Alloys – Look on the White Side

Enter the alloy designers. Some praise them. Some pity them. For the alloy designers have a tough job on their hands: They have to design metals that are not only aesthetically pleasing, but also offer mechanical properties suitable for making jewelry. This is not an easy task.

In the jewelry industry, it’s rare to have an alloy without drawbacks, particularly when the alloy is not yellow. When designing alloys, there are a number of hurdles to jump. First, color is absolutely critical. Unfortunately, we alloy designers have a relatively limited palette to work with. Gold and copper are the only two metals that are truly “colored;” all others are varying shades of gray. So when we design a white alloy, we don’t make it “whiter,” we make it “less yellow.”

Next, once the alloy is formulated to achieve the desired color, it must still have suitable mechanical properties. The jeweler must be able to cast and fabricate product without major problems.

It would be nice to think that we could find a formulation that would achieve both the best possible color and ideal mechanical properties, but in the real world, there are always compromises- especially when it comes to creating the perfect nickel-free white gold.

WHITE GOLD ALLOYS

The predominant type of white gold alloy used in the U.S. today is nickel white. Nickel white golds are relatively inexpensive, and if the alloy contains enough nickel, the white color is good. However, nickel whites aren’t the friendliest alloys to work with: they oxidize and can be difficult to clean due to the tenacious nature of nickel oxide; they segregate into two phases when cooled from casting, annealing, or soldering temperatures, causing discoloration, corrosion, and tarnish; and they can fire crack during heating, quench crack during cooling, or stress corrosion crack.

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Such mechanical properties would be reason enough for metal suppliers to try to develop nickel-free alternatives, but there is another major consideration: A percentage of the population is allergic to nickel. For these individuals, contact with nickel can result in dermatological problems that can range from a mild skin rash to severe open sores and permanent scaring. In Europe, there are now laws governing the use of nickel in jewelry sold in the European Union member states. (See “The Nickel Directive,” page 46.)

Between the mechanical failures and health hazards of nickel white golds, it would seem appropriate to develop a nickel-free alternative.

Enter the alloy designers.

NICKEL-FREE WHITE GOLDS

Nickel-free white gold alloys were originally developed in the 1920s using palladium as the primary bleaching agent. For the most part, this is still the case today, particularly in 14k and 18k gold. Palladium has very good corrosion and tarnish resistance, and it mixes well with gold, offering almost complete homogenization throughout the range of gold-palladium compositions. It also yields alloys with excellent mechanical properties- superior in many respects to the nickel-whites- which are especially good for deforming applications.

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But palladium does have one significant limiting factor: cost. The recent rise in the price of palladium has made palladium white gold alloys quite expensive. As a result, the alloy you use often depends on the karat range you’re working in and the type of jewelry you’re producing.

For example, most manufacturers of 10k jewelry would be delighted to find a 10k white gold that does not require rhodium plating, does not tarnish, can be either investment cast or mechanically worked using standard techniques, and is appropriate for all product types. Although they can get those properties from a 10k palladium white gold that contains about 20 percent palladium, they aren’t willing to pay the hefty price tag: Since palladium sells for $300 to $450 per ounce, the 10k alloy in question would cost considerably more than a 14k yellow alloy- around the 16k mark.

On the other hand, many manufacturers would be willing to pay 22k prices for an 18k alloy that met the same criteria. This is often the case because consumers will usually pay more for an unusual piece of jewelry in the higher karats, such as 18k or 22k. They are less likely to move on a price for a 10k piece.

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But for those who find the price prohibitive, there are other elements that will bleach gold to varying lesser degrees, but most have undesirable properties. Poor corrosion and tarnish resistance, high degrees of segregation, embrittlement, and poor mechanical deformation properties are often experienced with these palladium alternatives.

Recently, though, alloy designers have introduced palladium alternatives that have improved dramatically over their predecessors. There are several alloys that replace some or all of the palladium with other elements, such as manganese. (These were used in Europe a number of years ago but were either modified or withdrawn due to processing difficulties.) A nickel- and palladium-free alloy that contains manganese is available in the United States. This is a relatively new product and I am not very familiar with its processing.

For the sake of this article, I will focus on the nickel-free alloys that I know best, providing overviews of 10k, 14k, and 18k options.

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10k

Most 10k nickel-free white gods on the market rely on high silver and zinc levels to bleach the gold. Some may contain a small amount of palladium, typically up to 5 percent, but cost is usually the limiting factor here.

Rhodium plating – Most 10k nickel-free alloys require rhodium plating as a final finishing operation to get the optimum white color.

Hardness – These alloys have low hardness values in the annealed condition, typically 95 HV to 115 HV. (Unlike copper, silver and zinc are poor hardeners of gold.) They also have low working-hardening rates, making high deformations between anneals easily achievable. They can be fabricated into all product forms using standard jewelry techniques.

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Palladium has very good corrosion and tarnish resistance, and it mixes well with gold.

As a general rule, 10k nickel-frees do not age-harden because copper is low or absent; therefore no special considerations during heating and cooling are required to achieve optimum working characteristics. On the down side, this means that the finished piece cannot be hardened for increased wear and durability.

Soldering – The alloys solder well with no known vices. Since the solders generally appear to be somewhat yellow, joints should be kept to a minimum.

Casting – There can be a tendency to form dross when investment casting, a problem of zinc oxidation that is often encountered with high silver alloys. This can lead to crucible degradation, problems with pyrometric temperature measurement, and zinc oxide inclusions in the cast metal.

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Oxidized zinc can also make the liquid metal sluggish, which may result in incomplete fills and metal failure, particularly in small sections like prongs. Zinc may also present a problem if you are using a pressure-over-vacuum casting system. Most of these systems evacuate the top chamber prior to back-fill, and volatilized zinc is not good for your vacuum system.

Special Considerations – The high percentage of silver in these alloys results in poor tarnish resistance. Sulfur in the environment readily combines with silver and the resulting sulfides produce the typical brown discoloration we call tarnish. Rhodium plating will protect the jewelry from tarnish, but only for so long. When the plating wears off, tarnish will soon follow.

14k

The majority of commercially available 14k nickel-free white gold alloys use palladium as the primary bleaching agent. How white an alloy you choose to purchase will depend on how much money you are willing to spend. The palladium-based 14k alloy s available are generally classed as one of two sub-families: low-palladium alloys and high-palladium alloys, the properties of which are outlined below.

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Low-Palladium Alloys

Due to their lower palladium content, these alloys are the less expensive of the two 14k options. They are usually alloyed with silver, copper, and zinc. Silver is an important secondary bleacher. With the higher gold content, however, it is not very efficient in a primary role, since it tends to impart a green hue.

Low-palladium alloys contain up to 10 percent palladium and the resulting color is not truly white, but what is often referred to as “straw white” or “cream white.”

Because of some color flaws, these alloys are best suited to certain applications. For example, if you are quantity producer, the relatively low palladium content will offer a significant unit cost savings.

Rhodium plating – Rhodium plating offers the best results, but the alloy color will be noticeable when the plating wears off.

Hardness – These alloys are ductile and easy to deform; their typical annealed hardness is 100 HV. They have low work-hardening rates, and some can be age-hardened to a minimal degree. Such characteristics make low-palladium alloys particularly suitable for high deformation applications, such as high deformation applications, such as high relief die striking and hand raising, because in-process anneals can be kept to a minimum.

Soldering – These alloys solder well, either with yellow solders or palladium white hard solders.

Casting The low palladium alloys pose no significant challenges to investment casting. Adding palladium will always increase the melting range of a karat gold alloy, but the typical melting ranges for low-palladium alloys are 1,830°F to 2,010°F (1,000°C to 1,100°C). They should not present the caster with any major problems.

High-Palladium Alloys

The second sub-family of 14k nickel-free alloys is the high-palladium alloys. The big advantage here is color: These alloys are in excess of 10 percent palladium, and as the level increases, the alloy gets “whiter.”

Unfortunately, nothing is free. The downside to these alloys is cost and greater age- and work-hardening characteristics, which will require additional steps in the production process. Higher melting temperatures also require special considerations in investment casting. Depending on your market sector, these characteristics may be worth working around, or they could be a right royal pain in the behind.

None of the 14k low- or high-palladium white gold alloys appear to be susceptible to stress corrosion, a problem that frequently occurs with 14k nickel white settings. (Although it has not been scientifically proven, the likelihood of this happening appears to be directly proportional to the value of the stone.)

Both 14k palladium-white alloys have considerably higher densities than their equivalent nickel white alloys, resulting in items being heavier by up to 15 percent. Once again, this may or may not be important, depending on your market.

Rhodium plating – Although how “white” an alloy is depends on its composition, 15 percent palladium alloys generally give an excellent white color, negating the need for rhodium plating in most circumstances. If pieces made from this alloy are rhodium-plated, wear is far less noticeable as compared to the low-palladium alloys.

Hardness – These alloys can be much harder than the low-palladium alloys, depending on composition. (Formulations with more copper will be harder than those containing less copper and more silver.) Higher palladium contents also encourage age-hardening, so quenching after annealing is advised for optimum ductility. Aged hardness values can be in the region of 200 HV, whereas when quench annealed, the ally shows hardness values closer to 150 HV.

In addition, high-palladium alloys work-harden faster than low-palladium alloys, so more in-process anneals may be required.

Soldering – Same as Low-Palladium 14k.

Casting – Investment casting is another area where high-palladium alloys may require adjustments to manufacturing processes. As previously noted, adding palladium to a karat gold will raise the melting range. As you add more, the melting range moves even higher, typically to between 2,010°F and 2,190°F (1,100°C to 1,200°C), and possibly even higher if more palladium is added.

This high melting temperature presents potential problems for the investment caster. More superheat may be required to prevent premature freeze-off and allow complete fills. (Superheat is the difference between the liquidus temperature of the alloy and the temperature at which you cast it.)

Casting equipment used for high-palladium white golds must be capable of reaching the required superheat for these alloys, which may be as high as 300°F (150°C) over liquidus point of the alloys. In addition, the temperature measurement system must be capable of reading a temperature this high. (A type S thermocouple, for example, may be required.)

Also, different sprue and gate configurations may be necessary: Those that work on a particular design in 14k yellow may be inadequate for a 14k high-palladium white alloy. Sprues may need to be thickened and shortened to prevent premature freeze-off and guarantee progressive solidifications; multiple sprues and gates may also be required.

The higher casting temperature also creates potential problems with sulfate investments. When cast, the hot metal decomposes the standard sulfate investment to generate sulfur dioxide gas. This gas “pushes” the liquid metal away from the mold wall and leads to a rough surface texture and a heavier oxide layer, which means you have to do more finishing work. This problem can be avoided with the use of a high temperature investment.

For both spruing and investing, individual specifications will vary. The appropriate method will be determined through experience.

18k

As with the 14k alloys, the majority of commercially available 18k nickel-free white gold alloys are designed using palladium as the primary bleaching agent. Once again, they fall into two major subfamilies: low-palladium alloys and high palladium alloys. The 18k alloys are generally 10 to 15 percent more dense than other 18k white gold alloys.

Color may vary somewhat with 18k alloys, even when the palladium content is the same. Secondary elements can have a noticeable effect on the color of the alloy, and “whiter” colors can be designed with less palladium and lower melting ranges. There is sometimes a trade-off between excellent color and a tendency to form heavier oxide layers, but many manufacturers consider the extra work worthwhile for the better color. Many of these formulations are proprietary, however, so the best approach is to talk to your metal supplier about what will work best for you.

Low-palladium alloys

Rhodium plating – These alloys need to be rhodium plated because of their poor color.

Hardness – Annealed hardness values of 120 HV and work-hardening behavior typical of general purpose 18k alloys are common.

Soldering – Depending on the application, either yellow solders or hard palladium white solders may be used, although the palladium white solders are the best color match.

Casting – Melting temperatures are in the range of 1,740°F to 1,920°F (950°C to 1,050°C), so there are no surprises for the investment caster.

High-palladium alloys

The 18k high-palladium alloys are closet in color to platinum alloys. Mechanical properties are similar to the low-palladium family.

Rhodium plating – Not usually required.

Hardness – All the alloys are ductile and take high degrees of deformation between anneals. The annealed hardness is generally between 120 HV and 130 HV.

By Stewart Grice - © 2003
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