Applications of additive technologies in jewelry production

Laser metal fusion is also known as 3d metal printing and is now the field of additive manufacturing industry with the greatest potential for development and growth.

A high intensity laser selectively melts the metallic powders following a course defined by cad design and creating three-dimensional metal objects.

A variety of metals can be used with LMF technology including steel and bronze, which are the most common, and gold and platinum which are now being researched and closely examined. Our experience in additive printing began from this research a few years ago with the collaboration of sisma.

We started with a few tests and we immediately realized that additive production has certain characteristics that make it unique.

Let’s take a closer look

Additive production has particular characteristics that make it unique:

No cost for creating production moulds, tools and equipment

Reduction in costs and in design to prototype times

Reduced production scrap and environmental protection

Maximum freedom in the design stage. In fact, LMF can create extremely complex shapes that cannot be made with other technologies.

In detail:

– 1 – no cost for creating production moulds, tools and equipment

With LMF, precise machine centring, to be used, for example, for laser engraving, can be done simply and at a contained cost and blocking mechanisms for creating serial items and small prints can be used that reduce both costs in the production phase and re-production times.

– 2 – reduction in costs and in design to prototype times

Creating a model and arriving at the master copy is much faster with LMF than going through the classical production channels. The visual perception on video of the created item is followed a few hours later by its tactile perception, notably cutting verification times for creating the master copy.

– 3 -reduced production scrap and environmental protection

Producing with LMF means reducing waste and energy consumption. In traditional processing methods, micro-casting, for example, many steps are required to create an object involving wax, rubber and plaster scrap. The time of the entire production process with LMF is radically reduced and scrap with
it.

– 4 -maximum freedom in the design phase. LMF can create extremely complex shapes that cannot be made with other technologies

The advantages of direct metal printing compared to lost wax casting are incredible both in terms of precision and geometric possibilities. One example is jewellery that looks like sculpture. Nuovi Gioielli’s experience with LMF stemmed from the desire to approach new technologies and production
techniques to accompany artisan workmanship. With LMF, the design is always new and many creative limitations, often due to traditional processing techniques and the many production stages, can be avoided with this technique.

An object like the one you can see, can be made with one single process.

Having control over the results that we want to achieve, creativity can be expressed without limits, uniting aesthetics, production and architectonic technique into one single item.

The designer controls the whole production process, from the creative phase to powder verification, from fusion to the post process.

METALPIXEL

Research and development into LMF processes have led us to create METALPIXEL

In 1800, jacquard revolutionized the 19
^th century textile sector and, thanks to this special loom, we can still create complex designs and innumerable patterns for the clothes we wear.

More than 200 years later, Nuovi Gioielli, using LMF technology, applies the softness of fabric and jaquard patterns to metal. The result is something quite unique that has many applications, not only in the jewellery sector but also in the much wider fashion and design industry.

Imagine a shirt cuff, imagine an insert in a bag or simply imagine, because METALPIXEL can be anything and everything you want.

Fabrics and their patterns have always been the essence of fashion. Metal, due to its solidness, has always taken a backseat in the accessory world. Now, with this new processing technique, it can become an integral part of a new way of conceiving textiles, softness and customs.

Responsibly Sourced Artisanal Gold: A Tour of Peruvian and Colombian Mining Communities.

What is ASM or Artisanal and small-scale mining?

Artisanal and small-scale mining can best be described as ‘low intensity’ mining, encompassing all types of mining from alluvial deposits to hard rock extraction1. Estimates vary, but in general it is accepted that ASM’s provide about 20% of the gold mined each year but employ about 80% of the mining
population. Operations tend to be informal, very low technology based and labor intensive. In many cases, miners use hand tools such as hammers and chisels, with the more advanced operations using nothing more complex than small scale excavation equipment such as back-hoes and dump trucks. ASM communities
also tend to be very marginalized and have little access to resources, particularly state or industry support. In many cases the need to mine is poverty driven and mining operations are often excluded from the normal banking sector, many having no bank accounts or access to credit. A sad consequence
of this is that many are subject to criminal activity in many forms.

The aim of both the Fairmined and Fairtrade organizations is to improve the above situation. By introducing standards which ensure that all gold and silver produced at artisanal and small-scale mining operations certified by the respective organization are mined in a responsible manner, they improve
the lives and working conditions of miners and mining communities. Once the mine becomes certified, they are not only guaranteed a fair price for the metals they sell, but also a premium to be paid directly to the mine by the purchaser for every gram of gold and silver purchased.

It must be noted that there is not enough ASM gold mined to fulfill all of the requirements for gold, whether for jewelry, electronics or medical uses. The point of responsible sourcing is not to eradicate industrial scale mining, it is to improve the lives and working conditions of miners and mining
communities in the developing world. Many customers, particularly those described as Generation X and Millennials, have different, less traditional buying habits. They want a story, something they can relate to, they want their purchases to make a difference socially and environmentally. This makes
responsible sourcing of gold and silver very relevant in today’s jewelry market.

Figure 1. Miners at the Sotrami mine, Peru.

Figure 1 shows a husband and wife who work at the Sotrami mine at Santa Filomena in Peru. This is a small scale mine employing 700 workers of which 160 are miners working at the face. Some mines are privately owned, and some, like Sotrami, operate as cooperatives while others are lone operators or single-family
operations.

Mercury use.

Both Fairmined and Fairtrade are concerned with mining communities in the developing world, and with this comes many problems with unregulated artisanal and small-scale mining. One of the biggest problems is the use of mercury. Extracting gold using mercury is a relatively inexpensive, simple and quick
process. Gold bearing ore is crushed into a sand like consistency and mercury is mixed in, usually with your hands and in many cases your feet as well. The resulting amalgam is then separated from the waste products and the mercury burnt off to leave gold typically of around 75% to 80% pure.

Figure 2. Using mercury to extract gold from ore
².

Figure 3. Mercury-gold amalgam
².

Figure 4. Burning off mercury
³.

Unfortunately, mercury is also highly toxic, and not just the liquid mercury. The fumes from burning off mercury are also highly toxic. Heavy and prolonged exposure causes irreversible damage to the human body. Mercury poisoning damages the brain, heart, lungs, kidneys and immune system. It results in
birth defects with many children born to mothers who have mercury poisoning being educationally subnormal with decreased intelligence. Mercury contamination also results in a toxicized ecosystem ‘ water, plant life, fish and animals are all affected. Because of bioaccumulation, mercury works its way
up the food chain and can bio-magnify and increase concentration in certain plant and animal species4. This is a major reason to use certified ASM gold and silver ‘ to guarantee that mercury has not been used in the gold extraction process.

Responsibilities for Mine Certification.

For a mine to become Fairmined or Fairtrade certified, certain criteria must be fulfilled (1,5,6):

Mines are required to participate in the social development of their communities.

Mines must eliminate child labor from their organization. No one under the age of 15 can be contracted to work in the mining organization and under 18’s must work in non-hazardous conditions.

There must be health and safety training for all employees and minimum health and safety standards must be met. Mandatory use of personal protective gear must be observed and enforced at all times, and working conditions continuously improved.

The mines must recognize and respect the rights of employees to form or join trade unions and collectively negotiate their working conditions – freedom of association and collective bargaining.

The responsible use of chemicals is mandatory. If mercury is used in the extraction process it must be controlled and plans/timelines agreed to eliminate its use altogether. If cyanide processing is used it must be handled using responsible practices. Chemicals have to be reduced to a minimum and wherever
possible eliminated over an agreed time period.

Responsible use of any premiums paid for under the Fairmined and Fairtrade certification schemes.

Why do Fairmined and Fairtrade metals cost so much?

The previous section describes the basic framework that mining organizations have to agree to and implement to comply to the standards and qualify for the premiums. From the jeweler’s point of view, a few of the questions most ask when looking to purchase responsibly sourced artisanal gold and silver
are why does it cost so much, where does this additional money go to and what is it used for? These are all very fair questions because certified ASM gold and silver is expensive and the jeweler pays considerably above market rate for the materials they purchase.

Normally, the gold and silver purchased by jewelry material suppliers is already in-country, they just call up their bullion supplier or refiner and buy it. However, with Fairmined and Fairtrade metals, unless your business is in Peru, Colombia, Bolivia, Mongolia or a few African countries, it isn’t
in-country and it has to be exported from the mine and then imported into your country of business.

Both Fairmined and Fairtrade use the same basic system. A fixed percentage of the gold or silver market price is agreed between the mine and the purchasing company, guaranteeing a fair price to the mine for their product. The Fairmined and Fairtrade purchasing procedures regulate this to prevent mines
being taken advantage of. As well as this agreed percentage of the gold or silver market price, the purchaser also pays a price per gram of fine metal purchased, and this is paid directly to the mine. All fees are paid prior to the metals being shipped from the mine. Additionally, there are transport,
insurance and logistics costs to get the metals from the mine to an airport, through customs export, onto a plane to your destination country, through customs import then from arrival airport to your plant.

This can be somewhat complex, however by doing this, the miners get a fair market value for the metals sold, and the payment of the premium directly to the mine which is then spent on both mine and community improvements. This premium typically, but not exclusively, gets banked and when it reaches a
certain amount, or after a certain time, the managers who run the mine or owners who run the cooperative decide what they should spend it on. They’re typically responsible in their choices, which are monitored, and listen to the community before deciding what to do but it is always spent for the good
of the mine and community.

Examples of how the premiums are spent range from safety equipment to medical needs, bringing mains electricity or running water to the community, or building schools and places of worship. In the case of one of the mines visited by the author, they decided the money would be best spent on mains electricity,
so they wired up the town. The next year they were looking into supplying mains water ‘all water has to be delivered by truck to the mine daily. But, after consultation, they decided to purchase a cell phone tower. Many of the mine workers do not permanently live on site but work there for three week
shifts, so communication with home and family is important to them, hence the cell phone tower.

Another item they purchased was a football pitch. They built a plateau out of waste rock from the mining operation and then had the pitch built on it ‘ they actually have two at this mine, one at about 1,900 meters and another at 2,750 meters. For the mine workers and community in general, this was an
extremely good spend. The workers now have a sporting activity they can do for both fitness and entertainment, and they have their own league that involves other mines, improving inter-community relations.

Figure 5. The cell phone tower at the Sotrami mine, Peru.

Figure 6. A football pitch at the Sotrami mine, Peru.

These are just two examples of what Fairmined and Fairtrade money has contributed to for a mining community. This money does good, helping to improve the work, the social environment and the community. This is the case with all Fairmined and Fairtrade mines.

In both Peru and Colombia there are two different types of ASM mining activity, these being regulated and unregulated mining activity.

Regulated Mining.

Mining accounts for only a small percentage of the GDP in Colombia however for Peru, about 15% of the GDP is from ‘on the books’ mining. There are no reliable figures regarding how much ‘off the books’ or ‘unregulated’ mining there is. The Government wants all mining activities to be legal and regulated
for a number of reasons. Primarily it will increase the tax base and help grow the economy. This tax base is used for many infrastructure projects and services such as building roads and construction, medical care and education to name a few. They also want to implement health and safety regulations
to safeguard the miners, and eradicate crime and exploitation. To encourage and incentivize miners to become legal and regulated, the government offers a 5% tax refund to a regulated mine for every kilo of gold exported.

Unregulated mining.

Unregulated mines are exactly as described and considered illegal. There are no rules and miners often work for little money under poor working conditions. Unregulated miners can die due to their working conditions, from poor health and safety, and the use of mercury and other chemicals. They are also
taken advantage of in many ways by criminals who exploit them, and unfortunately for many there is no way out of this cycle.

Unregulated mines are typically remote and difficult to reach. Those observed by the author in Peru were often high up in mountainous areas with no roads or tracks leading to them, and tens of kilometers away from the nearest paved roads. Everything needed for the mining operation has to be carried up
the mountainside either by donkey or by hand. Supplies such as food and water, tools and equipment, any wood for shoring up tunnels to make them safe, mercury to process the ore, all of these need to be transported to the remote areas where these mines exist. The mortality rate of the miners is relatively
high. Tunnels are unsupported ‘ they have to get wood to the general area first, then up the mountain, so often they don’t bother and the inevitable happens and tunnels can cave in. Problems with mercury poisoning are also high. In recent years at the request of the Peruvian government, the doctor at
Sotrami’s medical center tested blood samples from unregulated miners and found all to be over the safe legal limit for mercury. As a result of this the owners of the unregulated mines did not allow further tests to be carried out for fear of being closed down. The unregulated miners need the work and
so, as previously stated, get caught in the cycle.

Macdesa & Sotrami Mines, Peru.

The Macdesa and Sotrami mines lie within the Chaparra and Lucanas provinces respectively in Peru, about 600km south of Lima and roughly in between Nazca and Arequipa. Both of these provinces are in the Atacama Desert, which is the driest places on Earth. This is predominantly a mountainous and dusty
region, with the dust being the fineness of cement. It is quite a challenging environment in which to mine and to live.’

The Macdesa mine started life as a hole in the ground like all the other mines in the region. Many of these mines were left idle after Peruvian independence from Spain in 1821. When people started resuming mining, a relatively recent thing, the area was very much the ‘wild west’ with rival mining groups
fighting each other. Miners were barely making a living, safety was non-existent and life was very hard. As a consequence of this, at Macdesa, about 350 miners agreed that the only way to improve was to work together and form a community.

Macdesa is about 1,500 meters in altitude and at the beginning, the miners used hand tools and wheelbarrows to move ore from the mine. They would load approximately 60kg of ore from the mine into sacks and carry these to their donkeys. Once these animals were loaded up, they were walked 50 kilometers
down the mountain to the processing plant near the coast and main highway. However, the miners were taken advantage of and were only given about 80% of the market value. To compound this, the scales were rigged, the assays were rigged, and the ore was processed incorrectly. As a result, the miners rarely
saw any more than 50% of the value of the gold they mined.

So, in order to improve their yields and payouts, the miners started processing ore themselves by hand with mercury, but didn’t really know what they were doing. They determined that they needed to invest in their own technology and hire people who did know what they were doing. The story was very similar
at Sotrami. The original 350 ‘Socio’s’ ‘ founders – took out just enough wages to live by. The remainder of the money they received for their gold was reinvested to solve their problems and improve their processes and yields. Other things they did were:

Purchased proper mining drills.

Made rock and dirt roads to get trucks up and down from the processing plant to the mine.

Improved housing for mine workers and their families.

Invested in safety equipment and implemented a training program for all workers.

Improved conditions in the mine. Air was pumped into the mining shafts to help prevent pulmonary problems. The walls & ceilings were shored up to make the tunnels safe. Rail tracks were installed in each tunnel to enable them to easily move the ore out of the mine.

They began to assay on-site. This way, they knew exactly what purity their dore bars were and so what to expect as a return.

They developed processes to become mercury-free.

They purchased a boring machine, enabling them to test drill and see where the vein goes, greatly reducing the amount of mining needed and increasing their yield of gold recovered to ore mined.

The story for the Sotrami mine is similar. The founders formed the mine thirty one years ago and the village of Santa Filomena has grown around the mine at about 2,750 meters elevation. The population of Santa Filomena is ≈ 2,000, of which 700 work directly for the mine, with the remaining population
working support jobs. Similar to Macdesa, all the profits are invested back into the mine and the community. Workers work 20 days on, 10 days off which are unpaid, and usually eleven hour shifts. Many miners live away from the mine and travel home for their 10 days off to see family, but some have their
families with them, especially if both parents work at the mine.

Very often, local and national governments see mining as a kickstart industry. Thanks to the Mayor in the local town of Challa, who also involved the central government, 40km of asphalt road has been laid from the main Pacific Coast Highway, cutting the drive time to the mine in half. People in the lower
areas have taken advantage of the road and started crop farming, because now they can grow their produce and easily transport it to market without damaging it en route. In many cases where mining is a success story, its presence indirectly gives opportunities to others. The road is also important for
other reasons, one being that for Sotrami there is no running water onsite as such. All the water they need is transported up the mountain three times a day using three trucks.

Typically, nothing goes to waste. Rather than just discarding the rock, they use it for improvements. Waste rock is used to create flat areas which are then built on to expand the mine buildings and the town.

Figure 7. Waste rock used to create level building space at Sotrami, Peru.

Macdesa has three tunnels working their mine, whereas Sotrami only has one and all miners have to enter and exit the mine through this entrance and down this shaft. The mine is 630 meters deep, has 13 levels, and it takes the miners 35 minutes to climb down and 45 minutes to climb back up after completing
their shift.

Figure 8. The mine entrance at Sotrami, Peru.

Figure 9. The entrance shaft at Sotrami, Peru.

Figure 10. One of the mine tunnels at Sotrami, Peru.

Inside the mine, timbers are used to shore up the roof but generally there’s only lighting at the workface and for safety at the ore extraction rails. Although these timbers all have to be brought up from the coast because there are no trees growing in the vicinity of the mines, the good thing is that
the climate is so dry that there are no bugs to eat away at the wood and rot is generally not an issue.

Safety equipment must be worn by all when down the mines, particularly hard hats and especially respirators because the one thing that is a constant is the dust. The particles can cause serious lung problems if miners are unprotected. This is very different from the mines in Colombia, which are in an
entirely different environment.

Ore Processing: Hard Rock.

As far as ore processing is concerned both mines use similar processes. They both originally used mercury but understood the toxicity of this to themselves and the environment and so switched to a much more environmentally friendly cyanide process.

Figure 11. Large rotation barrel.

The ore is first crushed and then processed to the consistency of sand in a very large rotation barrel. Water is introduced to help the grinding process and also to form a slurry, which exits the barrel through a filter, making sure that the particles are small enough for downstream processing. Once
filtered, the slurry is pumped into cyanide treatment tanks. The gold present becomes suspended in the cyanide solution and is then pumped into reaction tanks containing activated carbon particles. The carbon particles are about the size of a grain of rice and they attract the gold, taking it out of
suspension. Typically, three successive tanks are used to maximize the gold yield; the process overall yields 96% of the gold entering the process as crushed ore.

At this point, the used cyanide solution is disposed of into cyanide waste pools. This may sound drastic, but the process is environmentally safe and not to be confused with the extremely environmentally damaging process of cyanide leaching, where the chemical is exposed to the ore in an untreated way
and comes into direct contact with the ecosystem with few or no constraints or containment. These cyanide waste pools have very durable membranes and the waste is chemically treated so that when it is exposed to ultra violet light it eventually converts to carbonates, making the waste pools non-toxic.
Once full, these pools are filled in then either planted or built on.

Figure 12. The slurry filter.

Figure 13. Activated carbon tanks.

Figure 14. A waste cyanide pool. The photograph is taken from a previous pool that has been filled, covered and planted.

Figure 15. Steel mesh electrodes in the plating tank.

The next part of the process is to filter out the gold laden carbon particles from the tanks, transfer these to an electrolytic cell and then plate the gold onto steel mesh. Once this part of the process is complete, the final part is to load the gold laden mesh into an oven and the gold is melted off
and cast into dore bars. The gold purity is now of the order of 80% and the product can be further processed or sold into the market as dore.

Spending the premiums.

As discussed earlier, premiums have been spent on various projects: mains electricity, running water, cell phone towers and football pitches. If the miners are asked why they do what they do and why they are happy to get certified by Fairmined and Fairtrade, there are numerous reasons. These include
environmental responsibility, better working conditions, prosperity, but they all agree that a major reason is for their children. These are relatively young enterprises and the mine workers want their children to have better lives and opportunities than they had. These miners came from virtually nothing,
created businesses and founded communities, and are flourishing.

A great example of this is their schools. At Macdesa, they have spent premium money on their kindergarten and elementary school, which are permanent block buildings, fenced in, clean, safe, and filled with computers. The children are taught how to use both the computers and the internet at a very early
age, and after school, the adults are also taught how to use computers, so they are put to very good use.

Figure 16. The elementary school at Macdesa.

Figure 17. Computers purchased with Fairmined and Fairtrade premiums.

The miners at Sotrami have used their premiums in similar ways. Sotrami has an excellent medical centre and this is very important not only for the mine, but also for the entire surrounding community. The mine funds this with their premiums, but they let anyone who needs it use it, regardless of whether
they are employed by Sotrami or not. They see it as their community duty to do so. It’s well equipped and they have a doctor assigned to them permanently. The next item on their wish list is an X-ray machine. Imagine having to travel 70km down a mountain on unpaved roads for X-rays on a broken bone.

The Peruvian mines discussed are success stories, and the same can be said for the Colombian mines.

Iquira, Coodmilla and Gualconda mines, Colombia.

The gold mines in Colombia discussed here are much smaller in scale than the Peruvian mines. The Iquira mine is in the Huila district, and the Coodmilla and Gualconda mines are in the Narino district, both areas being very lush and at altitude. Similar to the Peruvian mines, the mines in Narino are remote
and about a four-hour drive on unpaved roads from the nearest major highway.

Many of the same problems regarding regulated and unregulated mining in Colombia exist as noted for Peru, and until recently many areas of Colombia were dangerous and off limits to outsiders. There is still a great deal of illegal mining that the Colombian government wants to regulate, but implementation
has been a problem. Just telling miners they can’t mine anymore without providing any alternatives does not work. As a consequence, most communities don’t have an alternative and so go back to illegal mining just to survive. Colombia now has a formal ban on mercury but this is difficult to enforce for
various reasons. The Colombian government has also recently changed the banking laws as an anti-corruption and anti-terrorism countermeasure. A consequence of this is that several mining organizations have lost their ability to export their gold, however Fairmined is working with the mines and the government
to change this.

The Iquira Cooperative is southwest of Bogota in the Huila region of Colombia. This region is famous for coffee and a number of the miners at Iquira are also coffee farmers. The mines were on their land, they knew the gold was there, but they did not begin mining until 2004 when they organized as a cooperative
of 11 shareholders. By 2010 there were 35 cooperative shareholder members of which 8 are women and the co-op has 11 legally registered mines. Initially they would sell their gold informally to the local market, however becoming Fairmined certified has given them the ability to export.

Figure 18. Narino, Colombia
⁷.

Each mine is a separate business, but they all work as a cooperative for banking, selling and exporting purposes, resulting in better deals and reduced costs. These are relatively small mines and typically go about 500 meters into the mountain on one, two or three levels. About 20% of employment in Iquira
is by the cooperative and all of the workers come from the region.

Figure 19. A mine entrance at Iquira, Colombia.

The Colombian mines are also very different from those in Peru as far as environmental conditions are concerned. While the Peruvian mines are very dry and dusty due to the arid region, the Colombian mines are very damp. Water runs down through porous rock and hits a gold-containing quartz layer, which
is impervious, and so runs down the layer into the mine. The miners say they look for this because where there’s water, there’s gold.

The mines all have safety systems in place as part of their certification and qualification for the premiums. Each mine has a gas detection and alarm system to protect the workers, along with safe rooms in case of gas leaks or collapses. All mines and processing plants have first aid kits and all workers
have the required personal protective equipment. A number of mines in the cooperative employ women to handle the explosives and do the blasting, as well as work at the mine face.

Figure 20. A safe room in the mine at XXXX

Figure 21. Miners in Colombia
¹.

Figure 22. Sacks of ore waiting to be transported to the processing facility.

Figure 23. Sacks of ore in the crusher, Iquira, Colombia.

Once the ore is mined and bagged it is transported to the processing plant. Each bag contains approximately 50kg of ore which is processed in a similar manner to that used at the Peruvian mines, except on a smaller scale. Due to the nature of the deposits, after crushing, a flotation table is used to
separate out the heavier elemental gold and gold-containing particles. They do not use carbon rice but settlement tanks and let the gold rich solutions separate out by gravity before further processing into dore bars. There is a very similar process used at the Coodmilla and Gualconda mines in the Narino
district ‘ crushing followed by flotation table then cyanide treatment.

Figure 24. The flotation table at Gualconda, Colombia.

Figure 25. Gold separated from waste from the settlement tanks.

The Coodmilla cooperative, which is run as a non-profit organization, has been in operation for forty years and has four mine titles of which two have been certified by Fairmined. They have one hundred hectares of land covered by their titles and they are currently only working about three hectares.
Unfortunately, both Coodmilla and Gualconda are currently out of certification, but not due to mining practices. They are having difficulty meeting the new banking requirements outlined earlier. Fairmined is working with them and the Colombian government to resolve this.

Figure 26. A gold bearing quartz vein.

Figure 27. The Gualconda mine, Narino, Colombia.

Both of these mines are remote. Gualconda has been built into the jungle using the natural slope of the hillside to help with their processes. For Gualconda at least it has been a difficult journey up until this stage. Back in 1974 the mine was mined completely using hand tools and mercury, requiring
466 grams of mercury to process one ton of ore. Once used, this waste mercury was discarded directly into the river and therefore into the ecosystem. The mine has made great progress in cleaning up the mercury contaminated sites however there is still evidence of the mercury processing used that requires
cleaning up.

Figure 28. The mercury contaminated site at Gualconda.

Between 2001 and 2006 the mine was shut down due to armed conflict involving paramilitary forces and coca growing. Because of this, about one hundred families were displaced, leaving the area for the local city, but the miners state that life in the city did not suit them. In 2006 they formed their cooperative
but their ASM policy was in its infancy and very poor with regard to processing and environmental care. Between 2009 and 2013 they still did not have power for equipment, so they built a water mill using parts sourced from a junkyard. At this point they were still using mercury but had determined that
to be a responsible ASM source, they needed to phase out the use of mercury altogether. Initially, their policy was to reuse the mercury rather than just discharge it into the river, preventing environmental damage and reducing the quantity required to process one ton of ore from the previous 466 grams
to 25 grams. In 2015 they finally got electricity at the mine and this allowed them to redesign their processing plant, fully eliminating the need for mercury and adopting the more environmentally friendly and safe cyanide-based process that they use today. This gave them an increase of 20% in efficiency,
but also increased costs ‘ cyanide is more expensive than mercury when used to process gold bearing ore. However, the increase in efficiency and the knowledge that with Fairmined certification they would get better prices for their gold, plus a premium, made this process the chosen route to help them
fulfill their desire to be a responsible ASM source. After three years this is just coming to fruition.

The aim of the miners is not to stand still but to improve continuously, which includes decontaminating the remaining area at the mine where mercury was used for processing. This mine is now considered a model for responsible ASM mining and they have tours through every two weeks to see what they have
done and how they have done it. This is all within the backdrop of Colombia still having many social conflicts and corruption ‘ violence and drug trafficking is still prevalent in these areas. It is still the case that many mining titles are given to large mines but the smaller independent mines often
go ignored.

Why should you use responsibly sourced ASM gold?

There are many reasons to buy responsibly sourced and certified ASM gold. It may be a good way to get more customers into your store and so can be good for your business. If your customer base is made up of older generations, introducing a product line using ASM gold can be very attractive to both the
younger jewelry buying consumer and also customers who may not have considered gold jewelry, but making a purchase that makes a difference is attractive to them. There are jewelers who have decided to convert completely to Fairtrade and Fairmined gold, or at least as much as possible, but you don’t
have to do this. Every little bit helps and there’s absolutely no obligation, total conversion of every gold product you make is not necessary. Many jewelers who use ASM gold try one line or one collection to begin with, and if they have success, or the idea shows promise with their customer demographic,
they expand their offerings. The story is there: where it comes from, what the extra money is used for. It’s giving back to developing world c ommunities to improve their quality of life. This fact alone often gives customers a reason to buy jewelry made using ASM gold.

How can you purchase ASM gold?

The best way to find out where to purchase ASM gold is to contact Fairtrade or Fairmined (ARM), which can be done via their respective websites, and they will provide you with a list of suppliers and jewelers who are licensees and registered with them. Both organizations have similar systems: if you
want to use the name and mark, you have to be licensed. Depending on volume of business, you may have to pay to be licensed, undergo an audit, and pay a fee to the organization for every gram sold. If you do not want to use the name or mark, you can still purchase the gold ‘ there are no restrictions
on buying it. In this case you can still call it responsibly sourced gold from artisanal mines, you just can’t call it Fairmined or Fairtrade.

The aim of all businesses is to make money. However, this gold is relatively expensive and so to keep it as attractive as possible to the consumer, when pricing jewelry made from Fairmined and Fairtrade gold, it is advised not to mark-up the premium that has been paid to the mine. To keep the cost of
the piece as low as possible, ask the metal supplier to give you the cost of the gold purchased if it was not Fairtrade or Fairmined. This price can be used to calculate mark-ups and then the cost of the premium added in at the end. The more Fairmined and Fairtrade gold sold, the more money goes to
help improve the lives of miners and the mining communities.

Conclusions.

Where gold is sourced from to make jewelry is a personal choice for the jeweler. Suppliers of gold decide which direction they want to go with ‘ responsible or not – and so jewelers can also decide where and who to buy from. You have a choice. Jewelers who want to take part in the responsible sourcing
initiative can do so. There are three main sources to choose from:

Gold that has been mined responsibly but on an industrial scale. Where there is an audit trail, you know where the gold is coming from and you can inform your customers of this.

Gold that is produced from 100% recycled sources. This source of gold has already paid its environmental mining price, and by choosing this option you are getting, arguably, the most environmentally friendly gold supply by using what is already above ground.

Gold that has been mined responsibly from ASM communities. This choice directly helps developing world mining communities improve both their working conditions, their environment, and their lives in general.

Or, you can choose not to take part – it’s your choice. But consider the following:

• Being responsible can be good for business.
• All gold has been mined at some point.
• The overall aim should be to remove irresponsible mining from our industry.
• There must be transparency and traceability from mine and/or recycler to retailer.
• There is still a need for both industry and consumer education.

Finally, is ASM gold any different from any other gold? The answer, of course, has to be no, it isn’t. Gold is gold, wherever it comes from. However, it makes where it came from different, it makes how you get it different, and it makes the environmental and social price it has paid to get to you different.

References:

www.fairmined.org

Photo credit:
www.artisanalgold.org

Photo credit: Marieke Heemskerk

en.wikipedia.org/Mercury_poisoning

www.fairtrade.org.uk

www.responsiblemines.org

Photo credit: Fairmined Family

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

Figure 82. cavity in micro-cast ring section

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

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

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™

Improving abrasion and corrosion resistance of black finishes in the decorative sector

Introduction

Black decorative surface finishes are existing in several variations of accessible technologies and used every day covering a wide range of application methods. Though these methods have been used on an industrial scale for years, each method carries its own limitation. As the base substrates treated
have a high probability to be white in color in the case of aluminum, steel, and silver, or in other cases pre-plated with a white layer used to protect the base substrate as in the case of brass and zinc alloy, abrasion resistance is a key qualitative factor used to benchmark all categories
of black decorative surface treatments. As the black layer wears, the white layer is exposed, leaving a color contrast that is well defined by the human eye making it a rather simple task for the final buyer to evaluate the quality of the product as it ages. 

Specific to electroplating, black plating process have seen several modifications in recent years. With the European REACH compliance in full effect, some chemicals previously used as blackening additives or oxidizing agents can no longer be used. Metals historically used for black electroplating
such as nickel, cobalt, and chromium remain under constant scrutiny, increasing awareness of brands, buyers, and government agencies resulting in requests for limited use or no use of these metals increasing year by year fundamentally in the cases where the final object is to be worn or in contact
with human skin.

Some commonly used REACH compliant black processes in the decorative electroplating sector are black ruthenium and black rhodium. These treatments are generally considered flash with a maximum obtainable thickness of 0.2 micron and rather easy to be worn in time even with the elevated hardness of
both metals.

This paper will follow the development of a black gold electroplating process, run comparative trials against commonly used black decorative applications in our industries today, and explore its possible advantages both as a final and technical layer,    

Black decorative finishing technologies

Numerous types of black finishing methods are available and vary depending on the original substrate to be treated and the industry where applied. Though these processes are quite abundant in number, application, design, and cost limitations come to fruition when examined.

 Anodizing is a process for finishing aluminum alloys that employs electrolytic oxidation of the aluminum surface to produce a protective oxide coating. These coatings can be colored to a limited range including black. Though anodizing has a vast functional usage industrially, the process is used
heavily as a decorative method in many sectors to include home appliance, electronics, and most notably the mobile phone sector due to its cost/quality ratio. As the process is essentially a coloration of the metals natural oxide layer, anodizing is part of the aluminum itself providing total
bonding and excellent adhesion properties. The principal drawbacks to the process in general is that it tends to disrupt detail therefore it use is partial to simple shapes and the application itself is limited to aluminum and very few other non-ferrous metals restraining the processes overall
usage decoratively speaking.  

Paint and coatings are solvent or water diluted resin processes that provide the widest range of colors and effects of all the methods discussed. Though processing sequences may differ, paint and coating can be applied to any metallic or even plastic substrate. Due to this fact its usage can be found
in just about every sector examined however the spectacle frame market can be identified as having the most mature use of paint in the higher quality decorative ornament market. Many variations of black are obtainable with this type of application.  Single layer cataphoretic coatings (e-coatings)
are quite cost effective however suffer in either durability or oxidation resistance depending on the type of resin used. This set back can be circumvented with the use of 2 and 3 paint layer systems applied by spray using multiple resins, however in the end produces a higher cost due application
time and a raw material loss of up to 70%. The downside to the use of paint would be the fact that it disrupts the mechanical movement of functional parts such as claps, chains, and snap hooks. In the specific case of the spray application, it bonds to any substrate it contacts therefore jewelry
use is limited due to the constant presence of gemstones. Paint has also been stigmatized as “cheap” due to the plastic feel it tends to give to metal therefore many luxury goods sectors, depending on brand position, will not consider its use.

Physical vapor deposition (PVD) is a process that uses a vacuum chamber to pressurize a target into vapor which is then altered into a condensed film on the objects surface. Applicable substrates are limited to the temperature required to make the particular deposition which can be quite high limiting
use. This process is able to yield several colors to include black. One of the more recent process used is diamond like carbon (DLC) which produces a jet-black layer that is extremely hard and resistant to abrasion. Initial equipment investment is rather high making PVD by far the most expensive
technology discussed being difficult to find economically outside of China. Though very resistant to abrasion and wear, the chemical resistance of PVD tends to be rather weak due to the formation of microscopic pin holes formed in processing. This is eventually overcome with multiple treatments,
however the cost of the treatment is also dramatically affected. Given the watch sector uses stainless steel as a base material, maintains an elevated abrasion resistance requirement, and the geometries treated are standardized, makes it the perfect candidate for PVD and it use is widespread
throughout the sector.

Black electroplating treatments are vast using a large variety of metals to obtain the targeted result. Two of the most demanding decorative markets for this type of plating are the jewelry industry and the fashion accessories sector. Both sectors can be divided into high and low end segments. The
low end segments utilize metals such as chrome, nickel, tin, cobalt, or an alloy thereof principally due to cost. Though these metals are cost effective, the finishing obtained is not enough to pass normative climatic testing required by some brands. This fact paired with the increasing request
for nickel free finishes and stricter regulation as in the case of the European Unions REACH legislation, the higher end segments lean on metals such as palladium, rhodium, and ruthenium to obtain a black color. High end jewelry tends to use black rhodium where as high end fashion accessories
utilize black ruthenium. Where black ruthenium and black rhodium are able to pass most climatic testing if used with the proper pre-plating sequences, both have a maximum obtainable thickness of 0.2-0.3 micron so tend to present abrasion resistance issues. This is circumvented at the moment in
both high end sectors buy covering the metal with a paint to improve on abrasion or just by accepting the fact that the color will wear in time.

Which leads one to ask, is an abrasion resistant black layer obtainable without the use of paint, while maintaining both nickel free processing and REACH compliance? If so, can the market cost be bared?          

Nickel free black gold

Both rhodium and ruthenium have a maximum obtainable thickness of 0.2-0.3 micron mainly due to the fact that they are mono-metal systems that become brittle with thickness. With a target to improve abrasion resistance, the first objectives would be to maintain a good hardness while making the layer
more malleable and opening the range of reachable thickness. Omitting the use of nickel and cobalt to remain hypoallergenic as well as the use of copper which remains a principal source of oxidation, a bi-metal electrolyte was studied that included gold, palladium, and iron. The system was studied
at an alkaline pH to well receive the selected metals with gold in the highest concentration and iron with the lowest concentration of the metals used to develop the chemical.

Table 1 – Black gold electrolytic characteristics

Surface evaluation of the black gold alloy

When processed, the electrolyte produces an  alloy that is black in color and consists of 49% palladium, 39% gold, and 12% iron by weight making the deposit roughly 12.5 KT gold by title. To evaluate the alloy color,
the CIELab color coordinate system is used with principal focus on the L coordinate when evaluating black finishes.

Table 2 – Color coordinate comparison of black gold, black ruthenium, and black rhodium

The L coordinate is the luminosity value which in this case determines the overall darkness of the black deposit with the lower L value equating to a darker shade of black. The L value of 58 was measured with the black gold alloy.

This fits into the higher range of the industries standard black colors with L coordinates ranging from 50-60 in the case of black rhodium and a much wider availability of black shades with ruthenium having L coordinates ranging from 32-60. The most commonly used formulas of both rhodium and
ruthenium have L coordinates of 58 or 59.

Hull cell testing was conducted to identify the current density range, resulting in a mirror finished black panel produced at
1.0 A/dm
² after 10 minutes of deposition time. This paired with the fact the electrolyte demonstrated good reach and throwing power, 1.0 A/dm
² optimum current density value.

Thickness tests were done with the use of SEM/EDX microscope. Lab trials concluded that a level deposit of up to 2 micron was obtainable while maintaining a mirror like finish expected in the high end decorative plating sector.  With accurate thickness measurements, plating speeds were calculated
obtaining 1 micron in 12-15 minutes at 1.0 A/dm².

Chemical observations

The chemical has proven stable over a 12 month duration of time exhibiting no participation chemical elements if the proper pH is maintained. A fluctuating pH was detected in the electrolyte observing the pH dropping by roughly 0.2 each day.

The lowering pH can be corrected with the addition of pH adjustment salts, however would entail that operators of the black gold solution would have to monitor the pH solution of the daily, giving the electrolyte similar maintenance characteristics to a gold sulfite bath.

Figure 1 – pH variance in an 18-day period

Market segment plating sequences

Two of the more mature and standard black plating sequences were evaluated in each high-end market segment. In the case of jewelry, black finishing over silver was assessed as one of the more standardized cases. Starting from a base material of .925 silver, palladium is applied as the initial plating
layer followed by black rhodium in the most common finishing cycles. In this example, palladium is applied as a technical layer serving a dual use. The first improving the overall resistance to oxidation acting as a barrier to copper migration as well as preventing corrosion from environmental
factors. The thicker the palladium layer, the stronger is the resistance to climatic testing. The second reason for palladiums use is electrolyte protection. Palladium as a metal does not corrode in an acidic environment, whereas silver and the copper that it is alloyed with does. If the palladium
step is skipped, silver and copper will eventually disrupt the final quality deposited as their presence increase with time in the rhodium bath in the form of metallic contamination.

The high-end fashion accessories sector uses a completely different finishing cycle to obtain a similar color. The principal reason being that the initial base materials are typically either brass or zinc alloy and require a different plating sequence to finish to a high-quality result.  The most
commonly used cycles consist of 5-7 plating processes involving many layers for technical benefits. The selected sequence starts with alkaline copper for adhesion, followed by acid copper for surface brightness, moving to white bronze for improved hardness, then to palladium for oxidation resistance,
and finally 1-2 layers of ruthenium. In the case 2 ruthenium layers are used, the first layer is light grey ruthenium which is then followed by black ruthenium, improving the overall products wear ability in the end.   

To incorporate the black gold alloy into the selected plating cycles would allow for the addition of the new layer to be measured using the two industries most standard processes as the qualitative benchmark. 

Black Gold jewelry plating sequence

In the case of the proposed changes to the jewelry process cycle, the black gold was selected to replace palladium as the technical layer given the black gold alloy itself contains a high percentage of palladium along with the additional fact that both electrolytes have a similar alkaline pH would
mean minor processing deviations.

Sterling silver .925 parts were processed directly with 0.3 micron of the black gold alloy followed by 0.2 micron of black rhodium. Additional parts were processed with 0.3 micron of palladium and 0.2 micron of black rhodium to be used as benchmark in abrasion and corrosion testing. Following the
treatments, the samples processed using the black gold as an under layer had a visibly darker color that those processed with palladium, even though the final finish was the same black rhodium processed using the same parameters. This process defined in this paper as process sequence 1.

Figure 2 – Top row: Plating sequence chosen as reference cycle; bottom row: Test cycle with the implementation of black gold.

Black Gold fashion accessory plating sequence

The selected changes to the fashion accessory process cycle were different given the complexities of the plating process itself. In this case, we focused on the finishing cycle which incorporates 1 layer of ruthenium in the sequence. The black gold deposit was tested as a technical layer by using
it to replace the palladium layer in order to keep a similar cost, making quantitative room for the black gold. This process defined in this paper as process sequence 2.

Figure 3 – Top row: Plating sequence chosen as reference cycle; bottom row: Test cycle with the implementation of black gold.

In addition, the black gold was implemented into the process as a final finish given that it shares a similar color to gunmetal grey ruthenium. As both deposits have an L coordinate of 58, this would be used to benchmark the black golds resistance to oxidation with a market segment standard.

Figure 4 – Test cycle with the implementation of black gold as a final layer to test in contrast to ruthenium.

Brass parts were processed both with standard market segment plating sequences, and the relative black gold process deviations for qualitative testing.  This process defined in this paper as process sequence 3. 

Comparative testing

On the basis of the above information, the parts processed with more common plating cycles from the two selected market segments were comparatively tested against those processed with the black gold layer positioned in the sequence. Standardized normative testing methods were used to simulate oxidation
and abrasion.

Abrasion resistance

The test method used to gauge abrasion resistance in the high fashion accessory market is the Turbula test, designed to simulate wear or abrasion. A specific machine is used in which the rotation is mounted on a pivot creating a more aggressive environment compared to a standard tumbler.

The media used is elongated pyramid shaped ceramic, with a distinct form granting both sharp and flat contact points simulating two aggressive forms of abrasion. Testing is preformed with the media weighing roughly 41 grams per 100 pieces. Five parts are treated each test, with a fixed evaluation
phase following a turbula cycle at a fixed time and speed of 72 RPM.

Jewelry sequence results

Following one 3-minute abrasion cycle, the .925 silver specimens which utilized the black gold deposit as an intermediate layer proved to be more resistant to abrasion than the parts treated with the traditional palladium process. Further evaluation determined a better bonding between the black gold
and black rhodium layer when compared to traditional plating methods.

Figure 8 – Top row: palladium as an intermediate layer; bottom row: black gold as an intermediate layer both after abrasion cycle

  

Fashion sequence results

Following one 30-minute abrasion cycle, the brass samples which utilized the black gold deposit as an intermediate layer proved to be more resistant to abrasion than the parts treated with the traditional palladium process which demonstrated complete loss of the ruthenium layer and a failed result.

Figure 9 – Left: black gold as an intermediate layer; right –  palladium as an intermediate layer both after 30 minute abrasion cycle 

Synthetic sweat resistance

The synthetic sweat normative the high-end accessories perform is NFS 80-772:2010-10 which is a direct contact test and more aggressive than atmospheric versions of the test. Following this normative, samples are put into direct contact with an absorbent felt which has been doused with an artificial
sweat solution. The sample is sealed air tight and held at a consistent temperature of 55⁰C. The samples are then held for a predetermined duration of time in increments of 24 hours.

Following one 24-hour synthetic sweat cycle, the .925 silver samples which employed the black gold deposit as the middle layer demonstrated superior resistance compared to the parts treated with the customary palladium procedure. The surface of the pieces using the palladium process revealed elaborate
signs of chemical aggression.

Figure 9 – Top row: palladium as an intermediate layer; bottom row: black gold as an intermediate layer both following 24-hour synthetic sweat cycle.

Salt spray

Following the ISO normative 9227, objects to be tested are suspended in a sealed chamber and submitted to a constant salt spray for a predetermined duration of time. The test is designed to simulate a corrosive environment and test the substrates resistance in a cycle time 96 hour.

To test the salt spray resistance of the black gold alloy, process sequence 3 was used because in this case the black gold alloy is exposed as the final layer.

Following one 96 hour salt spray cycle, the brass  samples finished with both ruthenium and the black gold alloy showed no signs of corrosion.

Figure 9 – Left: Black Gold as top layer; Right:: Ruthenium as top layer both following 96 hour salt spray cycle

Conclusions

Initial laboratory testing has shown that the black gold alloy improved qualitative results when used as a technical/intermediate layer most notably, synthetic sweat resistance when applied over silver as a substitute for palladium and finished with black rhodium. When tested as a final layer, the
corrosion resistance was similar to that of ruthenium, but given the large cost differences when comparing palladium and gold with that of ruthenium and paired with the fact that the two electrolytes yield the same color, the use in this sense is unlikely.

The fluctuating pH of the black gold electrolyte can be seen as a limitation as it restricts the products use to qualified plating factories as it cannot be used in beaker plating which is quite common in the jewelry sector.

 Testing will continue into the future as the black gold electrolyte offers a wide range of obtainable thicknesses and considering the many different process sequences used throughout the decorative finishing industry, open up many doors of opportunity for this process, if combining electroplating
with other technologies such as PVD continues to gain popularity.

In the end, enhanced abrasion and corrosion resistance will remain a central target for many companies in the surface finishing sector. Qualitative improvements to commercialized goods improve customer satisfaction, increase brand reputation, and most importantly decrease our overall footprint to
the ecosystem with longer lasting products.

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)

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.

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):

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.

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).

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

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).

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).

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).

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.

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.

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):

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).

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).

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):

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):

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).

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.

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

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).

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).

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).

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).

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.

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.

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