acquired in the dental field to the goldsmith’s.

In 2007 a study of the investment casting of 950Pd alloys was conducted by fem on behalf of Palladium Alliance International (PAI). The results were presented at the Santa Fe Symposium in 2008 and published in 2009 [3]. In the following section the main findings of these studies will be summarized.

2.1 Typical casting challenges with 950 Palladium alloys

Commercial 950 palladium alloys contain Ru, Ga or Co as main alloying elements. Table 1 provides a list of alloys with their most important properties and characteristics.

Table 1:                Overview on commercial palladium alloys taken from manufacturer’s alloy datasheets. AC = as cast, CW = cold worked, AN = annealed.

Ruthenium (Ru) is a platinum group metal with a white color that alloys well to palladium. Ru containing alloys show a higher melting temperature, because Ru increases the solidus and liquidus temperature according to the binary phase diagram. Ru has a limited solubility in palladium and therefore only
950Pd-Ru alloys are manufactured. The increase of the melting range by the addition of Ru requires higher casting temperatures compared to pure palladium. This results in higher thermal stress to the crucibles and investment materials during casting. The melting range of 950PdRu is very small, only
a few degrees Celsius. Some values given in manufacturer’s data sheets for 950PdRu are contradictory to phase diagram information. This is attributed to further alloying additions that are not specified or to the difficulty in the determination of the real melting range.

During solidification dendrites are formed and Ru segregates to the dendrite core and the remaining melt enriches in Pd. Usually the melting range increases due to segregation, but this is very limited for 950PdRu. As a consequence the alloy shows a nearly isothermal freezing and therefore a very limited
form filling during casting. This process was investigated in detail for 950PtRu [15].

Ru dissolves in palladium to form a solid solution. Pd-Ru alloys are relatively soft, because of the small difference in the atomic size of Pd and Ru. The typical hardness is about 100-120 HV1 in the annealed or as-cast condition. In order to improve the mechanical properties additions of Gallium
(Ga) are frequently used.

The binary phase diagram of Pd with Ga is shown in Figure 1. Ga has a very low melting point (29°C) and its addition significantly lowers the solidus and liquidus temperature of Pd. The maximum solubility in Pd is ca. 8 mass% Ga. At higher Ga concentration many intermetallic compounds form in complex
phase reactions. No systematic studies are reported on the precipitation hardening of Pd-rich alloys in the open literature. However, such studies are available for Pt alloys [16] and the results can be transferred to Pd.

Precipitation hardening is well-known and applied for 950Pt alloys [16, 17]. However, the solubility of Ga in Pd is higher than in Pt. Therefore, higher amounts of Ga are required in order to achieve the same hardness level – or for a given Pd content, e.g. 950Pd, the achievable hardness is lower. The
hardening response of Ga alloyed Pt is reported to be unstable and therefore classified as non-viable for reliable hardening by some authors [16].


Figure 1:              Conventional approach of hard palladium alloys with higher Ga content. Section of the Pd-Ga system (left) compared to the Pt-Ga system (right) calculated using ThermoCalc and TCNOBL1 database.

Experimental trials and corresponding investigations in a previous study at fem [3] focused on two alloys, one with Ru/Ga and the other with Ag/Ga/Cu. Hence no general conclusion can be made about the suitability of alloys for palladium casting depending on alloy composition. On the basis of defect analysis
on industrial castings, it seems that alloys having a relatively high Gallium content tend to have a higher susceptibility to formation of cracks in as-cast parts. Crack formation turned out to be a complex issue. In depth failure analysis revealed, that the underlying mechanism is related to particular
casting conditions and properties of investment material. It should be noted that crack-free castings of the Ru/Ga alloy have been obtained in reproducible way during casting trials at fem and are also obtained in high quality and reproducible way by several industrial casters which cooperated in that
project.

Silicon is a typical impurity that occurs in investment casting processes. If scrap material is used for remelting the removal of any investment residues is of utmost importance [2]. Such oxide residues might decompose during melting, especially under reducing conditions (forming gas: Ar/H
2
or N
2/H
2) that must be avoided. The released oxygen gets into solid solution inside the melt and evaporates during solidification to form significant gas porosity. Si forms a deep melting eutectic (Pd + Pd
3Si) at a temperature of 782°C. Such a low melting eutectic at the grain boundaries is responsible for hot tearing. An example of the catastrophic result of silicon impurities is shown in Figure 2 . The casting tree is completely embrittled. Many cracks in the parts cause multiple fractures that
occur along the interdendritic grain boundaries. Position 4 in the lower right image shows increased Si concentration determined by EDX analysis.

Figure 2:              Hot cracking due to contamination with investment residues.

3 Development process

3.1 Identification of suitable alloying elements

Potential alloying elements have been selected from the periodic table. Some elements have to be excluded, because they are volatile, toxic, allergenic or radioactive, too reactive under the typical conditions of investment casting or insoluble. The main requirements to the new alloy were:

Sufficient melting range of min 25K

Medium Hardness (130-160HV1)

Fine grain structure

The alloy 950PtRu was defined as benchmark for the development of new 950Pd alloys. 950PdRu has a promising silver-grey color compared to the grey color of most 950Pd alloys. It contains 100% platinum group metals and does therefore not require protective gas during processing. However, the fluidity
of the alloy is very low and some manufacturers do not recommend it for casting.

Only few elements remain as candidates. In order to overcome the poor casting properties of 950PdRu the following improvements are required:

Widening of the melting range  Addition of Co, Fe or Cu

Improvement of casting properties, especially form filling            Addition of Co

Optimization of the segregation, reduction of investment reactions        Addition of Sn

Improvement of color and hardness       Addition of Cr, Fe, B

Grain refining    Addition of Fe, W, Zr

Figure 3 shows the changing melting range of different 950Pd alloys were Ru is replaced by a third element (Me). On the left side of the figure we find the binary 950PdRu alloy and on the right the binary 950PdMe alloys. Some elements such as Au have hardly any effect on the melting range and the liquidus
temperature. Other elements (Ag, Cu,Cr) have a medium effect on melting range and liquidus temperature. In case of Cu relatively high amounts are required to achieve an effect. The strongest effects are shown by the addition of Co and Fe. However, because of their tendency to oxidation the amount should
be limited to maximum 2%.


Figure 3:              Effect of alloying additions to 950PdRu. Ru is replaced by a metallic third element (Me). The x-axis gives the amount of the third element in mass-percent. Calculated using ThermoCalc and SNOB3 database.

The effect of segregation during the solidification process can be simulated by so-called Scheil-Gulliver simulations. The effective melting range of an alloy usually increases, because the complete thermal equilibrium that is assumed in the equilibrium phase diagrams is not achieved during a relatively
fast cooling process. This results in a continuous change of the chemical composition of the liquid phase as the solidification proceeds and this effect is what can be studied by the Scheil-Gulliver simulations . The effect of such non-equilibrium composition changes of the melt on the solidus temperature
is shown in Figure 4 for a series of 950Pd-30Ru-Co,Fe alloys. The binary 950Pd-Ru shows a very narrow solidification range. The addition of 20 ‰ of Fe+Co reduces the solidus temperature and the segregation processes become more pronounced. The segregation of Fe and Co to the liquid phase results in
a reduction of the effective solidus temperature and allows a melting interval of about 30-100 K. This appears promising in terms of improved form filling, better feeding (reduction of micro shrinkage) and reduced investment reactions.

Figure 4:              Scheil-Gulliver calculation. Segregation of 950Pd-30Ru alloys with varying Fe and Co content.

3.2 Investment casting trials

Based on the above described consideration a series of alloy compositions was derived as shown in Table 2. 950PdRu served as benchmark alloy and was purchased from C. Hafner, Pforzheim, Germany. The alloys were prepared by arc melting from pure elements with a purity of 99,9% or better (purchased from
HMW Hauner Metallische Werkstoffe, Germany). The button shaped sample was cold rolled to sheet that was used for the centrifugal investment casting of typical trees according to Figure 5 with a mass of approx. 100 g. On these trees a basic characterization was made that comprised the determination
of color, metal release, hardness, age hardening response and microstructure. The tree contained a series of typical jewelry parts that are prone to typical casting defects. The grid was used for the testing of form filling and the plate sample for color measurements and metal release tests. Ca. 35
alloy compositions were prepared and investigated. Table 2 provides a selection of these compositions. The most promising alloys were selected and modified in the following step.

Table 2:                Alloy compositions tested in small scale trials (selection)

1.	Three-ball ring
2.	Single gate ring 
3.	Light signet ring
4.	Plate
5.	Grid
6.	Solitaire ring
7.	Double gate ring
8.	Heavy signet ring

Figure 5:              Casting tree setup and cast parts

Casting required a sophisticated process control in order to guarantee reproducible and reliable casting conditions (Figure 7). The casting machine was the model TCE10 from Topcast, Italy that allowed melting and casting within 40-60 s from the beginning of the heating process. A high quality quartz
based crucible of type „KGZ“ from Porzellanfabrik Hermsdorf, Germany was used for all casting trials. This type of crucible was proofed as suitable for platinum alloys in a previous study. The metal temperature was controlled during melting and casting with a thermal imaging camera. This allowed a detailed
evaluation of the metal temperature that is superior to the pyrometer integrated into the casting machine. Even the flask temperature could be controlled by thermocouples mounted onto the tree or close to the interior flask surface to document investment overheating. However, such measurements require
very high effort and where therefore used only in a very limited amount of casting trials.

The flask temperature was selected depending on the size and shape of the parts and was 650°C in most casting trials. This temperature showed the best compromise of high form filling and low shrinkage porosity. In order to reduce investment reactions as far as possible a two-part phosphate bonded investment
powder was used (Ransom&Randolph Platinum). After casting, the parts underwent nondestructive testing by computer tomography and by conventional metallography.

Figure 6:              Casting machine and process control (description see text)

Optimized form filling requires a suitable tree-setup. Based on the experience of previous casting projects with platinum, the parts were mounted on the leading side relative to the spinning direction of the casting machine. Figure 7 illustrates the casting setup, the acting forces and an example of
the simulation of the form filling process. Due to the mounting of the parts on the leading side the metal is forced to flow to the tip of the tree. The parts are then filled gradually from the tip towards the ingate of the tree. Details on the investment casting process and the casting simulation can
be found in [18, 19].

Figure 7:              Casting conditions in centrifugal casting. Orientation of the parts on the leading side of the tree. relative to the acting forces. Blue arrows indicate the rotation direction. Arrows indicate the acting forces: orange (inertia), red (gravity) green (resulting force). Simulation
of the form filling process.

After casting the tree was cut and documented as shown in Figure 8. The surface quality was assessed using the appearance of the plate sample. The grid sample provided information about the form filling, which was given in percent of the filled node points of the grid. Metallographic inspection was made
on the single and double gate rings that are prone to shrinkage porosity. The metallographic section of the plate sample was used for color measurement before and after a metal release test in artificial saliva. The results were categorized into three categories that are given in
Figure 9 for form filling, investment reaction and porosity. The microstructure and grain size were determined using scanning electron microscopy. Possible defects such as cracks, chemical inhomogeneity or inclusions were investigated ( Figure 10).

Figure 8:              Casting results and evaluation routine (description see text)

 

Figure 9:              Evaluation criteria for form filling, surface quality and porosity

Figure 10:            Microstructure in the as-cast condition of selected alloys

Table
3:                Main results of selected alloys in full-size casting trials

Table 3 provides the results of some selected alloy compositions. 950PdRu shows good basic properties with medium grain size and only very few intergranular cracks. The addition of Co reduces grain size and helps to completely avoid cracking. Fe additions are promising in reducing grain size, but cause
issues with heavy gas porosity. Despite that fact Fe as kept as potential alloying element. Cu brought no real advantages and reduced hardness resulting in distorted rings already during devesting. This led to the exclusion of Cu. Adding Cr resulted in too strong reactions with the investment and heavy
cracking hence Cr was excluded from the study. The addition of Sn significantly changed the grain morphology, which was rated as disadvantageous. However, the form filling was very good and the overall performance was good which kept Sn on the list of promising alloying elements.

Finally, the alloys marked in green were studied further while those marked in red were excluded from the study. Based on this evaluation the most promising alloys were selected and optimized in further steps by adding additional alloying elements. All alloys showed low hardness at this stage of development,
therefore the focus of further improvement was on increasing the hardness.

In order to improve hardness the literature provided boron (B) and aluminum (Al) as promising alloying elements [20, 21]. However, both elements are not easy to add due to their high reactivity. This required the preparation of pre-alloys with carefully adjusted amounts of Al and B. Using such pre-alloys
enabled the caster to prepare alloys that contain Al and B without the risks of oxidation during initial melting of the alloy. It further enabled the manufacturer to control the amount of alloying addition very precisely. Figure 11 provides results at different levels of B and Al. Small additions of
1‰ B (alloy PD1502) increase strength and hardness significantly while maintaining ductility. Additions of Al have a similar effect, but much higher amounts of Al are required. An approximately linear increase of strength and hardness was found.

These results were determined by tensile testing of rods that were cast into copper molds. Such casting results in fast cooling and might not be representative for investment casting. Therefore, further tests were done by investment casting. Strength levels and hardness are maintained by investment casting,
but ductility is significantly lower. This is typical and is usually an effect of the coarser and columnar grain structure after investment casting.

Figure 11:            Mechanical properties of selected alloys based on 950PdRu with additions of B and Al

Further optimizations in additional casting trials resulted in the compositions provided in Table 4. The boron content was reduced for safety reasons, because higher level of B might cause hot cracking under inappropriate cooling conditions. A combination of Al and B provided the best and reliable properties
in repeated casting trials. The combination of metals was more effective than single additions of even higher amounts. Alloy variations using Fe and Sn provide the benefit of grain refinement and improved form filling, respectively.

Table
4:                Compositions of optimized alloys meeting the hardness requirements

A comparison with other high caratage white alloys (Figure 12) shows some specific benefits of the newly developed 950Pd alloys. Compared to 950PdRu the hardness is significantly increased to levels of 140-160 HV1, which is considered optimum. Higher hardness might be beneficial for improved scratch
resistance but compromises the formability of the material during stone setting. The alloys show some age hardening response that might be used, if higher hardness is required. The comparison with state of the art 950 platinum alloys (green columns) shows superior properties over 950PtRu, but lower
hardness than 950PtRuGa, which is sometimes considered as too hard. The hardness is also comparable to 18k Pd white gold alloys that contain zinc for improved hardness (yellow column).

Further properties that should be considered are color and density of the alloys. For white alloys the yellowness index (YI D1925) is the accepted standard for color assessment [22]. YI values below 18 are considered as “premium white”, which means that the alloys do not require rhodium plating. 950PdRu
based alloys show YI values of below 10, which is comparable to 950Pt alloys. The color difference between 950PdRu and 950PtRu is hardly visible by a human observer. In contrast, premium 18k Pd white gold alloys with a YI value around 18 appear much more yellow. The density of 950PdRu based alloys is
close to 12 g/cm³, which is 60% of 950Pt alloys and 75% of 18k white gold alloys. The lower density enables the manufacturer to produce bulkier jewelry at the same weight or light jewelry, e.g. earrings or pendants. The combination of the properties labels the newly developed 950PdRu alloys as
“light, bright and strong”.

Figure 12:            Comparison with commercial alloys (typical properties according to [23])

4   Summary and Conclusions

The present paper describes the development of 950Pd alloys with improved properties for jewelry applications. Promising alloy compositions were selected based on thermodynamic calculations, which were then melted and cast by centrifugal investment casting. Typical crucibles and investment materials
that proofed to be suitable for platinum alloys were used. These materials were found to be suitable for 950Pd alloys as well. The melt was overheated by ca. 80°C prior to casting. The flask temperature was 650°C in most casting trials. Alloys containing boron can be sensitive to flask quenching. Therefore,
the flasks should be cooled slowly to room temperature before devesting.

The new alloys are based on 950PdRu and contain additional alloying elements to widen the melting range (Co, Sn, B), to reduce grain size (Fe) and to increase hardness (Al, B). Typical liquidus temperature of such alloys was 1560-1570°C, which is slightly lower than 950Pd50Ru. The addition of the above
mentioned alloying elements increases the hardness from ca. 100HV1 of the soft binary alloy 950PdRu to 140-160 HV1. This hardness range is assumed to be the ideal hardness for stone setting and finishing that provides sufficient scratch resistance during jewelry wearing. Such hardness is also reached
for medium to hard 950Pt alloys of Ni-free 18k white gold alloys. The color of 950Pd alloys is comparable to 950Pt. Both groups of alloys show a yellowness index of about 1, which is significantly whiter than Premium white gold alloys (YI <18). The density of 950Pd alloys is ca. 40%and 25% lower
compared to 950Pt and 18k Pd white gold, respectively. The low density is an advantage for light or bulky jewelry items.

The binary alloy 950PdRu suffers from the low hardness. The new 950Pd alloys show superior hardness while maintaining good form filling ability, low crucible and investment reactions and sufficient resistance to hot cracking wit suitable process conditions. They might therefore enable 950Pd alloys to
become an option for jewelry applications.

5   Acknowledgements

Norilsk Nickel, Russia, is acknowledged for financial support of this work. Linus Drogs (AuEnterprises, USA) is acknowledged for supporting this project and consulting during its realization. The colleagues at fem are kindly acknowledged for their contribution to SEM investigation, metallography and
chemical analysis.

6   References

1.            Swan, N. and B.J. Williams.
Palladium – light, bright and precious – a world view. in
The Santa Fe Sympoisum. 2006. Albuquerque, USA.

2.            Battaini, P.
Investment casting behavior of palladium-based alloys. in
The Santa Fe Sympoisum. 2008. Albuquerque, USA.

3.            Fischer-Bühner, J., A. Basso, and M. Poliero.
Challenges for Palladium Casting Alloys. in
The Santa Fe Sympoisum. 2009. Albuquerque, USA.

4.            Fryé, T.
Palladium casting: an overview of essential considerations. in
The Santa Fe Sympoisum
. 2006. Albuquerque, USA.

5.            Battaini, P.
The working properties for jewelry fabrication using new hard 950 palladium alloys. in
The Santa Fe Sympoisum. 2006. Albuquerque, USA.

6.            Mann, M.B.
Palladium: manufacturing basics for servicing, assembly and finishing. in
The Santa Fe Sympoisum. 2007. Albuquerque, USA.

7.            Johnson-Matthey.
JM jewellery alloys. Available from:

http://www.noble.matthey.com/jewelry

.

8.            Hafner, C.
Palladium alloys
. Available from:


http://www.c-hafner.de/de/leistungen-und-produkte/edelmetall-produkte/werkstoffe/werkstoffe-fuer-schmuck-und-dekorative-anwendungen/

.

9.            Heimerle+Meule.
WKD_189-950. Available from:


http://www.heimerle-meule.com/fileadmin/dateien-global/02_Products/Schmuckhalbzeuge/Produktuebersicht/Werkstoffdatenblaetter/Legierungen/Pd_500_585_950/englisch/WKD_189-950eng.pdf

.

10.          Wieland. 2016; Available from:


http://www.wieland-edelmetalle.de/produkte/schmuckhalbzeuge/palladium/page.html?L=0

.

11.          Agosi.
Agosi palladium alloys
. Available from:


http://www.agosi.de/wp-content/uploads/2015/09/AG_AgosiManufaktur.pdf

.

12.          Legor.
Legor palladium alloys
. Available from:

http://products.legor.com/EN/download

.

13.          Hoover&Strong. 2016; Available from:


https://www.hooverandstrong.com/media/pdfs/MaterialSpecs-950Palladium-01-2015.pdf

.

14.          UnitedPMR.
950 Palladium Grain (PD950). 2016; Available from:

http://www.unitedpmr.com/palladium_950_grain.php

.

15.          Klotz, U.E., et al.

Platinum investment casting: material properties, casting simulation and optimum process parameters
. in
The Santa Fe Sympoisum
. 2015. Albuquerque, USA.

16.          Biggs, T., S.S. Taylor, and E. van der Lingen,
The Hardening of Platinum Alloys for Potential Jewellery Application.
Platinum Metals Review, 2005.
49(1): p. 2-15.

17.          Kretchmer, S.,
Heat-treatable platinum-gallium-palladium alloy for jewelry. 2003, Palenville, NY, US.

18.          Heiss, T., U.E. Klotz, and D. Tiberto,
Platinum Investment Casting, Part I: Simulation and Experimental Study of the Casting Process. Johnson Matthey Technology Review, 2015.
59(2): p. 95-108.

19.          Klotz, U.E., T. Heiss, and D. Tiberto,
Platinum Investment Casting, Part II: Alloy Optimisation by Thermodynamic Simulation and Experimental Verification.

Johnson Matthey Technology Review, 2015.
59(2): p. 132-141.

20.          Böhm, W.,
Legierung auf der Basis von Platin, Palladium oder Gold. 2011.

21.          Blatter, A., J. Brelle, and R. Ziegenhagen,
Allaige à base de palladium. 2006, PX Holding S.A., 2304 La-Chaux-de-Fonds, CH.

22.          Henderson, S. and D. Manchanda,
White gold alloys. Gold Bulletin, 2005.
38(2): p. 55-67.

23.          Fryé, T. and U.E. Klotz.
Mechanical properties and wear resistance of platinum alloys: a comparative study. in
The Santa Fe Symposium on Jewerly Manufacturing Technology. 2018. Albuquerque, NM, USA: Met-Chem Research.

Continue reading

Introduction: the galvanic process

Introduction: the galvanic process

Galvanic process refers to the depositing of a metal or metal alloy by using electrolysis during which the electrical energy developed inside the system is converted into chemical energy thus leading to a series of oxide-reduction reactions. The result of this phenomenon foresees that the electrical
current passing through an electrolytic system reduces the metal ions dissolved in the electrolytic solution to form a metal plating on an electrode. With this technique, in fact, the properties of an object’s surface can be modified and the system is therefore used in industry to protect structures
and metal objects from the effects of corrosion. The decorative purposes are no less important: in the jewellery and fashion sector, jewellery and fashion accessories are usually made by depositing layers of various thicknesses of the most noble metals onto less precious metal bases.

The simplest system for carrying out a galvanic process foresees (Figure 1):

Figura 1

Figure 1 : Diagram of an electrolytic system.


Constant voltage generator:

This is the actual plating bath engine in that it supplies the energy and current required for the oxide-reduction processes. To be more precise, the constant voltage generator uses a circuit rectifier which transforms the alternating current into direct current with alternating current residues of less
than 5%.


Cathode:

The negative electrode in the reduction process. It consists of the item to be galvanized and on which the dissolved metals in the electrolytic solution will be deposited. In fact, these are reduced at the interface between the electrode and the solution. The potential that leads to reducing the metal
around the cathode is called deposition potential. If the current distribution around the cathode is known, one can also have an idea of the thickness of the metal deposited on every part of the item being galvanized.


Anode

: The positive electrode in the oxidization process. The anodes can be active (or soluble) or inert (or insoluble). In the first case, the oxidization process foresees dissolving the metal that the electrode is made of which, from a state of nil oxidation, will transform into a dissolved ionic species
in the solution. In the case of inert anodes, these do not take part in the anodic reaction but play more of a supporting role in oxidation ensuring the electronic exchange at their surface and, therefore, the closure of the circuit.


Plating bath


: The electrolytic solution in which the metal salts to be deposited on the cathode are dissolved. It is the means that allows the current to be passed through the ions inside the solution. The electrolytic system will therefore consist of a solvent (water in the vast majority of cases) which has the
capacity to ionize the dissolved species within it. To be more precise, the solution contains the metal salts to be deposited and the conductive salts, i.e. easily ionizable species that are able to transmit current in the solution through ionic conduction. The current therefore passes through
the electrolytic system by means of the dissolved ionic species inside and allows the dissolved metals to be reduced onto the cathode. Usually the plating bath also contains other inorganic or organic additives that help to obtain more compact, smooth or shiny plating, thus influencing the plating
structure. Examples of these are surfactants that are able to lessen the surface tension between the solution and cathode in order to avoid gas permanency at the cathode-solution interface which could create plating defects. Others, however, are brighteners, or rather, substances able to produce
very fine grain electro-deposits and therefore give a shiny look.

Characteristic parameters of a galvanic process

Every type of plating bath will express the maximum potential of its performance if a series of parameters are respected. These depend on the type of metal or alloy being deposited and by the chemistry that the electrolytic system consists of. These characteristics are outlined below:


Potential difference:

This is the parameter through which the energy needed for the electro-plating process is supplied. Each metal ion has its own specific potential difference value as a result of which its reduction and consequent deposition on the cathode will occur. As a general rule, metals with a more negative value
than the standard reduction potential (Table 1) are also those that are more easily electro-plated. These potentials, however, are equilibrium values while galvanic processes are intrinsically dynamic processes, besides the fact that temperature and concentration parameters are often different
to the standard. The potential at which plating occurs is called the
deposition potential. This potential varies with the concentration of the metal in the bath and also depends on the current density. In fact, when current density increases, the polarization effects on the electrodes also increase and, as a consequence, conditions are more favourable for
depositing lower potential value metals than reduction standard. For this reason, metal deposition occurs within a more or less wide range of tension values.

Figura 2

Table 1: Standard reduction potentials for the most common chemical species.


Current density:


This parameter is much more important than tension in the galvanic process. Since the galvanic process is dynamic, the current generated by the potential difference is the parameter that is more greatly connected to the formation and growth of the metal deposit. The real parameter that determines the
quantity of electro-plating that forms on the cathode is the load quantity that flows during the electrolytic process. A more suitable parameter to check and which provides better load quantity management to ensure that it comes over the object being plated is certainly current density, or rather,
the load quantity that flows through a surface unit in a time unit measured in A/dm
2. The parts of items being treated that receive more current than the others are called
areas of high current density. In general, these are the pointed parts, those more exposed to the anodes, the initial or end parts of the item immersed in the plating bath. On the other hand,
areas of low current density are the exact opposite, that is, the central areas of the items and the most hidden parts. It is not always possible to work by controlling the current density because it is sometimes hard to determine the cathode surface, as in the case of barrel plating processes.
In these cases, the work is done by exclusively controlling the tension.


Temperature

: Although to a lesser extent, this parameter also contributes to supplying the necessary energy for carrying out the electro-plating process. It is a parameter linked to electrolytic process kinetics and determines effectiveness and speed. Together with conductive salts, the temperature helps to regulate
conductivity as well as plating bath penetration power. For these reasons, it must be accurately measured and maintained within a specific range of typical values for each plating bath.


Processing time


: This corresponds to the time needed to deposit the metal or alloy and to obtain a good quality plate of the required thickness. Obviously, the greater the processing time, the greater amount of metal will be deposited. An ideal processing time for each plating process is defined from a compromise between
the quality of the plate and the quantity of metal to be deposited.


Cathode efficiency


: Expressed in milligrams of deposit by Ampere-minute (mg/Amin), cathode efficiency indicates the quantity of metal or alloy deposited in one minute, working with a current of one ampere. It leads to understanding the effective possibility of depositing a metal by defining an estimate of how much current
is effectively responsible for forming the plate. In fact, in many cases, part of the current is consumed in accessory processes, like, for example, the development of hydrogen. The cathode efficiency of a plating bath depends on many factors and varies according to temperature, tension, metal
and additive concentration in the solution.

It is important to underline that the values of the typical parameters of a plating bath are not strict but it is generally possible to define a more or less wide range of good operability for each of the above-described parameters.

How to obtain good galvanic plating

Before going into the details of how to obtain good galvanic plating, it would be appropriate to define what exactly good quality plating is. Obviously, the quality of galvanic plating depends on the particular application for which the plating is required. In some cases, for example, it may be enough
for the metal to cover the objects being processed and therefore that the plating merely adheres well to the substratum. In the case of plating for decorative jewellery, the absence of porosity, which gives the plating its polished and shiny look, must also be added to the previously described
conditions as well as the need for good resistance to corrosion. In other cases, the thickness and hardness of the plating will also need to be evaluated.

To obtain good quality galvanic plating, it is undoubtedly necessary to have the right equipment as well as quality products but even so, this may not be sufficient. In most cases, imperfect plating results either from not respecting the parameter characteristics correctly for the specific plating bath
or from not preparing the items properly before performing the galvanic plating process.

2.1. Respecting the characteristic galvanic process parameters

In regard to the first aspect, it is important to keep within the optimal work
range
of each individual typical plating bath parameter in order to obtain a good quality plate. It is not certain that whether not respecting any one of these values will definitely result in a problem with the plate, but the galvanic solution will certainly have gone beyond its region of maximum
performance and this could lead to coming up against one or more plating defects or, in the worst situation, could even definitively compromise the plating and force the user to stop completely.

The most common precautions for obtaining a good quality galvanic plate are shown below, parameter by parameter:


Potential difference


: This, together with current density, is definitely the parameter to which more attention should be paid. Tension is easy to measure and often provides information regarding changes in the galvanic process, such as a reduction in conductive salt content or polarization effects at the anode. A tension
range within which a good quality plate can be obtained is usually defined for each process.


Current density:

In order to be certain of not incurring problems, the parameter on which to place total trust when checking the right energy needed to correctly form the deposit is current density. Working with current density values within the pre-established range certainly guarantees supplying the correct load to
the cathode and therefore of forming a plate with the right chemical-physical characteristics, like, for example, the right proportion of alloy, the right colour and the right grain finishing. The ideal current density range can be qualitatively and quantitatively evaluated by a Hull cell test
or bent-cathode test (Figure 2). If current distribution around the cathode is known, a good estimation can be made of how the metal will coat the entire object: the parts where the thickness will be greater or less. As shown in Figure 3, the galvanic plate will tend to form and grow mainly at
the corners and edges because these are the zones where the current is most greatly concentrated (high current density areas) and will collect less in hidden areas or, in general, in places furthest from the anode because it tends to concentrate less in these areas (low current density areas).

Figura 2

Figure 2 : Example of a Hull test (left) and a bent cathode (right).

Figura 3

Figure 3 : Diagram of the way in which a galvanic plate tends to grow. In the high current density areas (L), the deposit is greater compared to the low current density areas (D).

If the metal distribution obtained is not the one required, some precautions can be taken to improve it:


Modify the way in which the items are linked to the frames

: knowing that most of the current will accumulate in the corners of the items being plated and knowing that the current always tends to follow the shortest route between two conductors, in order to reduce the load accumulation at the corners of the objects, it is a good idea to place them as far away
from the anodes as possible. For this reason, if working in beakers, the recommendation is to place the objects parallel to the anode instead of perpendicular (Figure 4).

Figura 4

Figure 4 : To optimize galvanic plate homogeneity, it is a good idea to place the objects to be plated directly in front of the anode.


Modify the anode-cathode distance:

the anode should be shaped so that the anode-cathode distance is as similar as possible for all surface points and slightly further away from borders thus favouring current passage towards lower current density areas.


Take advantage of shielding effects:

when working with several objects at the same time, the pieces can be linked so that the corners shield one another thus creating effective obstacles against load accumulation in these areas (Figure 5).

Figura 5

Figure 5 : Diagram of the possible effect of reciprocal shielding of the items to be plated positioned parallel to the anode.


Temperature

: Each plating bath has its own precise working temperature. It is strictly correlated to solution conductivity: the higher the temperature, the greater the solution conductivity and the bath’s penetration power can therefore also be greater, i.e. the capacity to homogenously deposit metal even in areas
of very low current density. Any conductivity diversity is a critical aspect when several metals are being co-deposited to form an alloy: a change in temperature introduces a variation in the percentage of metals in plating alloys. In some case, as in rhodium plating, it is possible to work at
lower temperatures to those recommended but plating performances are notably reduced.  Besides lower efficiency, in fact, also the colour of the plating tends to be less bright because the brighteners activate at the suggested processing temperatures. On the other hand, working with excessive
temperatures could damage the system’s chemical components or excessively increase the efficiency of the process and lead to poor quality plating. In short, temperature is a parameter that should absolutely not be ignored and, consequently, effectively controlling it through thermostats and thermocouples
is required to avoid excessive temperature fluctuations.


Processing time:


There is a minimum processing time for each type of galvanic plating below which, the metal deposit will not be uniform. This time depends on the cathode efficiency of the solution: the higher it is, the less the minimum time for obtaining uniform plating will be. As is quite obvious, the longer the
processing time, the greater the plating thickness. It is, however useful to remember that not all galvanic baths have the capacity to produce extremely thick layers of plating. For this reason, especially in the case of baths devised to create thicknesses of less than 0.5 microns, a maximum
deposition time is defined beyond which the plating could be of poor quality. In the case of thick plating, the time needed to deposit one micron of plating is usually indicated and, also in this case, a maximum deposition time corresponding to the maximum plating thickness that the galvanic
bath guarantees for good quality plating, can be defined.


Cathode efficiency


: As already mentioned, this is not a real parameter to be set but rather an intrinsic characteristic of the solution that depends on other parameters. Nevertheless, it is an aspect that should not be ignored when aiming at obtaining a good galvanic plate since it provides an idea of the metal thicknesses
that the electrolytic solution is able to guarantee with good quality. A low efficiency value, in fact, indicates that the plating bath is suitable for a colour additive, a flash, and consequently, will not easily perform well for carrying out thicknesses of one micron (Figure 6).

Figura 6

Figure 6 : SEM image of the section of a sample that has various plating layers whose thicknesses have been measured.

Generally, thickness plating baths have medium-high cathode efficiency (greater than 15 mg/Amin), a higher concentration of metal (never less than 3 g/l) and also high density values. Without these three elements, it will be difficult to obtain good quality thick deposits.

2.2. Respecting the chemistry of the solution

In some cases, metal distribution can be improved by acting on the solution chemistry, for example, by using additives to modify the efficiency or conductivity.

A plating bath should always be kept within the values of reference regarding the concentration of its various components. The reasons for modifying the composition of a solution may be:


Decomposition of the chemical substances


– Drag-in and drag-out phenomena

It is quite rare that a solution will not need additives. Since they are necessary, the advice is to use them often and in small quantities so that the chemical substances never go outside the processing range. Adding them in large quantities is often not recommended due to undesirable accessory sub-reactions
that can occur or due to excess impurities that can contextually be included in the chemical species added to the bath.

The pollution aspect linked to the metal or organic type is also not a secondary aspect. The former is generally due to cross-contaminations between solutions or to anode and cathode crumbling or the breakdown of any other metal objects that may come into contact with the galvanic solution, or, lastly,
to improperly demineralized water. Organic contamination can also be due to cross-contamination with degreasing and neutralizations or simply to dirt that can accidentally appear in the galvanic solution or, lastly, to degraded additive residues that no longer work or contaminated water. The
risk, in fact, is that contaminations of this kind can be included in the galvanic plate, thus decreasing its quality. To avoid these inconveniences, the solution can be periodically filtered or treated with carbon active purifiers (b) or purification processes that take advantage of the electro-deposition
on large-sized sacrificial cathodes at low current densities (dummy plating). In the case of high-volume electrolytic solutions, it is always recommended to work with a constant filtering system while for small usage, filtering can be done with paper (Figure 7) and, obviously, one extremely useful
precaution is to cover the solution or tank when it is not in use for long periods of time.

Figura 7

Figure 7 : Examples of a paper filter used to filter ferrous precipitate (left) and filter cartridges used in plating system filtration (right).

2.3. Respecting preparation steps

The quality of the plate also depends on the condition of the item being plated and the preparatory phase. The objects to be treated must be polished in order to eliminate porosity and any other surface imperfection prior to galvanic deposition. It is therefore necessary that the objects to be plated
are of good quality and accurately prepared before carrying out the treatment (Figure 8). The preparatory phase is a highly underestimated aspect but, by following some simple basic rules, most plating problems can be resolved by proper preparation.

Figura 8

Figure 8 : Comparison between unpolished (left) and polished (right) brass rings.

When preparing the pieces to be plated, their surfaces must be thoroughly cleaned and free of any contaminants and their activation in order to optimize metal adhesion in the subsequent electro-plating phase. In theory, the steps to follow depend on the surfaces and the type of initial alloy on which
the metals are to be deposited. Keeping within the field of jewellery and fashion, the following steps for preparing the objects can be taken as standard (Figure 9):

Figura 9

Figure 9 : Diagram of the preliminary cleaning and surface activation steps for items to be plated.

– Ultrasonic cleaning

– Electrolytic degreasing

– Neutralization

The items are washed and rinsed after each of the previous phases. In fact, when the object is removed from a bath, its surface is coated with a liquid film of the solution in which it was previously immersed. This residue must therefore be removed in order to avoid cross-contamination.


– Ultrasonic cleaning


:

Ultrasonic cleaning eliminates any polishing procedure grease, oil and cleaning paste residue from the items to be plated. The functional principle is the cavitation generated by ultrasound: the vibration of the piezoelectric elements in the ultrasonic washing machine produces high frequency waves that,
in turn, generate bubbles in the solution that strike the surface of the objects at high energy, thus removing any contaminants that may be present. Normally the solution containing the detergent for ultrasound works at a specific temperature that favours dissolving the cleaning paste in strict
collaboration with the cleaning action carried out by the relative detergents and with a mechanical rather than an ultrasonic action. Consequently, in order for the ultrasonic cleaning procedure to be effective, the solution must contain the appropriate detergents and work at a specific temperature,
otherwise the degreasing action will not be sufficiently effective (Figure 10).

Figura 10

Figure 10 : Characteristic phases of the typical cavitation process in an ultrasonic washing machine.



Electrolytic degreasing
: This second preparatory phase requires using electrical current. Besides re-cleaning the items after their first ultrasonic cleaning, this process consists of a chemical activation of the surfaces to be electro-plated. After the electrolysis process, hydrogen
bubbles can develop on the pieces thus ensuring cleaning and the activation of the metal surfaces in order to optimize and maximize subsequent electro-deposition. This phase is essential for avoiding deposit adherence problems. The degreasing solution is usually alkaline and consists of a series
of chemical substances that attack dirt molecules, capturing them on the surface and thus stopping them from being re-deposited, and remove any oils and grease that ultrasonic cleaning has not removed adequately.


Neutralization: Neutralization is a simple chemical process that neutralizes all substances, usually from electrolytic degreasing, that pollute and are incompatible with subsequent plating processes. These residues are cleaning agents left on the surfaces of the items. The solution must be
chemically opposed to that of degreasing. Since degreasing is almost always alkaline, neutralization will require a more acid solution. With neutralization, the objects being treated are perfectly clean and the surface is neutral and ready for electro-plating.

A simple and effective way of checking that the preparation procedure has been carried out correctly consists of seeing if the object’s surface is water-break free. Indeed, if the object has been well activated, water will flow over its surface homogenously, forming a uniform liquid film. This provides
the assurance that the surface is sufficiently free of contaminants which might otherwise provoke plating differences on the surface. If the surface is not adequately clean, drips or areas where the uniformity of the liquid film is broken will appear (Figure 11).

Figura 11

Figure 11 : Comparison between an improperly prepared surface (left) and a correctly prepared surface (right) using the water free break check. The presence of drops in the picture on the left and the regular liquid film on the right well show that the first surface was improperly prepared compared to
the one on the right.

Working almost exclusively with water-based solutions, it is clear that, in order to obtain a good quality galvanic plating, the right water must be used. The quality of the water used in plating strongly affects the final result of the plating process. For these reasons, the water must be free of any
organic contamination and have a low saline content (less than 5 microsiemens). Industrial plating systems are usually equipped with columns with active carbon and ion exchange resin. For galvanic solutions, therefore, the best choice is to use deionized water.

Causes of galvanic plating defects

When plating is not of good quality, it is said to have defects. There are a vast range of imperfections that can appear on the surface of the object on which galvanic deposition has been carried out and which spoil the aesthetic aspect and affect the chemical-physical properties.

3.1. Types of galvanic plating defects

In an attempt to outline the defects, the following can be defined:
focal defects, or rather, non-extensive defects positioned in a more or less regular manner on the plate surface;
surface defects, that is, defects that homogeneously involve all the object’s surface or large continuous areas of it; and
adhesion and cohesion defects relating to the capacity of the galvanic deposit to adhere to the underlying metal and to stay intact, overcoming the forces of tension that necessarily arise during the nucleation and growth processes of a plating layer on a surface.

The most common focal defects are (Figure 12):

Figura 12

Figure 12 : Examples of different types of focal defect. Top from left: dark spots on the plate (circled in red), white spots on the plate (circled in red), dark post-oxidation spots (circled in red). Bottom from left: cloudiness, bubbles (circled in yellow) and pitting (circled in red).


Dark spots on the plate (burning spots):

these are irregular spots on the plate surface. They can be in the centre of the deposit but are more often found on the object’s extremities, in high current density areas. They are usually generated by over-high tension or by conductivity problems, such as the use of a damaged anode or a contaminated
solution.


White spots on the plate (stains):

these spots appear very close together and are not necessarily small. They are usually due to an incorrect preparation procedure or to conductivity problems in the bath due to the solution being contaminated or old, or also to the use of unsuitable tools, such as malfunctioning anodes.


Pitting:

This refers to micro-porosity or generally tiny round concave holes found irregularly on the plate. This defect is often due to an improper preparation procedure or to pre-existing imperfections on the surface of the items to be plated that were not eliminated in the polishing or tumbling phases. In
both cases, oxidative phenomena generated on the surface cause the defect.


Bubbles and blisters:

These are authentic, round accretion bubbles on the plate surface. They usually tend to form in high current density areas but can also be found in other parts involved in the galvanic plating process. In this case too, the reasons that lead to their formation are ascribable to an incorrect execution
of the galvanic process or the age of the plating solution.


Streaking:

Streaks can appear either as concentric rings that go from the high current density areas to the low current density parts or as streaks generating from the object’s edges. They mark the onset of burning phenomena due to excessive tension conditions or to a low metal content in the solution or the solution
generally being too old.


Haziness/cloudiness:

These are random plated surface areas where the plate is translucent and cloudy as if covered by a whitish veil. They are usually due to incorrect processing parameters such as the absence of solution movement, too low temperature and/or current density or improper object preparation or the use of inefficient
instruments or, lastly, organic contamination in the solution.


Dark post-oxidation spots:

This refers to the appearance of spots immediately after having carried out the deposition or in the phases immediately after drying. The occurrence of similar oxidative phenomena is usually due to poor adherence or distribution of the plate on the surface after improper preparation or because of the
original surface being excessively rough.

There now follows a list of the most common defects found on surfaces (Figure 13):

Figura 13

Figure 13 : Examples of various types of defect that affect large areas. From left: burning, dull deposit, discoloration.


Burning:

This occurs when the entire deposit or portions of it have a large grain finishing with a dull and not so shiny look, a coarse, rough and often poorly adhesive plate. This defect is usually due to the tension or current density being too high or when there are conductivity problems due to the absence
of additives in the solution, or to a low concentration of metal to be deposited. An incorrect procedure, such as not stirring the solution, can lead to this type of defect.


Dull deposit:


This deposit is not polished or shiny in large and well-defined areas of the object. In fact, when looking at the deposit in the high current density areas, the defect is usually due to similar causes as those that lead to burning, while, when the low current density areas are examined, the problem is
an improper preparation of the items to be plated or incorrect temperatures. A contaminated solution can also be the cause of a dull deposit.


Discoloration:

In some cases, areas of the same object can have different colours or the entire deposit can be lighter or darker than the required shade, especially in the case of alloy plating. This defect is usually due to using the wrong temperature and current density or tension, or to a low concentration of metals
in the solution. Deposit iridescence is also included in this type of defect, an aspect that is due to lower thicknesses than the recommended parameters or to conductivity problems linked to malfunctioning anodes or to electrolytic solutions with a low conductive salt content.


Low levelling:

This is characteristic of thick deposits if the deposit is not homogeneously distributed and discontinuities can be identified similar to a series of overlying planes or similar to uncoated porosities. This defect is usually caused by an inadequate concentration of additives in the solution or to their
age or, lastly, to the fact that the galvanic plating processing parameters have not been respected.


Low throwing power:

Penetration power refers to the electrolytic solution’s ability to deposit the metal correctly in the low current density areas with the most homogeneous thickness possible. If this does not occur, there will be discoloration or no deposit in these specific areas of the object being plated. This usually
happens when the processing parameters are not respected or when the solution needs conductive salts (Figure 14).

Figura 14

Figure 14 : Example of a chain plated with ruthenium at low temperature and tension compared to the reference values. The penetration problems are clear since some areas of the chain have no plating and others are irregular.

Lastly, defects which cause the electro-plated layer to come away from the substratum need to be taken into consideration. Adhesion defects are those that occur immediately after galvanic deposition or even at the same time as the galvanic process itself. Usually there are two ways in which the deposit
can detach: (Figure 15)


Peeling:
when the deposit flakes, depending on the foil


Blistering:
when the deposit crumbles altogether to form a fine powder, otherwise known as floury.

Figura 15

Figure 15 : Examples of adhesion defects. At top left, ruthenium-plate blistering, at bottom right, nickel-plate peeling.

Lack of adhesion is often due the objects being improperly prepared or to the absence of pre-strata underneath the final one, or the deposition parameters (specifically, temperature and tension) not being respected. However, when deposit loss occurs at a later moment, often following the application
of variable degrees of stress on the plated object, the defects are referred to as
cohesion defects
.  At the time of their electro-plating, the metals are subject to tension forces which can be so intense as to spoil the deposit in two ways (Figure 16):


Cracks


Flaking

Figura 16

Figure 16 : Examples of cohesion defects in nickel-plating. Top, cracks in the plating after sample bending; bottom, plate flaking after sample bending.

In some cases, cohesion problems not caused by internal stress between the metal contacts can occur. In these cases, the deposit becomes fragile due to hydrogen development
(hydrogen embrittlement), a common sub-product in galvanic processes. To avoid this, lower tensions can be adopted or appropriate solution agents that limit hydrogen development can be used.

3.2. Most common causes of defect

There are many reasons why a plate can have defects and, as described above, a defect can often be due to more than one thing. Vice versa, the cause of a defect can appear in more than one type of defect. Making a detailed list of all the possible defects and their causes without contextualizing them
to one specific galvanic process can be extremely complicated and incomplete. Nevertheless, generally speaking, it is possible to group defect causes into three categories:


Defects due to not respecting the parameters:

This category includes defects due to not having respected the characteristic parameters recommended on the technical form for the specific electrolytic system and defects due to using inadequate instruments, such as damaged anodes or different anodes to the recommended type, partially oxidized cables,
inadequate electrical equipment, etc…


Defects due to improper preparation:

This category groups defects that arise due to the absence or incorrect execution of one or more preparatory steps prior to electro-plating.


Defects due to using inadequate products:

This group refers to solutions formulated with low quality chemical substances or plating baths where not all the values are within the functional parameters (pH, density, metal qualities, etc…).

Except for rare and exceptional cases, statistically, almost all defects originate either due to not having correctly respected all the characteristic galvanic plating parameters or because the objects to be plated were not prepared by faithfully following the procedure.

3.2.1 Defects due to not respecting the characteristic galvanic process parameters

The defects due to not respecting the parameters of a specific galvanic solution are among the most immediate and most easy to resolve defects. In fact, all that needs to be done is to correct the parameter in order to return to obtaining a good quality plate. Therefore, going back over the typical parameters
of a solution, the most probable causes of the defect can be found:


Incorrect potential difference:

Usually, working with tension values that are too high or too low compared to recommendations leads to adhesion problems in the electro-formed plating. To be more precise, working at a potential that is too low leads to the possibility of an inhomogeneous distribution of the metal deposit and, in the
case of alloys, compromises its percentage composition with changes in colour and in chemical-physical properties. Working at a potential that is too high, on the other hand, besides possible deposit colour variation, can also lead to burning or dark spots on the plate (Figure 17).

Figura 17

Figure 17 : Pink gold plating carried out at different potential differences. Working with tensions below the range (right), the alloy becomes richer in gold content and the plating takes on a more yellow look than the correct alloy deposited by working with the right tension value (left).


Incorrect current density:

Substantially the same defects can occur when tension values are incorrect since the two parameters are correlated. That is to say, burning, poor deposit adhesion, spots on the plate or a different colour to that foreseen in the case of alloy plating.


Incorrect temperature:

It is important to work at the right temperature since excessive temperatures generally cause the deposit to burn, either due to the excessive amount of current at the cathode or because the heat tends to destroy the solution additives which, consequently, will no longer perform their levelling and brightening
actions on the deposit. On the other hand, if the temperature is too low, the deposit will not be uniform or, in the most extreme cases, may even be lacking. Working at incorrect temperatures produces different conductivity conditions that can generate significant effects when metal alloys are
being deposited, such as discoloration or a different composition of the alloy with the consequent variation in the chemical-physical properties or plating costs (Figure 18).

Figura 18

Figure 18 : Example of a sample plated with ruthenium at a lower temperature than the work range. The central area (low current density) has no deposition at all, the peripheral areas (high current density) have irregular plating.


Incorrect deposition time:

Increasing deposition time certainly gives greater thicknesses but exceeding the times can cause dull deposits, dark spots or deposit cohesion problems. Too short a time can generate adhesion defects or cloudiness or the colour of the plate can be irregular or not the right shade in the case of alloy
plating.


Incorrect cathode efficiency:

This is not a processing parameter but, as mentioned earlier, it depends on previous parameters and influences the quality of the plate. It may be lower compared to predictions due to a low metal or conductive salt concentration in the solution or because of too many additives. In these cases, problems
of adhesion and cloudiness may occur or the deposit may be thinner than expected. If cathode efficiency is too high, it may lead to burning.

3.2.2. Defects due to improper preparation

Improperly preparing the surfaces to be plated or using inadequate instruments are the most common causes of defects and, at the same time, are those that are the most neglected. Often, thinking that the preparation procedure is unimportant, the operator tries to improve the quality of the deposit by
acting on the deposition parameters or, to a worse degree, by intervening on the chemistry of the galvanic solution and therefore running the risk of definitively compromising the entire process.

Below is a list of the most common defects associated to an improper preparation of the items to be plated:


Polishing:

If the object to be treated is excessively porous, the galvanic plating will certainly not be able to eliminate the porosity, the plate will not be homogeneous and will probably have adhesion problems as well as dark spots or pitting. The deposit will simply follow the surface morphology and, in the
case of extremely bright deposits, this inhomogeneity will be even more evident (Figure 19).

Figura 19

Figure 19 : Comparison between a rhodium-plated sample that was not polished prior to plating and one that was.


Ultrasonic cleaning:


This cleaning removes polishing pastes and organic contamination. If not carried out, any residue on the surface can cause white or dark spots, adhesion problems or pitting.


Degreasing:

This is definitely the most important step in activating the surfaces to be electro-plated. If not correctly carried out, it can lead to irregular plating or adhesion problems and the majority of focal defects such as white spots, cloudiness, dark post-oxidation spots, pitting and bubbles (Figure 20).

Figura 20

Figure 20 : Examples of defects found on rhodium-plated samples previously degreased with expired degreaser.


Neutralization and washing:

Statistically these are the phases that are more often neglected during surface preparation. Not correctly carrying them out usually leads to white or dark spots, pitting, bubbles and also the possibility of contaminating the galvanic solution which can consequently lead to further defects generated
by contaminants.

3.2.3. Defects due to using inadequate products

These are the least probable causes of defects especially if extremely common plating baths are used like, for example, those for gold and rhodium plating. Nevertheless, when using the solution, organic or inorganic contaminations can be introduced which, in turn, can lead to defects such as white or
dark spots, cloudiness, bubbles or adhesion problems.

In the case of high-volume galvanic baths, the use of the bath and drag-out factors consume the electrolytic solution components, which then have to be reset. Not doing so will lead to a series of defects such as burning or cloudiness. Solution component consumption can also provoke a change in solution
pH, which can cause defects such as burning, cloudiness, spots, adhesion problems or, lastly, can also determine the onset of metal deterioration factors.

Resetting the galvanic bath components must be carried out by carefully following the technical instructions in order to prevent adding too much, which could generate further defects.

If there is an excess of organic additives, for example, the content can be reduced by active carbon treatments (g). This procedure is also useful for removing organic pollution. If the solution has been contaminated inorganically, selective precipitation methods can be used or dummy plating can be carried
out which will electro-deposit these inorganic contaminants, thus reducing their concentration in the electrolytic system.

Continue reading

The Italian jewellery sector in 2018

The Italian jewellery sector in 2018

The third quarter of 2018 registered a positive trend in global demand for gold jewellery with an authentic leap in quantity (+6%) after the feeble or negative data recorded in the previous quarters. According to the World Gold Council, the weakness in gold prices encouraged purchases, especially in
India (recovering after downturns during the previous quarters), in China (also favoured by Qixi, China’s equivalent of St. Valentine’s Day) and in many other markets in South East Asia. Middle Eastern countries, however, were still in difficulty, according to statistics released by the World Gold
Council.

Fig. 1 – Global demand

 Source: Intesa Sanpaolo World Gold Council data processing – Gold Demand Trend

The leap in global jewellery demand compared with a fall in prices, is a factor also reflected in Italian export dynamics. In the third quarter, in fact, with the negative trend in gold jewellery exports continuing (with a

-8.8% variation in Euro values) clashed with a particularly brilliant 2017. Quantity, on the other hand, developed with a significant leap forward (+35.7%), which implies brusque movements in average unit values (AUV, values divided by quantities).

 Fig. 2 – Gold jewellery export development * (% var. trend)

 Note: (*) Code 711319. Source: Intesa Sanpaolo Istat data processing

In the overall first nine months, foreign sales dropped by 4.1% in value, while they increased by 20.8% in quantity (to a total sum of 28 tons), implying a reduction in average unit values of about 20% in most countries.

The geographic details show how the fall in values in Euro was common to almost all the main destinations with the exception of the United States (+3.2%), the United Kingdom (+27.2%) and South Africa (+12%). In the same way, the growth trend in quantity involved every country with only the United
Arab Emirates registering a drop of 15.2%, confirming it as one of the markets where Made in Italy gold jewellery sales are suffering most greatly.

Table 1 – Development of gold jewellery exports * in the first nine months of  2018 (var. % trend var.)

Note: (*) Code 711319. Source: Intesa Sanpaolo Istat data processing

The French result particularly stands out: shipments of gold jewellery to France in the first nine months of 2018 actually grew by 86.6%, equal to 7 tons more compared to the same period in 2017, with a concurrent collapse in average unit values of over 50%, which could also reflect changes
in transfer price setting within French multi-national groups whose jewellery production is based in Italy. In fact, it should be pointed out how last year’s implied average unit values regarding export flows from Italy to France had reached particularly high levels compared to the average
national figure (with a leap of 32%). It would therefore seem to be a sort of “normalization” of the average values recorded in the previous years. The data concerning the third quarter highlight a recovery in positive export rates towards France, even in terms of values (+16%).

The quantity of gold jewellery exported towards the United States also registered a considerable increase (+51.4%, equal to 5 additional tons), underlining this market’s strong interest in Italian jewellery, in this case, also expressed in value data.

The positive trend in quantities sold abroad is coherent with the even greater growth development in the sector’s industrial production index (which includes costume jewellery and silverware). Production recorded an 8.3% increase in the first 10 months with a new significant acceleration
in the month of October, marking an extremely high development rate (although slower than the +17.4% average in 2017), clearly higher than the +2.6% average trend registered by Italian manufacturing in the same period.  

Fig.  3 – Development in industrial production index (trend % var., raw data)

Fig.  4 – Development in turnover index (trend % var., raw data)

 Source: ISTAT data processing

 Source: ISTAT data processing

The turnover index (which is in value) also continued to grow, although in this case, the slowdown compared to 2017 data was such as to take the fine and costume jewellery sector to the same rates as manufacturing.

On a territorial scale, where the data are only available in value and for the aggregate, which includes costume jewellery, the first nine months of the year confirmed the national downtrend. In more detail, in the overall nine months, Vicenza recorded the most pronounced reduction in
values (-4.8%), with a clear worsening in the summer months (the difference recorded between July and September 2018 and the same period in 2017 was -8.1%). The third quarter in Arezzo also saw a notable arrest in exports (-7.1%), which took the figure for the first nine months to
-2.3%. In the Valenza Po district, export rates (again negative) remained stable in the first part of the year (measured with the relative figure for the entire provincial area, as were the other production poles).

Fig. 5 – Fine and costume jewellery export development (trend % var. in value regarding provincial data)

Source: ISTAT data processing

To be more precise, in Vicenza the negative data involved many markets, especially the emerging ones: Hong Kong (-16.4% in the first nine months), United Arab Emirates (-19.2%), Jordan (‑32.4%), Romania

(-22.3%) and Turkey (-12.8%). Authentic collapses in export values to Jordan (-56.4%) and Turkey (-42%) played leading roles in the summer months and particularly stood out.

However, the opposite is also to be underlined for the United States (the top market outlet in 2017) which recorded an average stable trend in the first nine months of the year and an improvement in the third quarter. Signs of recovery also in exports to the United Kingdom (in the
wake of a negative result in 2017), which, with the third quarter data, seems to have stemmed the fall. Direct exports to South Africa were also good and clearly speeded up in the third quarter.

Tab. 2 –Vicenza gold district exports

Source: ISTAT data processing

Arezzo district’s exports are also continuing to feel the effect of significant and continual drops in the United Arab Emirates (-17.9% between January and September, with a -27% in the third quarter). Like Vicenza then, Arezzo also saw a brusque downturn of sales to Turkey with
a ‑26.7% reported over the summer months. Results relating to the USA, on the other hand, improved, taking the district back into positive figures between July and September, although enough to be able to report an overall growth rate for the first nine months of 2018
(-8.2%). Compared to Vicenza, however, Arezzo managed to maintain sales to Hong Kong at good levels: after the exceptional +21.6% of 2017, Arezzo exports grew by 1.8% in the first nine months. The development rate of the French market (which stayed around +20%) was also good.
Lastly, the excellent sales results, although at limited levels, in Panama and Lebanon are also well worth mentioning.

Tab. 3 –Arezzo gold district exports

Source: ISTAT data processing

Exports in the Valenza Po district, however, were decidedly more focused from a geographical point of view, with almost three quarters of sales made in Switzerland and France. As highlighted also on a national level, the result towards France was what particularly conditioned
the overall data.  In the first nine months of 2018, jewellery shipments from Valenza to France experienced a drop of 20.9% to then improve significantly over the summer. More recent data regarding exports to Switzerland, on the other hand, showed an abrupt downturn (-10.3%)
after a more energetic start to the year, something which also occurred for exports to the USA (-9.8% in the third quarter which, in any case, concluded the first nine months with an overall positive result of +3%). After the extraordinary +55.2% of 2017, direct exports
to Hong Kong finished badly (-14.6%, plummeting to -25.6% in the July to September period).

Tab. 4 –Valenza Po gold district exports

Source: ISTAT data processing

The prospects for the closing months of the year seem set towards cautious optimism. The awakening in global demand, although in a context of considerable uncertainty, leads to a belief in the probable continuation, also in the latter part of the year, of the trends revealed
by the first. We expect that demand from Asian markets and the United States will continue to be vigorous while demand in the Middle Eastern countries will continue to meet with difficulties

The prospects for 2019 appear more uncertain, influenced by the possible recovery in precious metal and gem prices (see the next paragraph) which could have a negative impact on the signs of jewellery demand recovery recorded in the second half of 2018. In any case, in
our scenario, the price of gold should remain – on an annual average – around 1,250 USD an ounce, a level just slightly lower than that of 2018.

Continue reading

Directions in 2020 and technological innovation for jewellery

DIRECTIONS IN 2020 AND TECHNOLOGICAL INNOVATION FOR JEWELLERY

Paola De Luca

Co-Founder & Creative Director at Trendvisions Jewellery + Forecasting, IEG Italy

Paola De Luca, in 2010, together with Italian Exhibition Group S.p.A. – organizer of VICENZAORO, one of the world’s leading international Jewellery Show – co-founded TRENDVISION Jewellery + Forecasting, an independent observatory focusing on trend forecasting for the jewellery industry where she supports the project as its Creative Director. Starting her career in New York in in 1989, she landed her first job as a designer for Fendi Jewellery and Watches. She later collaborated with leading luxury brands, such as Salvatore Ferragamo, Harry Winston, and Swarovski among other prestigious associations. In 2002, in partnership with CRU Group of London, she established TJF Group Ltd. For a decade, TJF Group focused on research and forecasting, launching the unique “TJF Trend book”, which became the world’s first and most authoritative jewellery trends publication. In 2010 Paola started an exciting collaboration with Rio Tinto Diamonds supporting as Design Director, their global design program. As Creative Director she leads design projects; educational programmes for buyers, government organisations; brands and manufacturers; she coordinates researches think tanks and market trends-focused projects. She is the Founder of The Futurist Ltd., a firm specialised in Forecasting and Creative Intelligence.

DIRECTIONS IN 2020 AND TECHNOLOGICAL INNOVATION FOR JEWELLERY

Download the presentation

 

Continue reading

Mechanical processing: focusing on critical points

MECHANICAL PROCESSING: FOCUSING ON CRITICAL POINTS

Andrea Friso

Product Division Manager – Legor Group S.p.A. – Bressanvido VI – IT

Andrea Friso performs at Legor Group S.p.A. as Sales Division Manager of Master Alloys. Materials engineering graduated in 2003, he starts his cooperation with Legor Group in 2004 by writing his graduation thesis about �innovatively colored gold alloys�. He is the company business role operating
between sales force, production and R&D, relying upon matured professional experience on the different product types and their positioning on different markets. He supports sales forces on commercial planning and the achievement of objectives together with their periodic control. He cooperates
with the technical and R&D dept. as for development, enhancement and promotion of products.

This piece aims to concentrate on a few fundamental aspects as to why the plastic deformation of an alloy is so important in the gold smithery production world and then exemplify typical errors which occur in production, triggering a chain of problems which are difficult to correct at the
end of the process. The aim of this piece is to accompany participants along a few logical paths they may find useful for thinking over certain cause/effect aspects pertaining to the plastic deformation production process. One of the most common cases of defects in mechanical processing
is obtaining semi-finished products with fragility or residual tension issues, with individual or a combination of causes and which may sometimes be difficult to pin-point. On average, current processes are also more complex than even a decade ago, due to their greater complexity or more
stringent quality controls.

Download the presentation

 

Continue reading

Improving your investment casting process

Improving your investment casting process

JÖRG FISCHER-BÜHNER

Consultant in Research & Development

Dr. Jörg Fischer-Bühner holds a PhD. in physical metallurgy and materials technology of the technical university RWTH Aachen. Since October 2007 he is active in Research & Development with Legor Group Srl, Italy, and Indutherm GmbH, Germany. Before he was Head of the Physical Metallurgy and Precious Metals Research division of FEM, the German Research Institute for Precious Metals and Metal Chemistry.

Download the presentation

 

Continue reading

Oro artigianale estratto in maniera responsabile: una panoramica delle comunità minerarie peruviane e colombiane

Oro artigianale estratto in maniera responsabile: una panoramica delle comunità minerarie peruviane e colombiane

Che cos’è l’ASM (Artisanal and small-scale mining), cioè l’estrazione artigianale e su piccola scala?

L’attività di estrazione artigianale e su piccola scala può essere descritta come un’estrazione a “bassa intensità”, che riguarda tutti i tipi di estrazione, dai depositi alluvionali all’estrazione in roccia dura1. Le stime variano, ma in generale si calcola che l’ASM fornisca circa il 20% dell’oro estratto
ogni anno, impiegando circa l’80% della popolazione dedita all’attività mineraria. Le attività tendono ad essere informali, con un ridottissimo utilizzo di tecnologie e ad alta intensità di lavoro. In molti casi i minatori utilizzano attrezzi manuali come martelli e scalpelli, mentre le attività più
avanzate si servono al massimo di piccole attrezzature da scavo come i retroescavatori e i dumper. Le comunità ASM tendono inoltre ad essere molto emarginate e hanno scarso accesso alle risorse, in particolare al sostegno statale o industriale. In molti casi la necessità di estrarre i metalli è determinata
dalla povertà e le attività minerarie sono spesso escluse dal normale circuito delle banche, tanto che molte di queste non hanno conti bancari o accesso al credito. Una triste conseguenza di tutto questo è che molte sono costrette a diverse forme di attività criminali.

L’obiettivo di entrambe le organizzazioni Fairmined e Fairtrade consiste proprio nel migliorare tale situazione. Grazie all’introduzione di norme in grado di garantire che tutto l’oro e l’argento prodotto nelle miniere artigianali e su piccola scala, certificate dall’organizzazione pertinente, siano
estratti in modo responsabile, è possibile migliorare la vita e le condizioni di lavoro dei minatori e delle comunità minerarie. Una volta che la miniera viene certificata, non solo è garantito un prezzo equo per i metalli che essa vende, ma anche un sovrapprezzo che l’acquirente deve pagare direttamente
alla miniera per ogni grammo di oro e argento acquistato.

Va sottolineato che non viene estratto abbastanza oro ASM per soddisfare l’intera domanda di oro, sia per la gioielleria che per l’elettronica o per usi medici. Lo scopo di un approvvigionamento responsabile non è quello di soppiantare l’industria mineraria su scala industriale, ma di migliorare le condizioni
di vita e di lavoro dei minatori e delle comunità minerarie nei paesi in via di sviluppo. Molti clienti, in particolare quelli descritti come Generation X e Millennials, hanno abitudini di acquisto diverse e meno tradizionali. Tali clienti pretendono una storia, qualcosa a cui potersi relazionare, vogliono
che i loro acquisti facciano la differenza dal punto di vista sociale e ambientale. Questo rende l’approvvigionamento responsabile di oro e argento molto importante nell’attuale mercato della gioielleria.

Figura 1. Minatori della miniera di Sotrami, Perù.

La figura 1 mostra un marito e una moglie che lavorano nella miniera di Sotrami a Santa Filomena in Perù. Si tratta di una miniera di piccole dimensioni che impiega 700 lavoratori, 160 dei quali sono minatori che lavorano al fronte della miniera. Alcune miniere sono di proprietà privata, altre, come
quella di Sotrami, operano come cooperative, mentre altre sono gestite da operatori indipendenti o da singole famiglie.

Utilizzo del mercurio.

Fairmined e Fairtrade si occupano delle comunità minerarie nei paesi in via di sviluppo, nelle quali vi sono molti problemi collegati all’estrazione artigianale e su piccola scala non regolamentata. Uno dei problemi più gravi è l’utilizzo del mercurio. L’estrazione dell’oro tramite il mercurio è un processo
relativamente economico, semplice e veloce. Il minerale contenente oro viene frantumato in un materiale di consistenza simile alla sabbia e il mercurio viene poi mescolato ad esso, di solito con le mani e in molti casi anche con i piedi. L’amalgama che ne risulta viene poi separato dagli scarti e il
mercurio viene bruciato al fine di ottenere l’oro che avrà una purezza del 75-80% circa.

Figura 2. Utilizzo di mercurio per estrarre oro dal minerale.2

Figura 3. Amalgama di mercurio e oro2.

Figura 4. Il mercurio viene bruciato3.

Purtroppo il mercurio è anche altamente tossico, e non solo il mercurio liquido. Anche i fumi derivanti dalla combustione del mercurio sono molto tossici. Un’esposizione massiccia e prolungata causa danni irreversibili al corpo umano. L’avvelenamento da mercurio danneggia il cervello, il cuore, i polmoni,
i reni e il sistema immunitario. Ciò determina malformazioni alla nascita per molti bambini nati da madri che hanno subito un avvelenamento da mercurio, bambini con ritardi nell’apprendimento e con livelli di intelligenza inferiori alla norma. La contaminazione da mercurio si ripercuote anche sull’ecosistema
circostante: acqua, piante, pesci e animali vengono tutti colpiti. A causa del fenomeno del bioaccumulo, il mercurio penetra tutta la catena alimentare e tende ad accumularsi e aumentare la sua concentrazione in alcune specie vegetali e animali.4 Questo è uno dei motivi principali per cui sarebbe bene
acquistare oro e argento ASM certificati, proprio per garantire che il mercurio non sia stato utilizzato nel processo di estrazione dell’oro.

Criteri per la certificazione delle miniere.

Affinché una miniera possa ottenere la certificazione Fairmined o Fairtrade, devono essere soddisfatti alcuni criteri (1,5,6):

Le miniere sono tenute a partecipare allo sviluppo sociale delle loro comunità,

e devono perciò eliminare il lavoro minorile dalla loro organizzazione. Nessun soggetto con età inferiore ai 15 anni può essere assunto a lavorare nell’organizzazione mineraria e i minori di 18 anni devono lavorare in condizioni non pericolose.

Deve essere implementata una procedura di formazione in materia di salute e sicurezza per tutti i dipendenti e devono essere rispettati gli standard minimi di salute e sicurezza. L’uso obbligatorio di attrezzature per la protezione dell’individuo deve essere sempre rispettato e applicato, e le condizioni
di lavoro devono essere costantemente migliorate.

Le miniere devono riconoscere e rispettare il diritto dei lavoratori a formare o aderire a sindacati e negoziare collettivamente le loro condizioni di lavoro, cioè la libertà di associazione e di contrattazione collettiva.

L’uso responsabile di prodotti chimici è obbligatorio. Se viene utilizzato del mercurio nel processo di estrazione deve essere controllato e si devono concordare piani e scadenze per eliminarne del tutto l’uso. Qualora venga utilizzato il trattamento con il cianuro, è essenziale che venga maneggiato
in modo responsabile. Le sostanze chimiche devono essere ridotte al minimo e, ove possibile, eliminate entro un periodo di tempo concordato.

Qualunque sovrapprezzo pagato nell’ambito dei sistemi di certificazione Fairmined e Fairtrade deve essere gestito in modo responsabile.

Perché i metalli Fairmined e Fairtrade costano così tanto?

La sezione precedente descrive le regole base che le organizzazioni minerarie devono accettare e implementare al fine di conformarsi agli standard e ed essere idonee ad applicare il sovrapprezzo. Dal punto di vista del gioielliere, alcune delle domande più frequenti quando si cerca di acquistare oro
e argento artigianali di provenienza responsabile sono: perché costa così tanto? Dove vanno a finire questi soldi aggiuntivi? E per cosa vengono utilizzati? Sono tutte domande corrette perché l’oro e l’argento con certificazione ASM sono costosi e il gioielliere paga un prezzo superiore a quello di
mercato per i materiali che acquista.

Normalmente l’oro e l’argento acquistati dai fornitori di materiali per gioielleria sono già presenti nel paese. Basta chiamare il proprio fornitore di lingotti o il proprio raffinatore per acquistarli. Tuttavia, a meno che la vostra azienda non si trovi in Perù, Colombia, Bolivia, Mongolia o in alcuni
paesi africani, i metalli Fairmined e Fairtrade non sono presenti nel paese e devono essere esportati dalla miniera e poi importati nel paese in cui svolgete la vostra attività.

Sia Fairmined che Fairtrade utilizzano lo stesso sistema di base. Una percentuale fissa del prezzo di mercato dell’oro o dell’argento viene concordata tra la miniera e l’azienda che acquista i metalli, garantendo così alla miniera un prezzo equo per il suo prodotto. Le procedure di acquisto Fairmined
e Fairtrade regolano questo aspetto per evitare che le miniere vengano sfruttate. Oltre a tale percentuale concordata rispetto al prezzo di mercato dell’oro o dell’argento, l’acquirente paga direttamente alla miniera anche un prezzo per ogni grammo di metallo pregiato acquistato. Tutte le commissioni
sono pagate prima della spedizione dei metalli dalla miniera. Inoltre ci sono costi di trasporto, assicurazione e logistica necessari per trasportare i metalli dalla miniera all’aeroporto, passando per la dogana, su un aereo diretto nel paese di destinazione, passando per un’altra dogana, e infine dall’aeroporto
di arrivo al vostro stabilimento.

Tutto questo può sembrare un po’ complesso, ma così facendo, i minatori ottengono un equo valore di mercato per i metalli venduti e il pagamento del sovrapprezzo direttamente alla miniera, sovrapprezzo che viene poi speso sia per i miglioramenti della miniera che della comunità. Tale somma aggiuntiva
viene normalmente, ma non esclusivamente, depositata presso una banca e quando raggiunge un certo importo, o dopo che è trascorso un certo periodo di tempo, i gestori che si occupano della miniera o i proprietari della cooperativa decidono come spenderla. Di solito i gestori sono responsabili nelle
loro scelte, che vengono monitorate, e ascoltano la comunità prima di decidere sul da farsi, ma le somme vengono comunque spese per il bene della miniera e della comunità.

Alcuni esempi di come vengono spesi tali somme aggiuntive sono le attrezzature per la sicurezza, gli interventi medici, i collegamenti alla rete elettrica o all’acqua corrente, o la costruzione di scuole e luoghi di culto. Nel caso di una delle miniere visitate dall’autore, i responsabili hanno deciso
che il denaro sarebbe stato meglio speso per l’energia elettrica, e perciò hanno cablato la cittadina. L’anno successivo stavano valutando l’opportunità di creare una rete idrica, poiché tutta l’acqua deve essere consegnata giornalmente con dei camion fino alla miniera. Ma, dopo essersi consultati,
hanno deciso di acquistare una torre per cellulari. Molti dei lavoratori della miniera non vivono stabilmente sul posto ma vi lavorano in base a turni della durata di tre settimane, quindi la comunicazione con la casa e la famiglia è importante per loro, ed è per questo che si è optato per una torre
dedicata ai cellulari.

Un’altra delle opere realizzate è stato un campo da calcio. Hanno costruito un altopiano con le rocce di scarto derivanti dalle attività di estrazione mineraria e poi ci hanno costruito il campo sopra (in realtà ne hanno due presso questa miniera, uno a circa 1.900 metri e un altro a 2.750 metri). Per
i minatori e la comunità in generale, si è trattato di soldi spesi molto bene. I lavoratori hanno ora un’attività sportiva che possono praticare sia per mantenersi in forma sia per divertirsi, e hanno persino creato una propria lega che coinvolge altre miniere, migliorando così le relazioni fra le diverse
comunità.

Figura 5. La torre dei cellulari presso la miniera di Sotrami, in Perù.

Figura 6. Campo da calcio presso la miniera di Sotrami, Perù.

Questi sono solo due degli esempi relativi a ciò a cui il denaro Fairmined e Fairtrade ha contribuito per una comunità mineraria. Questo denaro produce effetti positivi contribuendo a migliorare il lavoro, l’ambiente sociale e la comunità. E tale aspetto riguarda tutte le miniere Fairmined e Fairtrade.

Sia in Perù che in Colombia esistono due diversi tipi di attività mineraria ASM, cioè l’attività mineraria regolamentata e quella non regolamentata.

Attività mineraria regolamentata

L’industria mineraria rappresenta solo una piccola percentuale del PIL in Colombia, ma per il Perù circa il 15% del PIL proviene dall’estrazione regolamentata. Non esistono cifre attendibili relative al numero di miniere “in nero” o “non regolamentate”. Il governo vuole che tutte le attività minerarie
siano legali e regolamentate per una serie di motivi. In primo luogo per incrementare la base imponibile e contribuire a far crescere l’economia. Le entrate derivanti dalla tassazione vengono utilizzate per molti progetti e servizi infrastrutturali come la costruzione di strade e l’edilizia in generale,
l’assistenza medica e l’istruzione, solo per citarne alcuni. Il governo intende inoltre implementare norme sanitarie e di sicurezza per salvaguardare i minatori e sradicare la criminalità e lo sfruttamento. Al fine di incoraggiare e incentivare i minatori a lavorare legalmente e in maniera regolamentata,
il governo offre alle miniere regolamentate un rimborso fiscale del 5% per ogni chilo d’oro esportato.

Attività mineraria non regolamentata

Le miniere non regolamentate sono esattamente come descritte e vengono considerate illegali. Non ci sono regole e i minatori spesso lavorano per pochi soldi in pessime condizioni. I minatori non regolamentati possono morire a causa delle loro condizioni di lavoro, della scarsa salute e sicurezza, dell’uso
di mercurio e di altre sostanze chimiche. Vengono inoltre sfruttati in vari modi dai criminali e purtroppo per molti di loro non c’è via d’uscita da questa situazione.

Le miniere non regolamentate sono normalmente ubicate in aree remote e difficili da raggiungere. Quelle osservate dall’autore in Perù erano spesso ad alta quota in zone montuose, prive di collegamenti o sentieri, e a decine di chilometri dalle strade asfaltate più vicine. Tutto ciò che serve per l’attività
estrattiva deve essere portato fino alla miniera a dorso d’asino o a mano. Provviste come cibo e acqua, utensili e attrezzature, legname per puntellare le gallerie al fine di renderle sicure, mercurio per la lavorazione del minerale, tutti questi rifornimenti devono essere trasportati nelle aree remote
dove sono collocate le miniere. Il tasso di mortalità dei minatori è relativamente alto. Le gallerie spesso non vengono puntellate perché è prima necessario portare il legname alla zona generale e poi su per la montagna, perciò spesso i minatori evitano di farlo e il peggio accade producendo un collasso
delle gallerie. Anche i problemi di avvelenamento da mercurio sono frequenti. Negli ultimi anni, su richiesta del governo peruviano, il dottore responsabile del centro medico di Sotrami ha analizzato campioni di sangue dei minatori non regolamentati e ha riscontrato livelli di mercurio superiori ai
limiti legali di sicurezza. Di conseguenza, i proprietari delle miniere non regolamentate non hanno permesso di effettuare ulteriori test per timore di subire una chiusura forzosa. I minatori non regolamentati hanno bisogno di lavoro e perciò, come detto in precedenza, sono intrappolati in questa situazione.

Miniere di Macdesa e Sotrami, Perù.

Le miniere di Macdesa e Sotrami si trovano rispettivamente nelle province di Chaparra e Lucanas in Perù, a circa 600 km a sud di Lima e all’incirca tra Nazca e Arequipa. Entrambe queste province si trovano nel deserto di Atacama, che è il luogo più secco della Terra. Si tratta di una regione prevalentemente
montuosa e desertica, dove la polvere ha la stessa finezza del cemento. E’ un ambiente molto difficile in cui vivere e lavorare. 

La miniera di Macdesa è nata da un semplice buco nel terreno come tutte le altre miniere della regione. Molte di queste miniere furono abbandonate dopo l’indipendenza del Perù dalla Spagna nel 1821. Quando la gente ha iniziato a riprendere le attività minerarie, fatto questo relativamente recente, l’area
è divenuta un vero e proprio far west, con gruppi minerari rivali che si combattevano fra loro. I minatori si guadagnavano a malapena da vivere, la sicurezza era inesistente e la vita era molto dura. Di conseguenza a Macdesa, circa 350 minatori concordarono che l’unico modo per migliorare la situazione
era quello di collaborare e formare una comunità.

Macdesa si trova a circa 1.500 metri di altitudine e all’inizio i minatori usavano attrezzi da lavoro e carriole per trasportare il minerale dalla miniera. Riempivano sacchi di minerale da circa 60 kg nella miniera e li caricavano poi sugli asini. Una volta che gli animali erano carichi, camminavano
per 50 chilometri giù per la montagna fino allo stabilimento di lavorazione vicino alla costa e all’autostrada principale. Tuttavia i minatori venivano sfruttati e ricevevano solo l’80% circa del valore di mercato del metallo. Come se questo non bastasse, le bilance e i campioni venivano truccati e
il minerale veniva lavorato in modo errato. Di conseguenza, solo raramente i minatori ricevevano un valore superiore al 50% dell’oro estratto.

Perciò, al fine di migliorare i rendimenti e i pagamenti, i minatori hanno iniziato a lavorare il minerale a mano con il mercurio, ma non avevano idea di quello che stavano facendo. Così hanno deciso di investire nelle loro tecnologie e di assumere persone che conoscevano queste tecniche. Anche nel caso
di Sotrami la situazione era analoga. I 350 “soci” – i fondatori – ricavavano appena il necessario per vivere. Il resto del denaro ricevuto per l’oro veniva reinvestito per risolvere problemi e migliorare i loro processi e i loro rendimenti. Altri interventi effettuati comprendevano:

Acquisto di adeguate attrezzature per la trivellazione.

Realizzazione di strade in pietra e sterrate per far salire e scendere i camion dall’impianto di lavorazione alla miniera.

Miglioramento degli alloggi per i minatori e le loro famiglie.

Investimento in attrezzature per la sicurezza e implementazione di un programma di formazione per tutti i lavoratori.

Miglioramento delle condizioni di lavoro presso la miniera. Pompaggio di aria all’interno delle miniere al fine di prevenire problemi polmonari. Puntellamento delle pareti e dei soffitti in modo da rendere sicuri i tunnel. Installazione di binari ferroviari per consentire ai minatori di spostare facilmente
il minerale dalla miniera all’interno dei tunnel.

Avvio di test sul minerale direttamente in loco. In questo modo hanno potuto sapere esattamente quale fosse la purezza del minerale estratto e quindi quali rendimenti attendersi.

Hanno inoltre sviluppato processi di trattamento del minerale senza l’uso del mercurio.

Hanno acquistato una macchina alesatrice, che permette loro di testare le perforazioni effettuate e vedere in che direzione si estende la vena, riducendo notevolmente la quantità di scavi necessaria e aumentando la resa di minerale recuperato rispetto a quello estratto.

La storia della miniera Sotrami è simile. I fondatori hanno creato la miniera trent’anni fa e il villaggio di Santa Filomena è cresciuto intorno alla miniera a circa 2.750 metri di altitudine. La popolazione di Santa Filomena è di circa 2.000 persone, di cui 700 lavorano direttamente per la miniera,
mentre la restante popolazione è impiegata in occupazioni volte a sostenere la produzione della miniera. Come a Macdesa, tutti i profitti vengono reinvestiti nella miniera e nella comunità. I lavoratori sono tenuti a prestare servizio per 20 giorni ed hanno diritto a 10 giorni di ferie non retribuite,
di solito con turni di undici ore. Molti minatori vivono lontano dalla miniera e nel corso dei 10 giorni di riposo tornano a casa per vedere la famiglia, anche se alcuni hanno portato lì le loro famiglie, soprattutto se entrambi i genitori lavorano in miniera.

Molto spesso, i governi locali e nazionali vedono l’industria mineraria come un’industria in crescita. Grazie al sindaco della città locale di Challa, che ha coinvolto anche il governo centrale, sono stati posati 40 km di strada asfaltata che collega la miniera alla principale autostrada della costa
pacifica, dimezzando così i tempi di percorrenza fino alla miniera. Gli abitanti delle zone ad altitudini meno elevate hanno approfittato della strada e hanno iniziato a coltivare, perché grazie alla strada possono coltivare i loro prodotti e trasportarli facilmente fino al mercato senza danneggiarli
durante il tragitto. In molti casi, quando le attività di estrazione mineraria hanno successo, sono in grado di generare indirettamente opportunità anche per altri. La strada è importante anche per altri motivi, uno dei quali è che a Sotrami non c’è acqua corrente. Tutta l’acqua di cui hanno bisogno
viene trasportata in montagna tre volte al giorno con tre autocarri.

In genere, nulla va sprecato. La roccia di scarto invece di essere buttata, viene utilizzata per apportare migliorie. La roccia di scarto viene utilizzata al fine di creare aree pianeggianti edificabili in modo da espandere le strutture intorno alla miniera e gli edifici del villaggio.

Figura 7. Rocce di scarto utilizzate per creare uno spazio edificabile pianeggiante a Sotrami, in Perù.

Macdesa ha tre gallerie all’interno della miniera, mentre Sotrami ne ha solo una e tutti i minatori devono entrare ed uscire dalla miniera attraverso questo ingresso e da questo pozzo. La miniera è profonda 630 metri, è composta da 13 livelli, e ci vogliono 35 minuti per scendere e 45 minuti per risalire
dopo aver terminato il proprio turno di lavoro.

Figura 8. L’entrata della miniera di Sotrami, in Perù.

Figura 9. Il pozzo di ingresso di Sotrami, in Perù.

Figura 10. Una delle gallerie di Sotrami, in Perù

All’interno della miniera, il legname viene utilizzato per puntellare la volta, ma generalmente l’illuminazione è presente solo nel luogo in cui si scava e, per sicurezza, lungo le rotaie destinate al trasporto del minerale estratto. Sebbene tutto il legname debba essere trasportato dalla costa perché
non ci sono alberi nelle vicinanze delle miniere, l’aspetto positivo è che il clima è così secco che non ci sono microbi in grado di intaccare il legno e i fenomeni di putrefazione non rappresentano in genere un problema.

Le attrezzature di sicurezza devono essere indossate da tutti quando si è in profondità nelle miniere, in particolare elmetti e soprattutto i respiratori, perché l’unica costante è la polvere. Le particelle di polvere possono causare seri problemi polmonari se i minatori non sono protetti. Si tratta
di una situazione molto differente rispetto alle miniere della Colombia, che si trovano in un ambiente completamente diverso.

Lavorazione del minerale: la roccia dura.

Per quanto riguarda la lavorazione del minerale, entrambe le miniere utilizzano processi simili. Entrambe le comunità in origine utilizzavano il mercurio, ma ne hanno poi compreso la tossicità per se stessi e per l’ambiente, ed hanno così optato per l’utilizzo del cianuro, molto più rispettoso dell’ambiente.

Figura 11. Grande cilindro a rotazione.

Il minerale viene prima frantumato e poi lavorato fino a raggiungere la consistenza della sabbia in un grande cilindro a rotazione. L’acqua viene introdotta per facilitare il processo di frantumazione e per formare un impasto, che esce dal cilindro attraverso un filtro, facendo in modo che le particelle
siano sufficientemente piccole per la lavorazione downstream. Una volta filtrato, il liquame viene pompato in vasche di trattamento contenenti cianuro. L’oro presente viene portato a galla nella soluzione di cianuro e viene poi pompato in vasche di reazione contenenti particelle di carbone attivo. Le
particelle di carbone attivo hanno all’incirca le dimensioni di un chicco di riso e attirano l’oro, catturandolo quando si trova in sospensione. Normalmente vengono utilizzati tre serbatoi in sequenza al fine di massimizzare il recupero dell’oro; tale procedura raccoglie complessivamente il 96% dell’oro
che viene immesso sotto forma di minerale frantumato.

A questo punto, la soluzione di cianuro usata viene smaltita nelle vasche per la raccolta del cianuro di scarto. Tutta questa procedura può sembrare pericolosa, ma è in realtà sicura per l’ambiente e non deve essere confusa con il processo di lisciviazione del cianuro, in cui la sostanza chimica viene
esposta al minerale senza alcun trattamento ed entra in contatto diretto con l’ecosistema senza nessun vincolo o contenimento, oppure con protezioni insufficienti. Le vasche per il cianuro di scarto sono dotate di membrane molto resistenti e i liquidi di scarto vengono trattati chimicamente in modo
che quando sono esposti alla luce ultravioletta si convertono in carbonati, così da eliminare le sostanze tossiche dalle vasche. Una volta riempite, le vasche vengono coperte e poi interrate o utilizzate per costruzioni.

Figura 12. Il filtro per i liquami.

Figura 13. Vasche con carbone attivo.

Figura 14. Vasca per il cianuro di scarto. La fotografia è stata scattata da una precedente vasca che è stata riempita, coperta e interrata.

Figura 15. Rete di elettrodi in acciaio all’interno della vasca per la placcatura.

La parte successiva del processo consiste nel filtrare le particelle di carbonio cariche d’oro dalle vasche, trasferirle in una cella elettrolitica e poi continuare con la placcatura dell’oro su una rete d’acciaio. Una volta che questa fase del processo è stata completata, la parte finale consiste nel
collocare la rete d’acciaio carica d’oro in un forno, in modo tale che l’oro si fonda e venga trasformato in lingotti di oro grezzo. La purezza dell’oro è a quel punto nell’ordine dell’80% e il prodotto può essere ulteriormente lavorato o venduto sul mercato come oro grezzo.

Come vengono spese le somme aggiuntive.

Come detto in precedenza, le somme aggiuntive sono state spese per vari progetti: reti elettriche, acqua corrente, torri per cellulari e campi da calcio. Se ai minatori viene chiesto perché fanno quello che fanno e perché sono felici di ottenere la certificazione Fairmined and Fairtrade, avranno molte
ragioni da addurre. Tra queste la salvaguardia ambientale, le migliori condizioni di lavoro, la prosperità, ma tutti concordano sul fatto che una delle ragioni principali sono i loro figli. Si tratta di imprese relativamente recenti e i minatori vogliono che i loro figli abbiano una vita e delle opportunità
migliori di quelle che hanno avuto loro. Questi minatori non avevano praticamente nulla, hanno creato imprese e fondato delle comunità, e stanno cominciando a vivere meglio.

Un ottimo esempio in questo senso sono le loro scuole. A Macdesa i soldi sono stati spesi per la scuola materna e la scuola elementare, che sono edifici in muratura permanenti, recintati, puliti, sicuri e dotati di computer. Ai bambini viene insegnato l’uso del computer e di internet in tenera età, e
nel doposcuola anche agli adulti viene insegnata l’informatica, e quindi è evidente che i soldi sono stati ben spesi.

Figura 16. La scuola elementare di Macdesa.

Figura 17. Computer acquistati grazie alle somme aggiuntive Fairmined e Fairtrade.

I minatori di Sotrami hanno utilizzato i soldi aggiuntivi in modo simile. Sotrami vanta un ottimo centro medico e tale centro è molto importante non solo per la miniera, ma anche per l’intera comunità circostante. La miniera finanzia questi progetti grazie alle somme aggiuntive pagate per il minerale,
ma permette a chiunque ne abbia bisogno di usufruirne, indipendentemente dal fatto che lavorino presso la miniera o meno. Viene visto come un dovere nei confronti della comunità. Il centro medico è ben attrezzato e ha un dottore presente sul posto in maniera permanente. Il prossimo oggetto che desiderano
è un’apparecchiatura a raggi X. Immaginate di dover percorrere 70 km giù per una montagna lungo strade sterrate per effettuare delle radiografie a causa di una frattura.

Le miniere peruviane discusse finora rappresentano delle storie di successo, e lo stesso si può dire per le miniere colombiane.

Le miniere di Iquira, Coodmilla e Gualconda in Colombia.

Le miniere d’oro in Colombia che tratteremo in questa sede sono più piccole delle miniere peruviane. La miniera di Iquira si trova nel distretto di Huila, mentre le miniere di Coodmilla e Gualconda si trovano nel distretto di Narino, entrambe aree molto rigogliose e a quote elevate. Come nel caso delle
miniere peruviane, le miniere di Narino sono ubicate in aree remote e a circa quattro ore di macchina lungo strade sterrate dalla più vicina autostrada.

In Colombia esistono molti degli stessi problemi relativi alle attività minerarie regolamentate e non regolamentate, come è stato notato per il Perù, e fino a poco tempo fa molte zone della Colombia erano pericolose e di fatto inaccessibili agli stranieri. Tuttora sono molte le attività minerarie illegali
che il governo colombiano vorrebbe regolamentare, ma al momento l’implementazione di tali regole si è rivelata un problema. Dire semplicemente ai minatori che non hanno più diritto ad estrarre il minerale senza fornire loro alcuna alternativa, non funziona. La maggior parte delle comunità non ha un’alternativa
e quindi torna a lavorare nelle miniere illegali per sopravvivere. La Colombia ha vietato per legge l’uso del mercurio, ma è difficile applicare tali norme per vari motivi. Il governo colombiano ha recentemente modificato le norme relative alle banche in funzione anticorruzione e antiterrorismo. Una
conseguenza di ciò è che diverse organizzazioni minerarie hanno perso la capacità di esportare il loro oro, anche se Fairmined sta collaborando con le miniere e il governo per cambiare questa situazione.

La cooperativa di Iquira si trova a sud-ovest di Bogotà, nella regione di Huila in Colombia. Questa regione è famosa per il caffè e infatti alcuni dei minatori di Iquira coltivano anche il caffè. Le miniere si trovavano sulle loro terre, e i contadini sapevano bene che l’oro era lì, ma non hanno iniziato
ad estrarre il minerale fino al 2004, quando si sono organizzati in una cooperativa di 11 azionisti. Già nel 2010 i soci della cooperativa erano diventati 35, di cui 8 donne, e la cooperativa possiede 11 miniere legalmente registrate. Inizialmente vendevano il loro oro in modo informale al mercato locale,
ma la certificazione Fairmined ha dato loro la possibilità di esportare.

Figura 18. Narino, Colombia7.

Ogni miniera rappresenta un’attività separata, ma tutte lavorano come una cooperativa a fini bancari, di vendita e di esportazione, ottenendo così prezzi migliori e costi ridotti. Si tratta di miniere relativamente piccole che scendono in profondità per circa 500 metri nella montagna su uno, due o tre
livelli. Circa il 20% dell’occupazione a Iquira è attribuibile alla cooperativa e tutti i lavoratori provengono dalla regione.

Figura 19. L’entrata di una miniera a Iquira in Colombia.

Le miniere colombiane sono inoltre molto diverse da quelle del Perù per quanto riguarda le condizioni ambientali. Mentre le miniere peruviane sono molto secche e polverose a causa dell’aridità della regione, le miniere colombiane sono molto umide. L’acqua scorre attraverso la roccia porosa e lungo uno
strato di quarzo contenente oro che è impermeabile, e di conseguenza l’acqua arriva fino alla miniera. I minatori raccontano che ricercano proprio quest’acqua, perché dove c’è acqua, c’è oro.

Tutte le miniere dispongono di sistemi di sicurezza, necessari per essere idonei a ottenere la certificazione e le somme aggiuntive. Ogni miniera è dotata di un sistema di rilevamento del gas e di un sistema di allarme per proteggere i lavoratori, oltre ad aree di sicurezza utilizzate in caso di fughe
di gas o crolli. Tutte le miniere e gli impianti di lavorazione sono dotati di kit di pronto soccorso e tutti i lavoratori dispongono delle necessarie attrezzature per la protezione individuale. Alcune miniere della cooperativa impiegano donne per maneggiare gli esplosivi ed effettuare le esplosioni,
ma anche per lavorare direttamente nella miniera.

Figura 20. Un’area di sicurezza presso la miniera di XXXX

Figura 21. Minatori in Colombia1.

Figura 22. Sacchi di minerale in attesa di essere trasportati all’impianto di lavorazione.

Figura 23. Sacchi di minerale nel laboratorio destinato alla frantumazione presso Iquira, in Colombia.

Una volta estratto e insaccato, il minerale viene trasportato all’impianto di lavorazione. Ogni sacco contiene circa 50 kg di minerale che viene lavorato in modo simile a quello utilizzato nelle miniere peruviane, tranne che su scala più piccola. A causa della natura dei depositi, dopo la frantumazione,
viene utilizzato un tavolo di flottazione per separare l’oro dalle particelle contenenti oro. Non utilizzano il carbone attivo ma vasche di decantazione e lasciano che le soluzioni ricche d’oro si separino per gravità prima di essere ulteriormente trasformate in lingotti di oro grezzo. Nelle miniere
di Coodmilla e Gualconda, nel distretto di Narino, si usa un processo molto simile: la frantumazione seguita da un tavolo di flottazione e da un trattamento al cianuro.

Figura 24. Il tavolo di flottazione di Gualconda, in Colombia.

Figura 25. L’oro separato dagli scarti dopo il trattamento nella vasca di decantazione.

La cooperativa di Coodmilla, un’organizzazione senza scopo di lucro, è attiva da quarant’anni e possiede quattro miniere, due delle quali hanno ottenuto la certificazione Fairmined. Le loro licenze comprendono cento ettari di terra, ma attualmente ne lavorano circa tre ettari. Purtroppo sia Coodmilla
che Gualconda non hanno alcuna certificazione al momento, ma non a causa delle procedure di estrazione del minerale. Hanno difficoltà a soddisfare i nuovi requisiti bancari descritti sopra. Fairmined sta collaborando con loro e con il governo colombiano per risolvere la questione.

Figura 26. Una vena di quarzo contenente oro.

Figura 27. La miniera di Gualconda a Narino, in Colombia.

Entrambe le miniere si trovano in aree remote. Gualconda è stata costruita nella giungla sfruttando il pendio naturale della collina per i processi di lavorazione. Almeno nel caso di Gualconda, il percorso di crescita si è rivelato fino ad oggi complesso. Nel 1974 l’estrazione avveniva completamente
a mano tramite attrezzi da lavoro e mercurio, ed erano necessari 466 grammi di mercurio per lavorare una tonnellata di minerale. Una volta utilizzato, questo mercurio di scarto veniva gettato direttamente nel fiume e quindi nell’ecosistema. La miniera ha fatto grandi progressi per quanto concerne la
bonifica dei siti contaminati da mercurio, ma vi sono ancora tracce del mercurio utilizzato per il processo di lavorazione che devono essere bonificate.

Figura 28. Il sito contaminato dal mercurio presso Gualconda.

Tra il 2001 e il 2006 la miniera è stata chiusa a causa del conflitto armato che ha coinvolto forze paramilitari e i coltivatori di coca. Per questo circa un centinaio di famiglie sono state sfollate ed hanno abbandonato la zona per andare ad abitare nella vicina città, ma i minatori affermano che la
vita in città non era fatta per loro. Nel 2006 hanno costituito la loro cooperativa, ma la loro politica ASM era appena agli inizi e molto carente per quanto riguarda la lavorazione del minerale e la tutela dell’ambiente. Tra il 2009 e il 2013 non avevano ancora a disposizione l’energia elettrica per
le attrezzature, e per questo hanno costruito un mulino ad acqua utilizzando parti provenienti da un deposito di rottami. In quel periodo utilizzavano ancora il mercurio, ma avevano compreso che, per diventare produttori ASM responsabili, era necessario eliminare gradualmente l’uso del mercurio. Inizialmente
la loro politica consisteva nel riutilizzare il mercurio invece di gettarlo semplicemente nel fiume, evitando i danni ambientali e riducendo la quantità necessaria per trasformare una tonnellata di minerale dai precedenti 466 grammi a 25 grammi. Nel 2015 hanno finalmente ottenuto l’energia elettrica
fino alla miniera e questo ha permesso loro di riprogettare l’impianto di lavorazione, eliminando completamente il mercurio e adottando il processo a base di cianuro che utilizzano oggi, più ecologico e sicuro. Questo ha permesso loro di aumentare del 20% l’efficienza, ma ha anche prodotto un incremento
dei costi – il cianuro è più costoso del mercurio quando viene utilizzato per la lavorazione di minerali contenenti oro. Tuttavia l’aumento dell’efficienza e la consapevolezza che con la certificazione Fairmined avrebbero ottenuto prezzi migliori per il loro oro, oltre ad un sovrapprezzo, hanno reso
questo processo la scelta privilegiata per aiutarli a diventare produttori ASM responsabili. Dopo tre anni tale processo si è finalmente realizzato.

L’obiettivo dei minatori non è quello di fermarsi, ma di migliorare continuamente, e ciò include anche la bonifica dell’area della miniera rimanente in cui il mercurio è stato utilizzato per la lavorazione. Questa miniera è ora considerata un modello per l’estrazione ASM responsabile e ogni due settimane
si organizzano visite guidate per vedere cosa è stato realizzato e in che modo è stato possibile realizzarlo. Tutto questo sullo sfondo di una Colombia che continua a presentare molti conflitti sociali ed un elevato grado di corruzione: violenza e traffico di droga sono ancora prevalenti in queste zone.
Anche oggi molte licenze minerarie vengono assegnate a grandi miniere, ma le miniere indipendenti più piccole spesso vengono ignorate.

Perché utilizzare oro ASM estratto in maniera responsabile?

Ci sono molte ragioni per acquistare oro ASM estratto in maniera responsabile e con certificazione ASM. Può essere un buon modo per attirare più clienti nel proprio negozio e quindi può rappresentare un fattore positivo per ogni attività. Se la vostra base clienti è composta da persone anziane, l’introduzione
di una linea di prodotti che utilizza l’oro ASM può rivelarsi molto interessante sia per i giovani consumatori che acquistano gioielli, sia per i clienti che non hanno ancora considerato l’acquisto di gioielli in oro, poiché acquistare qualcosa che fa la differenza nel mondo risulta interessante ai
loro occhi. Ci sono gioiellieri che hanno deciso di convertirsi completamente all’oro Fairtrade e Fairmined, o almeno il più possibile, anche se non è indispensabile apportare modifiche così drastiche. Qualunque cambiamento può aiutare e non c’è assolutamente nessun obbligo, non è necessaria la conversione
totale di ogni prodotto d’oro che si vende. Molti gioiellieri che optano per l’oro ASM iniziano con una linea o una collezione, e, se hanno successo o se l’idea si rivela adatta alla propria clientela, espandono la loro offerta. Si tratta di un prodotto con alle spalle una storia: da dove proviene l’oro,
a cosa serve il sovrapprezzo pagato. Acquistare questi prodotti significa ripagare le comunità dei paesi in via di sviluppo per migliorare la loro qualità di vita. Anche solo questo aspetto basta spesso ai clienti per acquistare gioielli realizzati con l’oro ASM.

Come è possibile acquistare l’oro ASM?

Il modo migliore per acquistare l’oro ASM è quello di contattare Fairtrade o Fairmined (ARM) tramite i rispettivi siti web, e in questo modo otterrete un elenco di fornitori e gioiellieri registrati e certificati dalle stesse organizzazioni. Entrambe le organizzazioni hanno sistemi simili: se si desidera
usare il nome e il marchio Fairtrade o Fairmined, è necessario essere in possesso di regolare licenza. A seconda del proprio volume d’affari, può essere necessario versare un contributo per ottenere la licenza, sottoporsi ad una revisione contabile e pagare una commissione all’organizzazione per ogni
grammo d’oro venduto. Se non si desidera utilizzare il nome o il marchio, è comunque possibile acquistare l’oro – non ci sono restrizioni sull’acquisto. In questo caso potrete comunque chiamarlo oro proveniente da miniere artigianali, ma non potrete chiamarlo Fairmined o Fairtrade.

L’obiettivo di ogni azienda consiste nel generare profitti. Tuttavia quest’oro è relativamente costoso e quindi per mantenerlo il più attraente possibile per il consumatore, si consiglia di non aggiungere al prezzo di vendita dei prodotti Fairmined e Fairtrade il sovrapprezzo che è stato pagato alla
miniera. Per mantenere il costo di ogni pezzo il più basso possibile, chiedete al fornitore di comunicarvi il costo dell’oro acquistato come se non avesse la certificazione Fairtrade o Fairmined. Tale prezzo può essere utilizzato per calcolare il ricarico e quindi il costo del sovrapprezzo aggiunto
alla fine. Più oro Fairmined e Fairtrade si vende, più grande sarà il contributo offerto per migliorare la vita dei minatori e delle comunità minerarie.

Conclusioni.

La provenienza dell’oro da gioielleria rappresenta una scelta personale per il gioielliere. I fornitori di oro decidono come comportarsi – in maniera responsabile o meno – e di conseguenza i gioiellieri possono decidere dove e presso chi acquistare. La scelta c’è. I gioiellieri che vogliono partecipare
all’iniziativa di approvvigionamento responsabile possono farlo. Ci sono tre soluzioni principali tra cui scegliere:

Oro estratto in modo responsabile ma su scala industriale. Laddove vi sono controlli e revisioni, sarete in grado di capire da dove proviene l’oro e potrete informare di ciò i vostri clienti.

Oro prodotto al 100% da materiali riciclati al 100%. Questa fonte di approvvigionamento ha già pagato il suo prezzo in termini ambientali, e scegliendo questa opzione si ottiene, probabilmente l’oro in assoluto più rispettoso dell’ambiente, poiché si utilizza quanto è già stato estratto.

Oro estratto in modo responsabile da comunità ASM. Questa scelta aiuta direttamente le comunità minerarie dei paesi in via di sviluppo a migliorare le loro condizioni di lavoro, il loro ambiente e la loro vita in generale.

Oppure si può scegliere di non partecipare affatto, è una scelta personale. Ma si tenga presente che

  • un approccio responsabile può ripagare anche in termini economici.
  • Tutto l’oro è stato estratto da qualche parte.
  • L’obiettivo di tutti dovrebbe essere quello di eliminare le attività di estrazione non responsabili dal nostro settore.
  • Devono esserci trasparenza e tracciabilità dalla miniera e/o dal riciclatore fino al rivenditore al dettaglio.
  • C’è ancora bisogno di una maggiore informazione sia all’interno del settore che tra i consumatori.

Infine, si può dire che l’oro ASM sia diverso da tutto il resto dell’oro? La risposta, ovviamente, è no, non è in alcun modo differente. L’oro è oro, in qualunque modo lo si produca. Tuttavia, ciò che lo rende diverso è il luogo di provenienza, il modo in cui lo si ottiene, e il prezzo ambientale e sociale
che viene pagato per farlo arrivare fino a noi.

Riferimenti

www.fairmined.org

Fotografie:
www.artisanalgold.org

Fotografie: Marieke Heemskerk

en.wikipedia.org/Mercury_poisoning

www.fairtrade.org.uk

www.responsiblemines.org

Fotografie: Fairmined Family

Continue reading

Applicazioni della metallurgia additiva nell’ambito della gioielleria

Laser metal fusion è anche conosciuta come stampa 3d metallica ed è oggi il campo dell’industria manifatturiera additiva con il maggior potenziale di sviluppo e crescita.

Un laser ad alta intensità fonde selettivamente le polveri metalliche secondo un percorso definito dal disegno CAD e creando oggetti tridimensionali in metallo.

I metalli utilizzabili con la tecnologia LMF sono svariati, tra i più diffusi l’acciaio e il bronzo e in fase di ricerca e approfondimento, l’oro e il platino. Da questa ricerca, iniziata qualche anno fa con la collaborazione di sisma, inizia la nostra esperienza nella stampa additiva.

Abbiamo iniziato con alcuni test e immediatamente ci siamo resi conto che la produzione additiva ha caratteristiche che la rendono unica.

Vediamole insieme

La produzione additiva ha caratteristiche peculiari che la rendono unica:

Nessun costo per la realizzazione di stampi utensili e attrezzature di produzione

Riduzione dei costi e dei tempi dal disegno al prototipo

Abbattimento degli scarti di produzione e tutela dell’ambiente

Massima libertà in fase di progettazione. LMF infatti permette la realizzazione di forme estremamente complesse, non realizzabili con altre tecnologie

Nello specifico:

– 1 – nessun costo per la realizzazione di stampi utensili e attrezzature di produzione

Con LMF sono stati realizzati in modo semplice e a costo contenuto centrature macchina precise, da utilizzare ad esempio per l’incisione laser, meccanismi di bloccaggio per la realizzazione di pezzi in serie e piccoli stampi che riducono in fase produttiva i costi e i tempi di riproduzione.

– 2 – riduzione dei costi e dei tempi dal disegno al prototipo

Realizzare un modello con LMF e arrivare al master risulterà molto più rapido che seguendo i classici canali produttivi. Alla percezione visiva dell’oggetto realizzato a video, seguirà poche ore dopo, la percezione tattile dello stesso snellendo notevolmente i tempi di verifica per la realizzazione
del master.

– 3 – abbattimento degli scarti di produzione e tutela dell’ambiente

Produrre con LMF significa ridurre gli sprechi e il consumo di energia. Nelle lavorazioni tradizionale, un esempio è la microfusione, i passaggi per realizzare un oggetto sono molteplici con scarti relativi a cere, gomme e gesso. Il tempo dell’intero processo produttivo con LMF si riduce radicalmente
e con esso gli scarti.

– 4 – massima libertà in fase di progettazione. LMF permette la realizzazione di forme estremamente complesse, non realizzabili con altre tecnologie

I vantaggi di una stampa diretta a metallo rispetto ad una fusione a cera persa sono incredibili sia in termini di precisione che in termini di possibilità geometriche. Un esempio sono i gioielli simili a sculture. L’esperienza Nuovi Gioielli con LMF è nata dal desiderio di avvicinarci a nuove tecnologie
e tecniche di produzione, affiancandole all’artigianalità del prodotto. Con LMF il design diventa sempre nuovo, e, molti limiti creativi spesso dovuti alle tecniche di lavorazione tradizionali e alla molteplicità dei passaggi produttivi, con questa tecnica sono elusi.

Un oggetto come quello che vedete può essere realizzato con un solo processo.

La creatività si può esprimere senza limiti, unendo estetica, tecnica produttiva e architettonica in un solo oggetto, avendo potere sui risultati che vogliamo ottenere.

Il designer controlla il processo completo di produzione, dalla fase creativa alla verifica delle polveri, dalla fusione al post process.

METALPIXEL

La ricerca e lo sviluppo nei processi LMF ci hanno portato alla realizzazione del METALPIXEL.

Nel 1800 il tessuto jacquard ha rivoluzionato il settore tessile nel xix secolo, ed è grazie a questo speciale telaio che ancora oggi possiamo creare complessi disegni ed innumerevoli fantasie negli abiti che indossiamo.

Più di 200 anni dopo, Nuovi Gioielli, avvalendosi della tecnologia LMF, applica la morbidezza del tessuto e il decoro jacquard al metallo. Il risultato è qualcosa di unico, dalle molteplici applicazioni, non solo nello specifico settore della gioielleria, ma nel più ampio settore della moda e del
design.

Immaginate il polsino di una camicia, immaginate l’inserto di una borsa, oppure, immaginate semplicemente, perché il METALPIXEL, può essere tutto ciò che volete.

Da sempre i tessuti e i suoi decori sono stati l’essenza della moda. Il metallo, per la sua solidità, ha sempre ricoperto il secondario ruolo di accessorio. Oggi, con questa nuova tecnica di lavorazione, può diventare parte integrante di un nuovo modo di concepire il tessuto, la morbidezza e il costume.

Continue reading

Potential and innovation of the selective laser melting technique in the manufacture of platinum jewelery

Potential and innovation of the selective laser melting technique in the manufacture of platinum jewelery

One of the revolutionary characteristics of selective laser melting is the possibility of processing materials that are notoriously difficult to use with other production techniques due to various melting mechanisms and modelling the alloys used in direct 3D metal printing. In the field of precious alloys,
this aspect is particularly interesting in the manufacture of platinum-based jewellery, a notoriously problematic element in the production phase due to its properties which include a high melting temperature of the alloys available on the market today and high reactivity with the materials it comes
into contact with. Consequently, production costs are higher, specific furnaces need to be used and, on average, the items produced often have defects. An assessment of the real revolutionary scope of using the SLM technique in making platinum jewellery was carried out by comparing SLM and traditional
production techniques, not only considering the technical aspects but also, and above all, the impacting economic/financial implications on the production line, in order to understand whether, and to what extent, introducing the SLM technique would lead to improvements in platinum jewellery manufacturing.

INTRODUCTION

In the article presented at the Santa Fè Symposium 2017 (1), we dealt with a general comparison between precious metal micro-casting and direct 3D printing processes (SLM) to understand whether this latter technique was effectively more advantageous than both classic and direct casting.

Among the production cases in which selective laser melting proved to be better, we identified the production of small series, the creation of hollow jewellery or items with complex geometries and no welding, or, when using difficult materials or those that are impossible to use in micro-casting.

Platinum jewellery production could be included in these cases since the casting of this material is famously more difficult than jewellery in gold or silver alloy (3). Moreover, the platinum jewellery market, despite renewed interest in this precious metal in the last 20 years, still has considerably
lower demand than gold or silver jewellery, therefore the machinery is not used to its full productive capacity.

In order to analyse whether, and to what extent, the SLM technique is competitive in relation to micro-casting in platinum jewellery production, we conducted a true-to-life production comparison between the two techniques in collaboration with Progol3D®️, a top reference for selective laser melting,
and Stilnovo S.r.l., a company in San Salvatore Monferrato (Valenza Po’ jewellery district), the producer of OEM jewellery and a reference for platinum casting.

The market segment chosen for comparing the two techniques was wedding and engagement rings since those are the most representative for platinum in the USA and Europe at the moment. The idea of eternal that has always been associated to platinum due to its resistance over time, means that this metal
is particularly in demand for wedding bands. This can be seen, for example, from data regarding the USA market in 2016 where, compared to a 10% drop in the production of platinum jewellery the year before, American spending on platinum engagement rings grew by 5% (2), making this segment even more predominant
than in the past.

WHAT HAS CHANGED COMPARED TO GOLD JEWELLERY PRODUCTION

Producing platinum jewellery with traditional methods notoriously involves more problems than producing items in gold, especially in regard to micro-casting. On the other hand, producing platinum jewellery using the Selective Laser Melting (SLM™) technique is not particularly any more difficult than
making items in gold alloy, which makes this technique interesting for platinum jewellery manufacture. In general, it can be seen that, for metals used in jewellery-making, the more difficult it is to produce with micro-casting, the easier it is to 3D print.

The greatest production difficulty found in micro-casting comes from differences in the thermo-physical properties of platinum alloys compared to gold alloys. First of all, the considerably higher temperature of liquid platinum alloys leads to the use of different heat-resistant materials for making
the moulds to withstand the greater temperatures. Instead of traditional materials based on calcium sulphate and cristobalite, other materials more resistant to high temperatures are required in which silica is blended with phosphate-based binding agents, which requires longer and more strenuous preparation
(4). Coating properties generally vary more drastically compared to traditional heat-resistant materials in the case of imperfect mixing, both in relation to components and processing times, making these materials more sensitive to storing and aging conditions, causing oscillations that are hard to
control in the surface quality and in the mechanical resistance of the cylinders (5).

Even if specific materials are used, cylinder resistance is still critical if they are heated to more than 900°C (6), a limit that leads to a much greater difference in temperature between molten metal and the cylinder during the casting phase compared to gold alloys with the metal consequently losing
heat faster on entry. This effect, together with viscosity and the surface tension of platinum alloys being greater than that of gold alloys, make it more difficult to fill the moulds completely, especially in the narrower areas, and it is therefore necessary to use centrifugal casting machines to partially
alleviate the problem (7). Increasing the centrifugal force helps the metal to fill the mould, but it also increases the chances of refractory fragment detachments which could then be incorporated into the metal when cooling. All these problems limit the quantity of metal that can be used on each tree
to a much lesser amount than can be used with gold or silver alloys, with the consequent reduction in production capacity. Filling problems and greater shrinkage in going from a liquid to a solid state (7) also mean that a more robust feeding system is needed, which, in turn, leads to a more unfavourable
ratio between scrap and pieces produced. A greater quantity of scrap implies higher production costs, which are further increased by the greater cost of refining platinum alloys compared to gold alloys, due to the more complex procedure and verification process. All these additional difficulties make
casting platinum jewellery more susceptible to variable results not to mention the need for more specifically skilled technicians.

The SLM™ process, on the other hand, has no particular problems compared to gold alloy production. In fact, the fundamental properties for metal-laser interaction, first and foremost, reflectivity and thermal conductibility, are more favourable for platinum alloys than for gold or silver alloys. This
means that less energy is required for laser melting and there is no need to add elements to the alloy to favour laser radiation absorption.

QUALITY COMPARISON

The quality comparison between platinum jewellery created through SLM™ and micro-casting was carried out by producing several ring models in Stilnovo’s BRIDAL series, a collection that best incorporates the concept of eternal associated to platinum jewellery since it comprises rings with the MULTISIZE
solution, covered by a patent (Application number 102017000104245, filed on 18 th September 2017).

The multisize patent is a system that leads to a new conception of the ring as an item that can easily change its diameter and therefore always be a perfect fit.

Changing the size of a ring has always been quite a problem for the jeweller as well as for the final wearer. Because a ring is long-lasting, and is sometimes even handed down from mother to daughter, it is quite possible that the need to change the size will arise sooner or later.

The operation is easy enough if the ring only has one mounted ring shank and a centre, but it becomes more and more complicated as shapes develop and is absolutely difficult when the whole shank is mounted: size modification, i.e. making the diameter smaller or larger is, in fact, obtained traditionally
by cutting the shank at the opposite side to the centre and adding or removing some of the metal. When the ring is mounted along the entire surface, it is dangerous to enlarge or tighten it by even just one size because the mounting may become insecure: in fact, changing the curve of the ring inevitably
modifies the diamond or precious stone setting, something with can compromise the reliability of the ring’s “hold” on them.

With the MULTISIZE RING solution, the internal part of the ring shank has a slot for an interchangeable B structure in various thicknesses (Figures 1, 2 and 3).

In our study, the A frames were made in platinum, while it was decided to use titanium for the sheet metal.

A simple KEY, a titanium hook, made into the shape of a treble clef, was used to pull out the interchangeable part from its slot in the fixed part when the size needed to be modified. Once the slot track in the fixed part A is empty, it is easy to position a new interchangeable part by hand and change
the size.

In order to compare micro-casting and direct metal printing, 10 models from the BRIDAL collection were chosen, comprising wedding bands, solitaires and trilogies, whose frames are shown in Figures 4 to 13. Production and the characteristics of the internal interchangeable parts were not taken into consideration
in this study since they were not made in platinum alloy but were preferentially made in gold or titanium due to the mechanical properties needed for the piece of the frame to be repeatedly inserted and removed without becoming deformed.

A wedding band model, called ETERNAL, which features 360° pavè (Figure 14), was initially chosen for the comparison but was later discarded due to the difficulty of removing the support required in SLM™ production.

6 rings of each model were made for both production technique in question of which 2 were to be sacrificed for destruction analysis, with the exception of the two wedding band models, of which three men’s size and three women’s size samples were made. The overall total of rings made for the study was
120 pieces of which 40 were to be sacrificed for destruction analysis. The list of pieces produced is summarized in Table 1.

In order to make the comparison more like a real production test, jewellery creation was divided between two producers: Stilnovo for micro-casting and Progol3D® for selective laser melting.  Each of the two producers is specialized in one of the two techniques being tested and is able to optimize the
process to obtain the best possible quality.

To assess quality differences given exclusively from the type of productive course and not ascribable to the different composition of the alloys used, the alloy 95PtGaInCu was used in both SLM™ and in micro-casting. Using the same composition for micro-casting and SLM™ made it possible not to give one
technique an advantage over the other thanks to the relative ease with which platinum can be melted by laser interaction so that no adjustments to the composition were needed for the SLM™ process, which would have been required had gold alloys been used. In fact, this composition is in the Progold range
as a micro-casting alloy and is also in use for SLM production at Progol3D®.

In regard to micro-casting production, the waxes were created with a Projet MJP 2500W 3D printing system using VisiJet M2 Cast wax. The cylinders were prepared with PRO HT Platinum Gold Star® keeping a water/plaster ratio of 33:100. The refractory firing cycle is outlined in Figure 15. Cylinder temperature
during casting was 850°C.

The cylinder plastering and firing phases were grouped as much as possible averaging between minimizing production times and the need to retrieve scrap.

For the melting and cylinder firing process, a Yasui VCC centrifugal casting machine was used with a casting temperature of 250°C above the alloy’s liquid state. After the cylinders were cooled, the refractory residues were removed from the metal by immersing them in hydrofluoric acid at room temperature.

A final sanding was carried out to complete refractory elimination.

As for selective laser melting, the jewellery was produced using a ReaLizer SLM50 laser printer equipped with a 100W fibre laser, collimated to a ray of 10 μm. The circular construction plate was 70 mm in diameter.

The layer thickness used for printing was 20 μm, favouring printing resolution over production speed, in consideration of the market segment involved in the study.

The printer was fed with 95PtGaInCu in powder form, obtained by gas atomization of the alloy and sieving to remove any coarse particles.

The shape of the powder particles was observed under a scanning electron microscope (SEM) and the particle size distribution was determined using a laser granulometer (Malvern, Hydro 2000S).

After the printing phase, the jewellery underwent shot peening to eliminate any partially melted powder on the surfaces which would cause the unrefined pieces to be rougher.

Both in micro-casting and direct metal printing, all the rings were re-fired to solubilize the alloy and reduce the internal tensions by furnace treatment at 1150°C for one hour, followed by rapid cooling in water.  In the case of wedding bands, the pieces were later hardened by furnace treatment at
650°C for an hour with slow cooling.

Whatever the production technique, every ring made was assessed using the following quality standards:

–          Surface appearance “as cast” or “as print”, impact of feed residues and supports.

–          Identification of any macroscopic non-conformity defects.

– Measurement of the internal diameter of the rings, variations to the nominal and deviations in measurements between rings of the same model.

On the two sacrificial samples for each model, the following was also carried out:

–          Measurement of surface roughness in both “as cast” or “as print” and after sanding or shot peening

–          Assessment of the internal quality by trimming and lapping the rings.

All the produced items that were not used for destruction analysis (altogether 40 micro-cast rings and 40 printed rings, subdivided into 10 models) were then polished and eventually mounted at Stilnovo for a final evaluation of the quality. The final quality assessment on the completed item was made
by Stilnovo’s internal quality control department which was not aware of the type of production technique used for each ring to be assessed. The standards normally adopted for high jewellery article control were applied.

At the same time, fundamental data were registered to compare the technological and economical aspects of micro-casting and direct metal printing, such as:

–          Production times

–          Production scrap

–          Technician impressions in the polishing phases

–          Technician impressions on the mounting

In order to correctly collect the data on finishing operations, an evaluation sheet, subdivided by phase, was attached to each ring and each technician was asked to complete it.

EVALUATION OF THE PHYSICAL, MECHANICAL AND TECHNOLOGICAL  CHARACTERISTICS

Surface appearance

The first comparison made between rings produced by micro-casting and by SLM™️ involved the appearance of the surfaces both when raw and after sanding or shot peening. This included assessing the impact on the surfaces of additional elements needed to create the item, in other words, feeders in
the case of micro-casting and supports in the case of SLM™️. The invasiveness and weight of these elements had direct repercussions on the quality of the rings due, for example, to the need to reconstruct the surfaces involved, and economically, because of being directly proportional to the percentage
of production scrap and process times.

This paragraph will evaluate the presence and invasiveness of feeders and supports in terms of the surface extension involved and the residue aspect, while the paragraph on the economic and financial repercussions will report the findings regarding scrap and production times.

Figures 18 to 25 compare the feeding and support systems of the 10 models selected for production.

From the comparison of the additional elements needed in production with the two techniques being examined, it was immediately obvious how the effect on the surfaces was completely different in the two cases. In micro-casting, where additional elements are considerable, the geometry of the directly fed
area of the jewellery item was totally lost, while in SLM™, the geometries below the residues of the supports, built as a grid, were generally visible.

Examples of support and feeding residues on the rings can be seen in Figures 26 and 27.

Support in SLM™ generally involves a greater surface area of the item, but, if the effective area of contact with the supports is taken into consideration, that is, the areas where the grid teeth actually touch the item and spoil the surface, the values are lower compared to the areas affected by feeding
in micro-casting.

There are cases, however, as in the example of the ETERNAL wedding band, in which, although the maximum geometry of the ring is maintained, the loss of detail due to the massive presence of support residue, makes production by selective laser melting, unsuitable.

For the solitaire 4 and trilogy 1 models, a good compromise was obtained in SLM™ by using a growth orientation that minimized the corner surfaces so that support was required, but with some supports in areas more difficult to reach than for other models when it came to removing them (Figures 28 and 29).
In these cases, a favourable use of support parameters leads to creating elements that are easier to detach thus partially compensating for the greater dexterity required for their removal.

In regard to the overall appearance of the surfaces, micro-cast rings were generally less rough in the raw state (example in Figures 30 and 31) and after surface treatment (Figures 32 and 33). However, surface irregularities were often observed, mainly between all the excess material burrs, which did
not appear with SLM™. These defects will be analysed in more detail in later paragraphs.

Roughness

To provide a quantitative evaluation of the differences between the surfaces, roughness measurements were carried out using a Taylor Hobson FTS INTRA 02 profilometer. The total roughness (Rt) of the profile was chosen as a comparison parameter corresponding to the difference between the highest and lowest
surface points. This value, in fact, represents the thickness of the precious material that must be removed in polishing to obtain an aesthetically satisfactory surface. The values were registered both for the items when raw (“as cast” in the case of micro-casting and “as print” in the case of SLM™)
and after surface sanding or shot peening.

Indeed, raw surface treatment is a production practice at both Progol3D®, by shot peening to reduce roughness and homogenize the surface appearance, and Stilnovo by sanding, mainly to eliminate refractory residues. The actual roughness that the jewellery-maker will come across in the roughing stages
is, in both cases, that of the treated item, and it will be from these values that the quantity of the material to be removed in order to obtain a smooth surface will depend.

Measurements were taken on several areas of the jewellery items corresponding to surfaces with various orientations in respect of the growth direction of SLM pieces and wax growth in micro-casting. Points with no obvious surface defects were measured in order to give an average Rt value net of macroscopic
surface irregularities.

In regard to wedding bands, the growth directions selected for 3D printing, for waxes in micro-casting and metal in SLM™ were the same, shown in Figure 34. Measurements were taken in direction 1 (surface parallel to the growth, direction perpendicular to z), in direction 2 (surface parallel to the growth,
direction parallel to z) and in direction 3 (surface perpendicular to the growth, direction perpendicular to z).

On the other hand, the solitaires and trilogies were printed with a different “standing” in SLM™ than the waxes in micro-casting due to the different type of support used. To be precise, growth was carried out with the pieces directed vertically in SLM™ and horizontally for waxes. In this case, measurement
directions were decided according to the growth direction, as shown in Figure 35 for SLM™ rings and in Figure 36 for micro-cast rings. The direction indicated with 4 therefore corresponds to a parallel surface in the growth direction, with the measurement taken perpendicularly to z, while 5 refers to
a variable surface due to inclination with measurement along z.

Table 2 shows the average values recorded on raw pieces divided by direction with the respective standard deviations, while Table 3 shows the values for items after sanding or shot peening.

The results are summarized in the graph in Figure 37.

As already noted in observing the raw surfaces, the roughness values are clearly greater in SLM™ compared to micro-casting. This is not a surprising result since surface roughness is one of the weak points of the SLM™ technique. In SLM™, the roughness registered is also greater on average than can usually
be found for gold alloys, a finding in line with the values reported in a study conducted by Progold® in 2015 (8), which noted how, compared to gold alloys, the greater presence of partially melted powder particles on the surfaces, leads to higher roughness on the raw item (Figure 38).

The higher degree of roughness registered in SLM™ in direction 3 compared to the other measurements is attributable to the surface progress caused by compounding the melting lines, which give a meniscus effect with greater height in the centre of the line and less height at the edges (Figure 39). In
the wax mould, the meniscus effect is much less pronounced (Figure 40), so much so that the roughness resulting from this effect, measured in direction 3, is much less than that caused by the subdivision into layers on the long z piece, which is the main cause of roughness in other directions.

The standard deviation recorded in SLM™ compared to micro-casting derives from an already higher deviation between different points of the same jewellery item belonging to equivalent areas.  These differences are mainly due to the different surface direction measured in respect of the movement that the
“wiper” makes during platform “recoating” (8), which results in the powder particles adhering differently to the surfaces.

The roughness on the items produced with micro-casting was, however, more constant both on individual items and in consideration of the various models.

Lastly, the effect of surface treatment, whether sanding or shot peening, on the roughness of the pieces in both production methods reduced the roughness values by about half compared to the “as cast” or “as print” condition.

The overall less surface roughness found in micro-casting generally implies that the jewellery-maker will have to remove less material in the roughing stage in order to achieve a compact surface. This is only true, however, if the piece has no areas with excess material, such as burrs, or spaces, like
surface dents. In these cases, the material lost and the processing time can increase considerably.

Defectology

Micro-casting

As already mentioned above, the jewellery items produced by micro-casting had a clearly higher incidence of macroscopic defects than those produced by SLM™, even after casting parameters were optimized.

The most commonly found defects were surface irregularities, such as burrs due to excess or lack of material.

In the first case (Figure 41), the cause was the partial rupture of the refractory so that cracks formed where they had filled with metal. This type of defect is generally very simple to correct since the excess material can easily be removed in a short time.

In some models, however, such as trilogy 1, the presence of details separated by tiny spaces, made this type of defect more critical, with cases like the one in Figure 42 where refractory rupture had caused different areas of the item to join up.

Phosphate-based refractory resistance variability, resulting from greater susceptibility to variations in storage conditions and the high temperatures of the casting metal, was the most probable source of other types of defect found.

The detachment of tiny portions of refractory led to the appearance of irregularities in some pieces in the form of cavities in cases where these micro-detachments became trapped in the metal (Figures 43 and 44), or of various-sized hollows whenever the micro-detachments were external to the metal and
created tiny round craters on the edges (Figure 45).

The high temperature of the metal, which causes reactions in the refractory, was probably responsible for the irregular surfaces and porosity found in some areas of the micro-cast jewellery items, like those in Figure 46 and in Figure 47, where the roughness was considerably greater than the average
of the surrounding areas.

In other items, surface defects seemed to have been caused by a combination of micro-detachments and refractory reaction (Figures 48 and 49).

The defects shown in Figures 43 to 49 were more damaging for the item compared to the previous since the problem was the lack of material rather than material in excess. In fact, this would have forced the technician to remove more material in order to obtain an even surface or to carry out repairs if
the cavities were deep, with a consequent greater loss of material and longer processing times.

Besides defects ascribable to metal-refractory interaction, problems were also found that derived from other production phases.

For example, the ovalling found in one of the micro-cast model 8 solitaires (Figure 50) was due to probable tension in the waxes or to problems in the plaster casting stage. Although deformed, in these cases, the jewellery-maker can quickly intervene to put the ring back into its original shape, practically
without altering the size so that this defect is of no particular consequence.

Another defect found was bent grips in the models where the grips were particularly long, especially in the model 4 solitaire. This problem (Figure 51), due in all probability to bending the waxes during plastering, can be resolved by adding a terminal ring to stop the grips from moving (Figure 52).

The rupture shown in Figure 53, on the other hand, was attributable to the mechanical stress that occurred in the cylinder cooling stage. In this case the ring was obviously non-compliant.

In order to further investigate the causes of rupture, the wedding band was sectioned horizontally and analysed under an electronic microscope.

In the internal part of the ring, where the fracture occurred, a cavity was found which was most likely due to refractory inclusion, given the results of the EDX analysis of the internal residues which highlighted the presence of silicone.

The cavity, which extended to both halves of the sectioned wedding band (Figures 54 and 55), had reduced the effective section of the ring thus drastically lowering the mechanical resistance, therefore the stress caused by the shrinkage in cooling exceeded the ultimate tensile strength causing the ring
to fracture.

SLM™

The macroscopic defects observed in the jewellery items produced with SLM™ were clearly fewer than those found with micro-casting. In fact, while the surfaces had a higher degree of roughness, only in the case of one ring produced was a real irregularity found in the form of swelling in one area of the
piece (Figures 56 and 57).

This type of defect occurs in SLM™ when the powder does not melt perfectly and so some partially non-melted particles remain and disrupt powder distribution in subsequent printing layers.

In this case in particular, since the defect only involved a small part of the item’s upper area, incomplete melting was probably caused by a variation in the average granulometry of the powder in the growth zone, due, for example, to the accumulation of agglomerates of partially molten particles within
the powder distributed by the “wipers” as the printing process continued.

Since the problem was material in excess and not a lack of it, correcting this type of defect was of no particular importance. However, it may happen that the swelling can be associated to widespread porosity in the area concerned, again caused by imperfect melting.

Dimensional coherence

An analysis of the correct nominal size and the deviations that could be found between the various same model rings was carried out on all the items produced by measuring the internal diameter, which can be directly correlated to the actual size of the ring.

For greater precision, the diameters were measured using a calibre (Mitutoyo), averaging three values in different positions, and also by means of photographic analysis using a Keyence digital microscope, suitably calibrated for maximum measurement accuracy.

Table 4 shows the data relating to the internal diameter of the rings. In the averages calculated for micro-casting, the ovalized ring in Figure 50 was not considered due to the difficulty of establishing the real diameter.

From the data obtained, it can be deduced that the internal diameter measurement in relation to the nominal value was always less in SLM™ compared to micro-casting for each of the ring models produced.

The origin of this reduction in internal diameter is obviously different for the two techniques: in SLM, it is caused by an imperfect correction of the width of the single laser trace while in micro-casting, it is caused by refractory shrinkage during the firing stage, from metal shrinkage as it goes
from liquid to solid and from the item’s contraction during cooling at room temperature.

In the case of SLM™, using platinum instead of gold was not a variable that could have affected dimensional variations, while in micro-casting, the higher temperatures and more notable shrinkage during phase change could have been cause for greater discrepancy in the nominal size for platinum rings rather
than gold rings. Repeatability on rings of the same model was generally greater in SLM™, with maximum standard deviations of ± 0.03 mm compared to ± 0.04 mm and more found in some micro-cast models. Given the greater oscillation found in micro-casting, any correction upstream of the internal dimensions,
by altering the design, for example, would be less effective.

Internal porosity

To analyse the porosity inside the items, the first technique considered was computerized tomography, a technique that has the advantage of not being destructive and able to investigate the entire volume of the jewellery item. However, the results obtained were not deemed satisfactory in terms of image
resolution, a problem caused by the high density of platinum which caused such elevated absorption of the beam, that analysing the thickness of the rings was extremely imprecise.

As an alternative to tomography, it was decided to make a direct analysis of the ring sections by cutting two out of the six rings produced for each model. In order to acquire a more complete evaluation of the internal volumes of the rings, sections from different areas of the items were analysed. To
be precise, one ring of the sacrificial pair was sectioned in the A plane represented in Figure 60, while the other was sectioned in the B planes (Figure 61), perpendicular to the first, in four different areas of the ring. After resin incorporation and lapping, the sections were photographed at 50X
to digitally analyse porosity using the software inside the Keyence microscope that had been used to take the images.

Figure 81, while Table 5 shows the percentage porosity values found in the various models considering porosity on the A and B planes of each ring, weighed along the entire surface of each analysed section.

The level of porosity found in the pieces can be quantified as medium-low in both production techniques with lower values in SLM™ compared to micro-casting, which, on average, had a twice as high porosity. For both cases, there was a notable variability between different pieces and between different
areas of the same sample, with sections that had practically total density and others that suffered from higher porosity.

In micro-casting, areas with singular high-volume porosity were observed, like, for example, the cavities in Figure 82, as well as a high number of small porosity clouds, as in the case of shrinkage porosity shown in Figure 83.

The porosity found in SLM™ was not in the form of cavities but single spherical pores (Figure 84), probably caused by gas, or areas with tiny, regularly placed spaces (Figure 85) due to imperfect melting between adjacent laser tracks.

Besides percentage porosity on all the items, locating any pores is also extremely important in jewellery-making: pieces with a dense interior but surface porosity are more difficult to finish than those that are more porous overall but have a more compact surface.

From this point of view, it can be seen how the porosity found in some areas of the SLM™ items was mainly inside the pieces and more rarely on the surface areas. This effect derives directly from the melting modality and item growth. Inside one single “layer”, the external surface is, in face, melted
as one single outer layer and the laser parameters are optimized in order to ensure the almost total absence of porosity in each laser track. The inside is then melted with parallel laser scans. Porosity tends to gather at the joints between the internal scans or between the outer and inner layers,
which are generally at least 150-200 μm from the surfaces, in an area that is unlikely to be removed in the polishing phase. In micro-casting, porosity distribution is more varied: there are surface cavities, visible also macroscopically on the external surfaces and mainly due to tiny refractory
detachments, and shrinkage porosity, which appears to concentrate more inside the items.

Lastly, the case of fracture found in one of the micro-cast wedding bands is to be considered: in this case, porosity, although concentrated inside the ring, was so extensive that it compromised the item’s mechanical endurance.

Metallographic appearance

To evaluate the dimension of the crystalline granules in the micro-cast rings and in those printed with SLM™, acid attacks were carried out on model 1 “as cast” and “as print” wedding bands.

Comparison confirmed what had already been seen in the past for samples in gold and platinum alloys: the average size of crystalline granules was drastically greater for micro-cast items (Figures 86 and 87) compared to SLM™ items (Figures 88 and 89). Therefore, in reality, signs of melting traces could
be made out but not of individual granules, even at high magnification.

The SLM™ sample showed the presence of micro-cracks, made visible by the acid (Figure 90). Mechanical tests, reported in the paragraph below, were carried out also to evaluate the effective impact of this defect on the properties of the SLM™ pieces.

Mechanical characteristics

The mechanical characteristics of jewellery, such as hardness, elongation and ultimate tensile strength, have direct repercussions not only on the item’s mechanical resistance but also on technological parameters, such as mounting and polishing.

For this reason, the mechanical performances of the items produced by micro-casting and SLM™ were compared, the alloy used being equal. Micro-hardness tests were carried out on “as print” or “as cast” model 1 wedding bands, both after re-firing (1 hour at 1150°C) and after hardening (1 hour at 650°C),
using a Vickers FUTURE-TECH hardness tester. The ultimate tensile strength (UTS) and elongation (A %) values were, on the other hand, obtained from traction tests carried out with an INSTRON dynamometer on specifically created specimens, shown in Figure… In this case the values were measures on “as
cast” or “as print” specimens and on specimens subjected to re-firing treatment, to evaluate possible mechanical differences that may affect the mounting phase.

The greater hardness found for “as print” items compared to “as cast” ones was more than likely due to the smaller dimensions of the crystalline granule in SLM™ and to higher internal tensions in printed items. The re-firing treatment, which, in the case of the alloy used, had the double effect of lowering
the samples’ internal tensions and of solubilizing, meant that, in both cases, hardness could be lowered to below 190 HV, therefore making mounting possible. After aging, in both cases, hardness increased considerably, although it was greater for micro-cast pieces, for which resistance to wear and tear
could therefore be greater than for SLM™ pieces. The difference observed could be caused by the presence of the micro-cracks seen in the SLM™ wedding bands when tested with acid, which favours indenter penetration into the sample.

In regard to traction tests, the samples printed by SLM™ had a greater ultimate tensile strength in the “as print” state than the “as cast” samples, to the expense, however, of lower ductility.

After the re-firing treatment, the ultimate tensile strength lowered for both types of sample but was still higher in the case of SLM™. The results of elongation at fracture, on the other hand, show an inversion between SLM™ and micro-casting. In fact, although ductility increased with thermal treatment
in both cases, the increase in SLM™ was considerably greater.

After re-firing, the samples produced by SLM™ therefore had higher UTS and elongation at fracture values, a fact that indicates how the micro-cracks observed after metallographic attack on printed samples can probably be attributed to mechanical properties rather than granule size and to possible internal
defects in the micro-cast samples.  Better performances after re-firing suggest a better behaviour of items during mounting.

Finishing: technician impressions

The impressions of sector technicians play a fundamental role in the possible success of a new production technique. Jewellery production is not exempt from this rule: even though the quality of a product may seem to be excellent according to technical analysis, if, during the processing phases, it does
not ‘convince’ the workers, with all probability, the production technique will not be adopted in the future. For this reason, we considered it essential to collect the opinions of the “jewellery makers” involved in item finishing in order to be able to add more subjective, but equally important to
the overall assessment of a new production method, evaluations to the quantitative ones, like times and losses due to finishing. The 80 rings created and not subject to destructive tests, were therefore sent for finishing and evaluation. Each of the working phases was carried out by the same technician
both for items created by micro-casting and by SLM™ in order to have the same judgment gauge for both techniques.

The first phase of the finishing process is eliminating any added element residues from the rings that were used in their production but which are not part of the item, i.e. feeders in micro-casting and supports in selective laser melting. As can be seen by the opinions expressed, summarized in Figure
87, from this point of view, the SLM™ technique suffered from some models having supports that were more complicated to remove, like those used for the model 4 solitaire in Figure 28 and in the model 1 trilogy in Figure 29. Removing supports from the internal areas of the item required greater dexterity
on the part of the technician and increased the probability of the item being spoiled in this phase. Substantially similar results were observed in evaluating the difficulty of roughing (Figure 90). In fact, opinions generally regarding the roughness and compactness of the item’s surface revealed that
rings produced by SLM were 80% less difficult to work than those made by micro-casting. The only significant difference was the presence of a non-compliant micro-cast ring identified in this phase, which had caused particular roughing difficulties before it was definitively put aside.  The results obtained
with this evaluation are particularly interesting if one considers that one of the weak points of selective laser melting is high surface roughness.

The impressions reported by technicians on this point revealed that, at least in the case of platinum, all that was needed was slightly greater pressure or thicker paper during this preliminary phase to eliminate the additional surface roughness with little additional effort compared to micro-cast pieces.

This extra effort was, however, amply compensated by the quality of the SLM metal that the technician found (Figure 94): the percentage of surfaces evaluated as excellent in SLM™ in terms of compactness was, in fact, close to 100%, while in micro-casting, evaluations were more varied with only about
60% of surfaces considered as excellent, 25% as average, 10% as low quality due to evident porosity, and two rings were judged as non-compliant.

No particular difference was observed in the cleaning phases (Figure 95), while in mounting, the overall evaluations in both cases went from low to none (Figure 96). The mechanical properties of the metal therefore resulted as more than good both for the micro-cast alloy and the printed alloy.

Quality Control: evaluation

The Quality Control judgment is fundamental for understanding if the jewellery items produced conform to residual porosity criteria and the aesthetics defined by high jewellery.    We therefore subdivided the rings into those that directly passed verification, those that needed quick laser repairs in
order to be compliant and those that were judged as non-repairable.

There was considerable diversity in the results obtained in SLM™ and in micro-casting: while three quarters of the printed rings immediately passed the checks, only half of those produced by micro-casting achieved the same result (Figure 97).

The judgments expressed by Quality Control confirmed the data obtained from analysing the macroscopic defects and internal porosity of the sacrificed samples: the items produced in platinum by SLM™ were immediately found to be less faulty than those produced by micro-casting.

In regard to non-compliances, no SLM™ item was found to be such compared to two items made through micro-casting and submitted for finishing. Furthermore, one sacrificial wedding band was found to be non-compliant due to breakage.

The final appearance of the ten ring models after mounting and polishing can be seen in Figures 98, 99 and 100 for micro-casting and in Figures 101, 102 and 103 for 3D printing.

ECONOMIC AND FINANCIAL ASSESSMENT

Semi-processed production times

Both micro-casting and selective laser melting production processes were organized to reflect the timing and subdivision into typical phases of real-life production. Micro-casting production was subdivided onto 11 trees, listed in Table 8. Cylinder firing cycles, which are the longest production phase
for micro-casting, were grouped in order to obtain the best compromise between production times and scrap recovery. In order to imitate what happens in real production, it was, in fact, decided to re-use the casting scrap, adding it to new alloy to compensate the percentage of material used for producing
authentic jewellery. This procedure is normally carried out to limit the quantity of precious metal needed, both because of the cost of the raw material itself and for the cost of refining the scrap. To be precise, new alloy was used for the first group of four trees while the scrap added to new alloy
to reach the weight of the tree to be cast, was used for the next three trees and the last four.

In SLM™, production was distributed over 7 printing plates (Table 9), created in descending order according to the height of the items to be produced. In fact, this optimizes the use of the powder by producing the highest items first for which the quantity of powder needed to fill the printing space
is greater.

The average times for each cylinder and the total times that the machinery was in use, subdivided by phase, are shown in Table 10 for micro-casting. Table 11, on the other hand, shows the average times for each printed plate and the total times that the machinery was in use, subdivided by phase, for
SLM™. The time it took the technicians to carry out production was also recorded. In fact, a higher total of man hours not only increases production costs, it also implies less possibility of having an automated production process.

From the data shown, it can be noted how machinery usage times are lower (-20%) in micro-casting compared to direct metal printing. In both cases, one production phase required a longer usage of machinery compared to the others: in micro-casting, firing the plaster casts took 55% of the total production
time, while in SLM™, the printing phase actually took 85% of the overall time. Both phases, however, do not require any intervention from technicians and only weigh on production costs in terms of machinery usage and electricity.

Looking at man hours however, the situation is the opposite: despite the longer machinery times, the SLM™ technique required less technician time than micro-casting (-20%), and is therefore more inclined towards automation.

Another important fact for evaluating a production technique is undoubtedly the total production time, considered as the actual time it took to create a jewellery lot. This timing takes into account the effective daily hours (8 hours subdivided into two groups of 4 with a 1-hour break in between), the
weekly working days (5) and which processes can continue during the night because no human supervision is required. Also taken into consideration are processes that can be done at the same time as well as any waiting times, such as the time needed for pickling in acid or for partially drying the cylinders,
which do not involve machinery or technicians, but are part of the production process all the same.

The time divisions for micro-casting and SLM production phases are shown in Tables 12 and 13 respectively, considering the production sequence actually used for making rings. This includes subdividing micro-casting into 3 groups of cylinders in order to be able to use less precious material by re-casting
scrap, and the creation of 3 SLM™ plates with the lowest last instead of inserting them with the others to save time, in order to be able to use less powder in the machine at the beginning.

The total production time was equal to 5 working days for micro-casting and 5.5 for SLM™, which was therefore slightly slower. It should, however, be considered that the operations done on the sixth day in SLM™ do not prevent starting the production of a second lot because they can be done at the same
time. This means that, in the case of consecutive lots, the production capacity of 60 rings, equal to those made for this study, with the subdivision between phases, can be considered the same in the two cases.

Finishing times

The overall finishing times are shown separately in terms of semi-processed production times because the finishing needed the same phases for the items made with micro-casting as those made with SLM™. The discriminating factor in this phase was therefore the ease with which residues from feeders and
supports could be removed and the quality of the items in terms of surface roughness and compactness and of residue porosity. In fact, generally speaking, external porous surfaces or particularly irregular surfaces force the technician to remove more material in order to reach more compact areas of
the item with the consequent increase in working times and finishing loss.

After analysing the times needed to remove feeder and support residues, it can be noted that the former, on average, was done faster given also the relative simplicity of the ring geometries in the fed areas. The average time required in this phase was also more homogenous in micro-casting, while in
SLM™, variability grew depending on the positioning of the supports with longer times required for models where technicians indicated residue removal as more complex on their work difficulty evaluation sheet.

However, observing roughing times, it appears that, except for a few cases, the rings produced with SLM™ required similar or even shorter working times than those produced by micro-casting. This fact agrees with the impressions on the complexity of this phase shown in Figure 93 which, on average, saw
SLM™ rings as easy to work as the micro-cast ones but with a better surface quality. Polishing phases did not reveal any substantial differences between the techniques even in terms of working times and the same can be said for mounting, with the exception of the model 4 solitaire, which recorded a
slightly longer time with micro-casting.

Repairs, needed to a greater extent on micro-cast items but which were of short duration, led to a slighter longer overall time for micro-cast jewellery.

Finishing loss

The material removed from the rings in the finishing phases has an immediate effect on production costs since it cannot be totally recovered by refining materials that come from processing. To calculate these costs, an average loss of 5% of precious metal from finishing was estimated. Table 16 shows
the average values for losses for each model and for each technique during all the finishing operations.

The overall losses were greater in SLM™ or in micro-casting, depending on the model being processed. Analysing the individual phases, it can however be noted that, in the feeder elimination phases, micro-casting had greater losses than SLM™, while in the roughing phases, selective laser melting always
lost more material. These results can be easily explained by the greater quantity of surface roughness in SLM™ in the latter case. The impact of the losses recorded in terms of production costs, assuming a loss of 5% in the polishing recovering phase, is summarized in Table 17.

Raw material production costs

For a correct evaluation of the final cost of the items, we also considered the differences in the raw material costs. In fact, the two production methods differ in terms of the price of the raw material and the number of refining actions required to make the same quantity of jewellery. In regard to
raw material costs, we estimated, by evaluating the market prices, a higher amount for buying ‘new’ raw materials of 0.3 €/g for the powder needed in the SLM™ process compared to the micro-casting alloy, due to the cost of atomization. The same cost difference was taken into consideration between the
breaking down and atomization of new raw material retrieved from refined platinum. In order to assess the impact of refining, the first calculation regarded the various yields of the two production processes in terms of the ratio between pieces produced and casting scrap. The weights recorded and percentage
production yields are listed in Table 18 for micro-casting and Table 19 for selective laser melting.

The different yields found in the two production processes had a direct repercussion on the refining needed in the two cases with the consequent effect on production costs.  The calculation of production costs due to refining was carried out assuming that:

the 60 rings produced for this study represented a typical production lot, equal to about 500g of raw jewellery. To make the 60 micro-cast rings for this study, production scrap was re-cast twice, starting from 1 Kg of the initial alloy. We considered that we would have to refine everything after one
production lot.

To consider a situation in SLM™ similar to that of micro-casting, we assumed that, also in this case, everything would need to be refined after having re-melted the scrap twice. Moreover, for this study, the printer was initially loaded with 2.8 kg of powder, a standard production condition.

Refining costs, both fixed and those that depend on the quantity of material, were calculated using the average of the prices applied by 6 different Italian market suppliers (Table 20).

Focusing on SLM™, the quantity of powder that was initially put into the printer to produce jewellery lots was 2.8 kg. When creating one single lot, no scrap needed to be re-melted and, at the end of the print, the quantity of powder still in the machine was about 2 Kg, the rest having been used to produce
the items (500g) and supports (300g approx.). The second lot was also created without re-melting while, in order to make a third production lot, the scrap (mainly from supports) needed to be re-melted and 1000g of new powder had to be added in order to fill the printing platform to cover the height
of the items to be printed. Re-melting the material twice was only required to produce a fifth lot and, only after the sixth lot did all the powder need to be refined. In order to start producing a seventh lot, 1000g of new powder had to be added to that made with refined material.

The data relating to the powder needed for production by SLM™ and the material to be refined are shown in Table 21 while Table 22 reports the relative costs.

As a comparison, the refining costs for 3 Kg of jewellery produced with micro-casting were calculated, bearing in mind that, after every 500g lot, about 0.5 Kg of scrap would need to be refined (Tables 23 and 24).

Despite the lower raw material costs, the cost per gram of jewellery produced was 7% higher in the case of micro-casting mainly due to the set amounts for each refining, which mainly corresponded to the costs for the concentration sample. These expenses were, of course, to be added to the hours of machinery
usage, man hours and energy consumptions in order to have a complete picture of the effective cost per gram of jewellery produced with the two techniques in question.

Total production costs of the rings

With the data presented in the previous paragraphs, including production times, production lots and yields, the final industrial costs for producing each individual model could be calculated. In order to do so and to make the comparison as close to reality as possible, the following assumptions were
made:

Production capacity was calculated for the two techniques based on the effective use of both systems, considering the ring lot made for this study as a quantity produced in a week.

We considered a total machinery amortization time of five years, taking into account the current average fiscal amortization period in Italy. We did not consider the potential duration of the systems in work hours because all the machinery will most probably become obsolete before the end of its lifecycle.

The costs linked to consumer materials were divided uniformly between the items made, calculating an average cost and not the specific cost of each item produced.

In the calculation hypothesis, we intentionally left out the physical space needed to carry out industrial activities which, in regard to 3D printing, are decidedly smaller than that taken up by a lost wax casting system. The same goes for the capital invested into the electrical and hydraulic systems
needed for micro-casting.

We also did not take into consideration any waste disposal costs (crucibles, plaster, acids, etc.)  deriving from the lost wax casting process.

We also lowered the benchmark by considering two hypothetical companies that exclusively produce items in platinum. This would mean lower exploitation of resources, which could be common to platinum, gold and silver processes.

Dividing machinery usage and human resource costs for each model was carried out based on the percentage weight of each ring produced in respect of the total cylinder cast or the print plate.

The hourly cost of the technicians was taken as the same for both SLM™ and micro-casting, and similar for each person involved in the production and finishing processes.

The consumer materials needed for SLM™ and micro-casting production are listed in Table 25.

The results of production cost calculations for each model were subdivided into semi-processed production costs, finishing costs (including losses of material during processing) and refining costs, shown respectively in Tables 26, 27 and 28.

What emerged from semi-processed production costs was the enormous impact of micro-casting system underuse which led to unfavourable amortization costs compared to the SLM™ technique. This resulted in a higher production cost for almost every ring model, with the exception of the wedding bands for men

System underuse is due to the widespread practice in many companies of internalizing the platinum casting phase mainly for strategic rather than economic reasons, preferring not to entrust the process to third parties. Moreover, the platinum jewellery segment is a precious metal jewellery niche with
production demand standing at about 60 times less than demand for gold, another element that contributes to not optimising the use of the machinery.

The overall finishing costs, however, showed a more varied trend with a general advantage for the SLM™ technique except for those items that involved greater difficulty in removing supports and in roughing.

In regard to refining costs, all the models resulted more favourable with SLM™ due to the higher cost per gram for micro-cast jewellery.

Lastly, looking at the overall costs (Table 29), SLM™ production was clearly less expensive compared to micro-casting in 5 solitaire models and for the ladies’ wedding band sizes, while trilogy 1 and wedding band 4 for men were less expensive in micro-casting. Finally, in three cases, the model 8 solitaire,
model 2 trilogy and wedding band 1 for men, the costs were practically identical, given that the differences found could easily be cancelled out by tiny variations in the production phases. It should also be underlined that, the additional cost linked to re-firing two non-compliant, micro-cast wedding
bands was not taken into consideration in the calculations, therefore only 57 out of the 60 micro-cast rings were truly suitable for sale compared to all 60 in SLM™.

Investment capital

The amount of investment capital required to start producing the semi-processed goods involved in this comparison, net of the necessary resources for the finishing phases, which are the same for micro-casting and SLM™, are slightly higher for SLM™. In fact, the greater quantity of machinery needed to
start micro-casting activities is only partially compensated by the high cost of a selective laser melting printer and by the need for more precious material, estimated as 2.8 Kg of powder against 1 Kg of alloy in micro-casting, to be able to print the jewellery items to their full height.

It is also true that, in the case of micro-casting, there is a better offer of machinery which could lead to a lower amount of investment capital, while in 3D printing, the investment capital calculated here is the minimum required for using this technique. Another difference is that the investment capital
for micro-casting is all instrumental while part of the capital in 3D printing is financial. This favours an SLM™ company in the case of liquidation since selling precious metal is easier than selling second-hand machinery.

It should be noted, however, that, as previously mentioned, we did not take into consideration the higher cost of the systems needed for the good working order of micro-casting machinery, which include a more complex electricity system, a hydraulic system that must be installed onto each machine with
chilled water and a vacuum system to convey the air and dispose of fumes during cylinder firing phases.

It should also be added that a lost wax casting system requires a working space of at least 50 m² which, at market price in Italy, would cost about 100,000 €, while the space need for 3D printing is potentially less than 1 m².

Environmental impact

The environmental impact is a standard that is becoming increasingly important in the overall assessment of a production process. For this comparative study, environmental impact was quantified for each production technique by calculating the Carbon Footprint (CF), which refers to the amount of greenhouse
gas (GHG) released during production, expressed in terms of CO2 equivalent mass.

The released GHG comparison was carried out considering all the phases and materials needed to complete jewellery production. Calculation of emissions caused by production and by disposing of the materials used was carried out using the data taken from the EcoInvent 2.2 database, while the data on greenhouse
gases deriving from the production of electricity to power the machinery were taken from the National System for Environmental Protection ((ISPRA), based on the production of electricity for the Italian network (9). Calculations did not include greenhouse gases caused by raw material extraction and
system and machinery construction.

It can be noted from the results how the total of greenhouse gases released into the environment during the production of 60 rings with SLM™ was half the amount generated with micro-casting. Greater electricity consumption, the gases released in plaster firing phases and the general use of materials
with a high environmental impact are the main cause of this result.

CONCLUSIONS

We can conclude from this study that, from a qualitative point of view, the jewellery produced with SLM™ is better both in terms of macro surface defects and internal porosity. This fact is confirmed by technician evaluation and the number of items that needed to be corrected by laser, as well as by
the lack of non-conformities compared to the three non-compliant wedding bands produced by micro-casting. The potential re-firing of a non-compliant piece is also more unfavourable in terms of time and costs compared to a hypothetical re-print.

Production times were slightly slower in SLM™, although the technique is more effectively exploited with small platinum production lots compared to a micro-casting system.  The greater production yield with selective laser melting also limits resorting to refining, with advantages on costs.

The overall costs were significantly in favour of the SLM™ technique for many of the models created, with only two evident advantageous cases in micro-casting. All this in the face of a slightly higher initial activity start-up investment and half the environmental impact.

In consideration of the data collected, we can conclude that, for companies that deal only in platinum productions and with weekly lots of about 500g of raw jewellery, the SLM™ technique is decidedly more advantageous than micro-casting, since it is more suitable for the small quantities of platinum
jewellery produced and has, on average, a better quality compared to the same items produced by micro-casting.

We can therefore confirm that, as our work presented at the Santa Fè Symposium 2017 hypothesized, platinum jewellery production can be included in cases in which the SLM™ technique is truly an added value compared to traditional casting.

References

Damiano Zito et al., “Why Should We Direct 3D Print Jewelry? A Comparison between Two Thoughts: Today and Tomorrow,”
The Santa Fe Symposium on Jewelry Manufacturing Technology 2017, ed. E. Bell et al. (Albuquerque: Met-Chem Research, Inc., 2017).

Teresa Fryé and Joerg Fischer-Buehner, “Platinum Alloys in the 21st Century: A Comparative Study,”
The Santa Fe Symposium on Jewelry Manufacturing Technology 2011, ed. E. Bell (Albuquerque: Met-Chem Research, Inc., 2011).

GFMS Platinum Group Metals Survey 2017, Thomson Reuters Eikon™.

G. Ainsley et al., “Platinum Investment Casting Alloys,”
Platinum Metals Review
22, no. 3 (London: Johnson Matthey & Co. Limited, July 1978): 78.

P. Lester et al., “The Effect of Different Investment Powders and Flask Temperatures on the Casting of Pt Alloys,”
The Santa Fe Symposium on Jewelry Manufacturing Technology 2002, ed. E. Bell (Albuquerque: Met-Chem Research, Inc., 2002): 321-334.

U.E. Klotz and T. Drago, “The Role of Process Parameters in Platinum Casting,”
The Santa Fe Symposium on Jewelry Manufacturing Technology 2010, ed. E. Bell (Albuquerque: Met-Chem Research, Inc., 2002): 287-326.

Damiano Zito et al., “Definition and Solidity of Gold and Platinum Jewelry Produced Using Selective Laser Melting (SLM™ ) Technology,”
The Santa Fe Symposium on Jewelry Manufacturing Technology 2015, ed. E. Bell et al. (Albuquerque: Met-Chem Research, Inc., 2015): 455-492.

Istituto Superiore per la Protezione Ambientale, “Fattori di Emissione Atmosferica di CO
2 e Sviluppo delle Fonti rinnovabili nel settore elettrico”, (2017).


Figure


1


. Eternal model frame and interchangeable part


Figure


2


. “KEY” for replacing the spring


Figure


3


. Sequence for changing the interchangeable part on Trilogy


Figure


4


. Model 1 wedding band frame


Figure


5


. Model 4 wedding band frame


Figure


6


. Model 4 solitaire frame


Figure


7


. Model 5 solitaire frame


Figure


8


. Model 7 solitaire frame


Figure


9


. Model 8 solitaire frame


Figure


10


. Model 15 solitaire frame


Figure


11


. Model 16 solitaire frame


Figure


12


. Model 1 trilogy frame


Figure


13


. Model 2 trilogy frame


Figure


14


. ETERNAL model wedding band


frame


Figure


15


. plaster firing cycles



Figure


16


. Feeders and supports used to produce the model 1 wedding band



Figure


17


. Feeders and supports used to produce the model 4 wedding band



Figure


18


. Feeders and supports used to produce the model 4 solitaire ring



Figure


19


. Feeders and supports used to produce the model 5 solitaire ring


Figure 20 . Feeders and supports used to produce the model 7 solitaire ring


Figure 21 . Feeders and supports used to produce the model 8 solitaire ring


Figure 22 . Feeders and supports used to produce the model 15 solitaire ring


Figure 23 . Feeders and supports used to produce the model 16 solitaire ring


Figure 24 . Feeders and supports used to produce the model 1 trilogy ring


Figure 25 . Feeders and supports to produce the model 2 trilogy ring

Figure 26 . SLM™ support residue on the surface of a ring

Figure 27 . Feeder residue on the surface of a ring


Figure


28


. Internal support in the model 4 solitaire


Figure


29


. internal support in the model 2 trilogy

Figure 30 . Raw model 4 wedding band produced by micro-casting

Figure 31 . Raw model 4 wedding band produced by SLM™

Figure 32 . Model 4 wedding band produced by micro-casting, after sanding

Figure 33 . Model 4 wedding band produced by SLM ™, after shot peening

Figure 34 . Roughness measurement directions on wedding bands

Figure 35 . Roughness measurement directions on SLM™ solitaires and trilogies

Figure 36 . Roughness measurement directions on micro-cast solitaires and trilogies

Figure 37 . Average roughness found in the various directions in micro-casting and SLM ™

Figure 38 . Surface roughness on the vertical wall of a raw SLM™ wedding band, 300X

Figure 39 . Surface roughness on the horizontal wall of a raw SLM™ wedding band, 300X.

The parallel tracks left by the laser scans can be seen.

Figure 40 . Surface roughness on the horizontal wall of a raw micro-cast wedding band, 300X.

Figure 41 . Excess material on the side of a micro-cast ring

Figure 42 . Example of refractory breakage

Figure 43 . Cavity on the surface of a model 4 solitaire, probably caused by a refractory micro-detachment encased in the molten metal

Figure 44 . enlargment of the defect in Figure 43

Figure 45 . Subsidence probably caused by fragments of detached refractory stuck on the surface of the molten metal

Figure 46 .

Figure 47 . Details of the surface in Figure 46

Figure 48 . Surface porosity in a micro-cast solitaire

Figure 49 . Details of the area in Figure 48

Figure 50 . Micro-cast model 8 solitaire with evident ovalling

Figure 51 . Deformation of the grips of a micro-cast model 4 solitaire

Figure 52 . Added ring to stabilize the position of the grips on model 4 micro-cast solitaires

Figure 53 . Breakage in one of the micro-cast wedding bands

Figure 54 . Internal cavity in a micro-cast wedding band.

Note the correspondence with the item’s fracture zone.

Figure 55 . Extension of the cavity in the other half of the sectioned wedding band

Figure 56 . Surface swelling in a model 2 trilogy produced by SLM™


Figure 57 . Surface swelling in the model 2 trilogy in Figure 56 compared with the standard profile

Figure 58 . Example of digital measurement



Figure 59 . Offset compared to the nominal internal diameter measurement with standard deviation

Figure 60 . Section A plane

Figure 61 . Section B planes

Figur e 62. Micro-cast wedding band 1

Figure 62 . SLM™ wedding band 1

Figure 63 . Micro-cast wedding band 4

Figure 64 . SLM™ wedding band 4

Figure 65 . Micro-cast solitaire 4

Figure 66 .  SLM™ solitaire 4

Figure 67 . Micro-cast solitaire 5

Figure 68 . SLM™ solitaire 5

Figure 69 . Micro-cast solitaire 7

Figure 70 . SLM™ solitaire 7

Figure 71 . Micro-cast solitaire 8

Figure 72 . SLM™ solitaire 8

Figure 73 . Micro-cast solitaire 15

Figure 74 . SLM™ solitaire 15

Figure 75 . Micro-cast solitaire 16

Figure 76 . SLM™ solitaire 16

Figure 77 . Micro-cast trilogy 1

Figure 78 . SLM™ trilogy 1

Figure 80. Micro-cast trilogy 2

Figure 81. SLM™ trilogy 2

solitario15_1_micro_01_50x_5

Figure 82. cavity in micro-cast ring section

solitario_4_micro_50x_10

Figure 83. porosity from shrinkage in micro-cast ring section

sol_7_SLM_2_01_50x

Figur e 84. Porosity from gas in SLM™ ring section

solitario16_1_SLM_01_50x_2

Figure 85. Porosity inter-hatches in SLM™ ring section

Figure 86. "as cast" micro-cast wedding band after metallographic attack, 50x

Figur e 87. “as cast” micro-cast wedding band after attack, 200x

Figure 88. "as print" SLM™ wedding band after metallographic attack, 50x

Figure 89.  "as print" SLM™ wedding band after attack, 200x

Figure e 90. Micro-cracks visible in the SLM wedding band after metallographic attack

Figure 91. Traction tester

Figure 92. Evaluation of the difficulty in removing supports/feeders

Figur e 93. Evaluation of roughing difficulties

Figure 94. Evaluation of the surface quality after roughing

Figure 95. Evaluation of polishing difficulties

Figure 96. Evaluation of mounting difficulties

Figure 97. Stilnovo s.r.l.’s internal Quality Control evaluation of the jewellery produced by SLM™ and by micro-casting

Figure 98. Micro-cast solitaires

Figure 99. Micro-cast trilogies

Figure 100. Micro-cast wedding bands

Figure 101. SLM™ solitaires

Figure 102. SLM™ trilogies

Figure 103. SLM™ wedding bands

Figure 104. Overall machine times for technique used and total man hours

Figure 105. Kg of CO2 equivalent produced by each of the two techniques


Table


1


. List of items produced for each model and production technique

Table 2 . as cast / as print roughness

Table 3 .Roughness after sanding or shot peening

Table 4 . Internal diameters measured for each model compared to nominal diameters

Table 5 . Average percentage porosity found for the two production techniques in question

Table 6. Vickers micro-hardnesses on micro-cast and printed model 1 wedding bands

Table 7. Mechanical characteristics

Table 8. Casting subdivision

Table 9. Print subdivision and relative production time

Table 10. Average and total machine and technician times for producing the rings with micro-casting

Table 11. Average and total machine and technician times for producing the rings with SLM™

Table 12. Subdivision of micro-casting production phase times

Table 13 . Subdivision of SLM production phase times

Table 14. Finishing operation times (in minutes) for rings produced by micro-casting

Table 15. Finishing operation times (in minutes) for rings produced by SLM™

Table 16. Overall finishing losses

Table 17. Costs relating to losses registered in the finishing phase,

assuming a 5% loss in the recovering phases

Table 18. Percentage production yield with micro-casting

Tab le19. Percentage production yield with SLM™

Tab le 20. Average refining costs on the Italian market

Table 21. Material to be refined for producing with SLM™

Table 22. Refining costs for producing with SLM™

Table 23. Material to be refined for producing with micro-casting

Table 24. Costs of refining for producing with micro-casting

Table 25 . Consumer materials needed for production

Table 27. Finishing costs, including losses, for each ring subdivided by model

Table 28. Refining costs for each ring subdivided by model

Table 29. Total production costs, including costs for producing semi-processed items,

finishing and refining

Table 30. Investment capital needed to start production with micro-casting and SLM™

Table 31. Kg of CO
2 equivalent produced in micro-casting

Table 32. Kg of CO
2 equivalent produced in SLM™

Continue reading

Miglioramento della resistenza ad abrasione e corrosione di finiture nere decorative

Introduzione

Le finiture superficiali decorative nere possono essere ottenute tramite diverse tecnologie accessibili e vengono realizzate quotidianamente ricorrendo a un’ampia gamma di metodi di applicazione. Anche se tali metodi vengono utilizzati da anni su scala industriale, ognuno di essi è caratterizzato da limiti. Essendo elevata la probabilità che i substrati di base trattati siano di colore bianco – come nel caso dell’alluminio, acciaio e argento – o pre-placcati con uno strato bianco utilizzato per proteggere il substrato – come nel caso della lega di zinco e dell’ottone -, la resistenza all’abrasione è un fattore qualitativo chiave utilizzato come indice di riferimento per tutte le categorie di trattamento superficiale decorativo nero. Poiché lo strato nero si usura, quello bianco resta esposto, mostrando un contrasto di colore che è ben percepito dall’occhio umano, cosa che semplifica all’acquirente finale la valutazione della qualità del prodotto via via che invecchia. 

Specifico della galvanoplastica, il processo di placcatura nera ha subito diverse modifiche negli ultimi anni. Con l’entrata in vigore del regolamento europeo REACH, alcune sostanze chimiche precedentemente utilizzate come additivi annerenti o agenti ossidanti non possono più essere utilizzate. Metalli storicamente utilizzati per la galvanoplastica nera come il nichel, il cobalto e il cromo sono costantemente sotto esame, incrementando la consapevolezza di marchi, acquirenti ed enti governativi, cosa che comporta, anno dopo anno, l’aumento di richieste legate a un uso limitato o all’eliminazione di questi metalli, soprattutto nei casi in cui lo scopo finale del prodotto è quello di essere indossato o essere a contatto con la pelle umana.

Alcuni processi di finitura nera conformi alla direttiva REACH comunemente utilizzati nel settore della galvanoplastica decorativa si basano sul rutenio nero e il rodio nero. Questi trattamenti vengono generalmente detti “flash”, con uno spessore massimo ottenibile di 0,2 micron, e si usurano piuttosto facilmente nel tempo malgrado la durezza elevata di entrambi i metalli.

Questo documento seguirà lo sviluppo di un processo di galvanoplastica basato su oro nero e la realizzazione di prove comparative relative alle applicazioni decorative nere più comunemente utilizzate al giorno d’oggi nel nostro settore e analizzerà i possibili vantaggi di tale processo in termini sia di strato finale che di strato tecnico.   

Tecnologie di finitura decorativa nera

I numerosi tipi di metodi di finitura nera disponibili variano a seconda del substrato originale dal trattare e dal settore di applicazione. Anche se questi processi sono piuttosto numerosi, al momento di esaminarli presentano limiti in termini di applicazione, design e costi.

 L’anodizzazione è un processo per la finitura di leghe in alluminio che utilizza l’ossidazione elettrolitica della superficie in alluminio per produrre un rivestimento protettivo di ossido. Questi rivestimenti possono essere colorati in una gamma di tonalità limitata, tra cui il nero. Anche se l’anodizzazione ha un vasto impiego funzionale a livello di industria, il processo viene utilizzato massicciamente come metodo decorativo in numerosi settori tra cui quello degli elettrodomestici, dell’elettronica e soprattutto quello dei telefoni cellulari in virtù del suo rapporto costo/qualità. Dal momento che il processo consiste essenzialmente nella colorazione dello strato di ossido naturale dei metalli, l’anodizzazione fa parte dell’alluminio in sé fornendo un legame completo e proprietà di adesione eccezionali. Gli svantaggi principali del processo in generale è che tende a esercitare un impatto negativo sui dettagli; il suo uso è pertanto parziale, ovvero limitato a forme semplici; l’applicazione stessa è limitata all’alluminio e a pochissimi altri metalli non ferrosi, il che restringe l’uso complessivo del processo dal punto di vista decorativo. 

I processi basati su vernici e rivestimenti sono procedimenti a base di resine diluite in acqua o solventi che offrono la più ampia gamma di colori ed effetti di tutti i metodi discussi. Anche se le sequenze di lavorazione possono variare, vernici e rivestimenti possono essere applicati a qualsiasi substrato metallico o addirittura plastico. In virtù di ciò vengono utilizzati praticamente in tutti i settori esaminati; fanno tuttavia la parte del leone – nel mercato degli ornamenti decorativi di qualità superiore – nel settore delle montature per occhiali. Con questo tipo di applicazione è possibile ottenere numerose varianti di nero.  I rivestimenti cataforetici monostrato (e-coating) sono piuttosto convenienti; tuttavia presentano problemi di durata o resistenza all’ossidazione a seconda del tipo di resina utilizzato. Questo inconveniente può essere eluso con l’uso di sistemi a 2 e 3 strati di vernice applicati a spruzzo utilizzando più resine; tuttavia alla fine il costo risulta più elevato a causa del tempo di applicazione e della perdita di materia prima fino al 70%. L’aspetto negativo dell’uso di vernice è dato dal fatto che ostacola il movimento meccanico di parti funzionali quali fermagli, catene e ganci. Nel caso specifico dell’applicazione a spruzzo, si lega con qualsiasi substrato con cui viene a contatto; pertanto l’uso in gioielleria è limitato a causa della presenza costante di pietre preziose. La vernice viene inoltre tacciata di “finitura a buon mercato” a causa dell’aspetto simile alla plastica che tende a conferire al metallo; pertanto molti settori di beni di lusso, a seconda del posizionamento del marchio, non ne prendono in considerazione l’uso.

La deposizione fisica da vapore (PVD) è un processo che utilizza una camera da vuoto per pressurizzare un target in vapore che viene quindi trasformato in un film condensato sulla superficie degli oggetti. I substrati applicabili sono limitati alla temperatura necessaria per realizzare una determinata deposizione, il che può costituire una grande limitazione per il suo utilizzo. Questo processo è in grado di produrre diversi colori, tra cui il nero. Uno dei processi più recenti utilizzati è il Diamond Like Carbon (DLC) che produce uno strato nero lucente estremamente duro e resistente all’abrasione. L’investimento iniziale in materia di attrezzature è piuttosto elevato, il che rende la PVD di gran lunga la tecnologia più costosa tra quelle discusse, essendo difficilmente fattibile dal punto di vista economico al di fuori della Cina. Anche se molto resistente ad abrasione e usura, la resistenza chimica della PVD tende a essere piuttosto debole a causa della formazione di fori microscopici durante la lavorazione. Questo problema può in ultima analisi essere superato con più trattamenti; tuttavia il costo del trattamento esercita un notevole impatto. Dal momento che il settore degli orologi utilizza l’acciaio inossidabile come materiale di base e richiede requisiti elevati in materia di resistenza all’abrasione e che le geometrie che tratta sono standardizzate, costituisce il candidato ideale per la PVD, che difatti viene ampiamente utilizzata in tutto il settore.

I trattamenti di galvanoplastica nera sono numerosi e utilizzano un’ampia varietà di metalli per ottenere il risultato target. Due dei mercati decorativi più esigenti per questo tipo di placcatura sono il settore della gioielleria e quello degli accessori moda. Entrambi i settori possono essere suddivisi in segmenti di fascia alta e bassa. I segmenti di fascia bassa utilizzano metalli quali il cromo, il nichel, lo stagno, il cobalto o leghe costituite da tali metalli principalmente per una questione di costi. Anche se questi metalli sono convenienti, la finitura ottenuta non è sufficiente per superare le prove climatiche normative richieste da alcuni marchi. Questo fatto, abbinato alla crescente richiesta di finiture prive di nichel e alle normative più severe come nel caso del regolamento REACH dell’Unione Europea, fa sì che i segmenti di fascia più alta ricorrano a metalli quali il palladio, il rodio e il rutenio per ottenere un colore nero. La gioielleria di fascia alta tende a utilizzare rodio nero mentre gli accessori moda di fascia alta si servono di rutenio nero. Anche se il rutenio nero e il rodio nero sono in grado di superare la maggior parte delle prove climatiche se utilizzati con le sequenze di pre-placcatura corrette, entrambi hanno uno spessore massimo ottenibile di 0,2-0,3 micron, per cui tendono a presentare problemi di resistenza all’abrasione. Questo inconveniente viene al momento eluso in entrambi i settori di fascia alta coprendo il metallo con una vernice per migliorare l’abrasione o semplicemente accettando il fatto che il colore si usurerà nel tempo.

Il che porta a chiedersi se sia possibile ottenere uno strato nero resistente all’abrasione senza l’uso di vernice, evitando al contempo l’uso di nichel nella lavorazione e mantenendo la conformità al regolamento REACH. Se è così, il mercato sarà in grado di sostenerne il costo?         

Oro nero senza nichel

Sia il rodio che il rutenio hanno uno spessore massimo ottenibile di 0,2-0,3, principalmente dovuto al fatto che si tratta di sistemi monometallici che si infragiliscono con l’aumentare dello spessore. Nell’ottica di migliorare la resistenza all’abrasione, il primo obiettivo sarà quello di mantenere una durezza soddisfacente rendendo al contempo lo strato più malleabile e ampliando la gamma dello spessore raggiungibile. Omettendo l’uso di nichel e cobalto in modo da preservare l’ipoallergenicità e l’uso di rame, che resta la fonte principale di ossidazione, è stato studiato un elettrolita bimetallico che includeva oro, palladio e ferro. Il sistema studiato presentava un pH alcalino per ricevere bene i metalli selezionati, con l’oro nella concentrazione più alta e il ferro in quella più bassa dei metalli utilizzati per sviluppare il composto chimico.

Tabella 1 – Caratteristiche elettrolitiche dell’oro nero

Valutazione della superficie della lega in oro nero

Durante la lavorazione, l’elettrolita produce una lega di colore nero costituita dal 49% di palladio, il 39% di oro e il 12% di ferro in peso, con un titolo in oro depositato di circa 12,5 kt. Per valutare il colore della lega viene utilizzato il sistema di coordinate colore CIELab, concentrandosi sulla coordinata L durante la valutazione di finiture nere.

La coordinata L è il valore della luminosità che in questo caso determina la gradazione scura complessiva del deposito nero, con il valore L più basso che equivale a una tonalità più scura di nero. Con la lega in oro nero è stato misurato un valore L pari a 58.

Tabella 2 – Confronto delle coordinate colore di oro nero, rutenio nero e rodio nero

Questo valore rientra nel range più elevato dei colori neri standard del settore con coordinate L che vanno da 50 a 60 nel caso del rodio nero e una disponibilità molto più ampia di tonalità di nero con il rutenio che presenta coordinate L che spaziano tra 32 e 60. Le formule più comunemente utilizzate sia del rodio che del rutenio hanno coordinate L pari a 58 o 59.

È stato realizzato un test basato su celle di Hull per identificare l’intervallo di densità corrente, il cui risultato è stato un pannello nero con finitura a specchio prodotto a 1,0 A/dm2 dopo 10 minuti di tempo di deposizione. A questo si aggiunge il fatto che l’elettrolita ha dimostrato un buon potere di penetrazione ed estensione, con un valore di densità corrente ottimale pari a 1,0 A/dm2.

Sono state eseguite prove di spessore con l’uso di un microscopio SEM/EDX. Prove di laboratorio hanno dimostrato la possibilità di ottenere un deposito fino a 2 micron mantenendo la finitura a specchio prevista per il settore della placcatura decorativa di fascia alta.  Con misurazioni accurate dello spessore, sono state calcolate le velocità di placcatura, ottenendo 1 micron in 12-15 minuti a 1,0 A/dm².

Osservazioni chimiche

La chimica si è dimostrata stabile in un arco di tempo di 12 mesi, dando prova dell’assenza di elementi chimici di partecipazione in caso di mantenimento corretto del pH. Nell’elettrolita è stato rilevato un pH fluttuante, soggetto a un calo giornaliero pari allo 0,2.

L’abbassamento del pH può essere corretto con l’aggiunta di sali di regolazione del pH; tuttavia ciò comporterà il monitoraggio giornaliero della soluzione del pH da parte degli operatori della soluzione di oro nero, conferendo all’elettrolita caratteristiche di manutenzione simili a quelle di un bagno al solfato per oro.

Figura 1 – Variazione del pH in un periodo di 18 giorni

Sequenze di placcatura nei segmenti di mercato

Sono state valutate due della sequenza di placcatura nera più consolidate e standard in ciascun segmento di mercato di fascia alta. Nel caso della gioielleria è stata valutata la finitura nera sopra l’argento come uno dei casi più standardizzati. Partendo da un materiale di base come l’argento 925, viene applicato palladio come strato di placcatura iniziale seguito da rodio nero nei cicli di finitura più comuni. In questo esempio il palladio viene applicato come strato tecnico con un duplice scopo. Il primo è il miglioramento della resistenza complessiva all’ossidazione fungendo da barriera alla migrazione del rame e impedendo la corrosione dovuta a fattori ambientali. Più spesso è lo strato di palladio, più forte è la resistenza alle prove climatiche. Il secondo motivo per l’uso del palladio è la protezione dell’elettrolita. Il palladio in quanto metallo non è soggetto a corrosione in un ambiente acido; lo sono invece l’argento e il rame con cui viene legato. Se viene saltato il passaggio del palladio, l’argento e il rame finiranno per deteriorare la qualità finale depositata in quanto la loro presenza aumenta con il tempo nel bagno di rodio sotto forma di contaminazione metallica.

Il settore degli accessori di alta moda utilizza un ciclo di finitura completamente diverso per ottenere un colore simile. Il motivo principale risiede nel fatto che i materiali di base iniziali sono tipicamente l’ottone o leghe di zinco, che richiedono una sequenza di placcatura diversa per il conseguimento di una finitura di qualità elevata.  I cicli più comunemente utilizzato sono costituiti da 5-7 processi di placcatura che comportano numerosi strati per ottenere vantaggi tecnici. La sequenza scelta inizia con il rame alcalino per l’adesione, seguito da rame acido per la brillantezza della superficie, passando al bronzo bianco per una migliorata durezza e quindi al palladio per la resistenza all’ossidazione; vengono infine applicati 1-2 strati di rutenio. In questo caso vengono utilizzati 2 strati di rutenio; il primo strato è costituito rutenio grigio chiaro che viene quindi seguito da rutenio nero, in modo da migliorare, in ultima analisi, l’indossabilità complessiva dei prodotti.  

L’integrazione della lega di oro nero nei cicli di placcatura selezionati consente di misurare l’aggiunta del nuovo strato utilizzando i due processi più standard del settore come benchmark qualitativo. 

Sequenza di placcatura della gioielleria in oro nero

Nel caso delle modifiche proposte al ciclo di lavorazione della gioielleria, l’oro nero era stato scelto per sostituire il palladio come strato tecnico, visto che la lega di oro nero in sé contiene una percentuale elevata di palladio, oltre al fatto che entrambi gli elettroliti hanno un pH alcalino simile, cosa che comporta deviazioni di lavorazione minori.

Parti di argento sterling 925 sono state trattate direttamente con 0,3 micron di lega di oro nero seguiti da 0,2 micron di rodio nero. Parti aggiuntive sono state trattate con 0,3 micron di palladio e 0,2 micron di rodio nero da utilizzare come riferimento nelle prove di abrasione e corrosione. A seguito dei trattamenti i campioni trattati utilizzando l’oro nero come substrato presentavano un colore visibilmente più scuro rispetto a quelli trattati con palladio, anche se la finitura finale era lo stesso rodio nero applicato utilizzando gli stessi parametri. Questo processo viene definito in questo documento come sequenza di processo 1.

Figura 2 – Fila superiore: sequenza di placcatura scelta come ciclo di riferimento, fila inferiore: ciclo di prova con l’implementazione di oro nero.

Sequenza di placcatura degli accessori moda in oro nero

Le modifiche selezionate per il ciclo del processo degli accessori moda erano diverse vista la complessità del processo di placcatura in sé. In questo caso ci siamo concentrati sul ciclo di finitura che include 1 strato di rutenio nella sequenza. Il deposito in oro nero è stato testato come strato tecnico utilizzandolo per sostituire lo strato in palladio per mantenere un costo simile, creando spazio quantitativo per l’oro nero. Questo processo viene definito in questo documento come sequenza di processo 2.

Figura 3 – Fila superiore: sequenza di placcatura scelta come ciclo di riferimento, fila inferiore: ciclo di prova con l’implementazione di oro nero.

Inoltre l’oro nero era stato implementato nel processo come finitura finale dato che condivide un colore simile al rutenio grigio canna di fucile. Poiché entrambi i depositi hanno una coordinata L di 58, questa verrà utilizzata come riferimento per la resistenza degli ori neri all’ossidazione con uno standard del segmento di mercato.

Figura 4 – Ciclo di test con l’implementazione dell’oro nero come strato finale da testare in contrasto al rutenio.

Le parti in ottone sono state lavorate con le sequenze di placcatura del segmento di mercato standard e le deviazioni di lavorazione dell’oro nero relative per l’analisi qualitativa.  Questo processo viene definito in questo documento come sequenza di processo 3.

Analisi comparativa

Sulla base delle suddette informazioni le parti trattate con i cicli di placcatura più comuni dai due segmenti di mercato selezionati sono state sottoposte ad analisi comparativa a fronte di quelle trattate con lo strato di oro nero inserito nella sequenza. Sono stati utilizzati metodi di analisi normativa standardizzata per simulare l’ossidazione e l’abrasione.

Resistenza all’abrasione

Il metodo di test utilizzato per misurare la resistenza all’abrasione nel mercato degli accessori di alta moda è il test Turbula, progettato per simulare usura o abrasione. Viene utilizzata una macchina specifica in cui la rotazione è basata su un perno, creando un ambiente più aggressivo rispetto a un miscelatore standard.

Il mezzo utilizzato è una ceramica a forma di piramide allungata con una forma diversa che garantisce punti di contatto sia acuminati che piatti, simulando due forme aggressive di abrasione. L’analisi viene eseguita con un mezzo che pesa circa 41 grammi per 100 pezzi. Per ogni analisi sono state trattate cinque parti con una fase di valutazione fissa seguita da un ciclo di Turbula a un tempo fisso e una velocità di 72 giri/min.

Risultati della sequenza di gioielleria

A seguito di un ciclo di abrasione di 3 minuti, i campioni di argento 925 che hanno utilizzato il deposito di oro nero come strato intermedio si sono dimostrati più resistenti all’abrasione rispetto alle parti trattate con il processo in palladio tradizionale. Un’ulteriore valutazione ha determinato un miglior legame tra lo strato in oro nero e rodio nero rispetto ai metodi di placcatura tradizionali.

Figura 8 – Fila superiore: palladio come strato intermedio; fila inferiore: oro nero come strato intermedio entrambi dopo il ciclo di abrasione

Risultati della sequenza degli accessori moda

A seguito di un ciclo di abrasione di 30 minuti, i campioni di ottone che hanno utilizzato il deposito di oro nero come strato intermedio si sono dimostrati più resistenti all’abrasione rispetto alle parti trattate con il processo in palladio tradizionale, che hanno mostrato la perdita completa dello strato di rutenio non superando in tal modo il test.

Figura 9 – A sinistra: oro nero come strato intermedio; a destra – palladio come strato intermedio entrambi dopo un ciclo di abrasione di 30 minuti 

Resistenza al sudore artificiale

Il test del sudore artificiale a cui sono sottoposti gli accessori di fascia alta è descritto in NFS 80-772:2010-10; si tratta di una prova basata sul contatto diretto, più aggressiva rispetto ai test atmosferici. In base a questo metodo di test, i campioni vengono messi a contatto diretto con un feltro assorbente che è stato immerso in una soluzione di sudore artificiale. Il campione viene sigillato ermeticamente e mantenuto a una temperatura costante di 55 °C. I campioni vengono quindi tenuti per una durata predeterminata di tempo a incrementi di 24 ore.

A seguito di un ciclo di sudore artificiale di 24 ore i campioni di argento 925 che hanno utilizzato il deposito di oro nero come strato centrale hanno dimostrato una resistenza superiore rispetto alle parti trattate con la procedura basata su palladio tradizionale. La superficie dei pezzi che utilizzano il processo in palladio ha mostrato segni notevoli di aggressione chimica.

Figura 9 – Fila superiore: palladio come strato intermedio; fila inferiore: oro nero come strato intermedio entrambi dopo un ciclo di sudore artificiale di 24 ore

Nebbia salina

A seguito della normativa ISO 9227, gli oggetti da testare vengono sospesi in una camera sigillata e sono sottoposti a una nebbia salina costante per una durata di tempo predeterminata. Il test è progettato per simulare un ambiente corrosivo e verifica la resistenza dei substrati in un tempo di ciclo di 96 ore.

Per testare la resistenza alla nebbia salina della lega di oro nero, è stata utilizzata la sequenza di processo 3 in quanto in questo caso la lega di oro nero viene esposta come strato finale.

A seguito del ciclo di nebbia salina di 96 ore, i campioni in ottone finiti sia in rutenio che in lega di oro nero non hanno mostrato segni di corrosione.

Figura 9 – A sinistra: oro nero come strato superiore; a destra: rutenio come strato superiore a seguito di un ciclo di nebbia salina di 96 ore

Conclusioni

L’analisi di laboratorio iniziale ha dimostrato che la lega in oro nero ha migliorato i risultati qualitativi se utilizzata come strato tecnico/intermedio, in particolare la resistenza al sudore artificiale, se applicata sull’argento come sostituto del palladio e finita con rodio nero. Quando testata come strato finale, la resistenza alla corrosione era simile a quella del rutenio, ma date le notevoli differenze di costo tra palladio/oro e rutenio e unitamente al fatto che i due elettroliti producono lo stesso colore, l’uso in questo senso è improbabile.

Il pH fluttuante dell’elettrolita di oro nero può essere visto come una limitazione in quanto restringe l’uso del prodotto a stabilimenti di placcatura qualificati non potendo essere utilizzato nei processi in beaker, che sono comuni nel settore della gioielleria.

 Le prove proseguiranno in futuro in quanto l’elettrolita di oro nero consente di ottenere un’ampia gamma di spessori e – considerando le numerose e diverse sequenze di processo utilizzate in tutto il settore delle finiture decorative – apre numerose opportunità per questo processo, anche attraverso la combinazione della galvanoplastica con altre tecnologie quali la PVD.

In ultima analisi, la migliorata resistenza all’abrasione e alla corrosione resterà un obiettivo centrale per molte aziende del settore della finitura delle superfici. Miglioramenti qualitativi agli articoli commercializzati migliorano la soddisfazione del cliente, rafforzano la reputazione del marchio e soprattutto riducono l’impronta complessiva sull’ecosistema grazie a prodotti che durano più a lungo.

Continue reading