August 6, 1999 United P.M.R. Inc.
The Current Technology of White Gold Casting Alloys.
Suggestions & Potential Solutions
The current alloys employed for white gold castings fall into 3 categories:
- Nickel Bearing
- Palladium Bearing
- Silver Bearing
The Nickel bearing casting alloys use nickel to bleach the gold white. The higher the nickel content the whiter the castings. The negative aspects of nickel white gold are as follows:
- Allergic reaction potential
- High casting temperature melting
- Potential for silicon additions to cause silicides (hardspots)
Nickel in Jewelry product is currently banned in Europe due to the increasing allergic reaction to the nickel contained. Once someone becomes sensitized to the nickel and has an allergic reaction, it is permanent. The allergic reaction, once it takes place can be quite severe, and can disfigure the individual. Reported cases in the United States have been few compared to the estimated 5% of individuals reacting in Europe. The current European solution is to ban Nickel as an additive, substituting Palladium to bleach the gold white.
Nickel white golds from a casting perspective mandate high melting temperatures, on the order of 1800-2000 degrees F. (980-1100 C). These temperatures are on the edge for causing sulfur dioxide reactions (gas porosity) with standard investment powders. The high temperatures also produce heavy oxide formations, which limit fluidity and reusability when attempting to re-melt.
Nickel white golds that use silicon as an additive to increase fluidity and reusability may cause a potential problem with silicide hardspots. A silicide is an intermetallic compound that forms when the element silicon is combined with certain other elements, in this case nickel, that in the presence of oxygen and pressure form their own compound at a given temperature. In the case of a nickel/silicide the transformation temperature is 1830 degrees F. (990 C). The size of the silicide formed is determined by the amount of oxygen (air), pressure the metal is exposed to and the time of exposure before solidification. The greater the oxygen exposure and the longer the cool time will create some considerably large silicides.
The Palladium bearing casting alloys use palladium to bleach the gold white. The higher the palladium the whiter the castings, however higher levels are still quite gray in color, not as white as nickel bearing alloys. The negative aspects of palladium white gold are as follows:
- High casting temperature melting
- Extremely limited reusability
- High cost
Palladium white golds from a casting perspective mandate high melting temperatures, on the order of 1900-2100 degrees F. (1040-1150 C). These temperaturescan cause sulfur dioxide reactions (gas porosity) with standard investment powders. In addition these alloys solidify very quickly, mandating perfect gating (sprue) technique for shrinkage porosity reasons. The limited reusability results from sulfur pickup by the palladium when using standard investment powder, which hardens the alloy considerably and not in a positive way. Additives like silicon cause similar problems with silicides as described above with the nickel alloys. The high cost of palladium, currently about $350.00 U.S. and the high cost of refining palladium make palladium alloys a very expensive alternative to nickel alloys. The cost of master alloy is about 50 times more expensive.
The Silver bearing white gold alloys use silver to bleach the gold white. The reality is these alloys are not white above 9K (used in England). The alloys are shades of green gold, becoming greener as the karat increases. The negative aspects of silver white gold are as follows:
Silver white gold’s only disadvantages are color & hardness, which makes it unacceptable without Rhodium Plating and possibly a little soft for some applications.
We have reviewed the negative aspects of white gold alloys used for jewelry manufacturing. When taking the current choices into account it is easy to understand the many difficulties the jewelry manufacturer faces when high production white gold casting is necessary. Ultimately, and practically speaking it would appear prudent that for all concerned, from the customer, retailer and manufacturer that a new approach be taken to use a master alloy that would provide the safety, quality and productivity to equal that of yellow gold.
Regulations and Guidelines do not appear to address White gold product. It would appear as well that most manufacturers Rhodium plate their white gold product due to the poor color of almost all white gold alloys. Due to the lack of guidelines regarding white gold, some manufacturers are starting to supply white gold castings using alloys that are intended for yellow or green gold. These castings mandate the use of a quality Rhodium plate. These somewhat rogue or in my opinion practical manufacturers realize it is in all concern’s best interest to use these alloys.
When discussions arise concerning the problems associated with current white gold alloys, it usually centers on 2 main issues. These are: My customers expect a whiter casting and The product will not be white when the Rhodium wears off. The issue of expectation of a whiter casting is mainly a communication issue within the jewelry industry. Jewelry being a small industry allows for much of the fragmentation we see amongst the manufacturers. It would be suggested that jewelry associations as well as leading manufacturers open a dialog to debate the white gold color issues.
The Rhodium plating issue also appears to be a matter of communication. Most jewelry finishers Rhodium plate their white gold. The question is how thick is the plating.
It would appear most finishers use what we will call flash plating. This involves plating for approximately 30 seconds. Flash plating is too thin to guarantee that the Rhodium will not wear off. Proper plating usually takes between 2-3 minutes to adequately assure that the Rhodium will not wear off. The proper time to Rhodium plate will vary to some degree on surface area and plating bath conditions. This can be determined by communication with your plating supplier. Rhodium is a very hard metal, with proper plating Rhodium should never wear off.
We have covered above the key issues involved with white gold casting alloys. Other issues that involve white gold are hardness of the gold when stone setting and finishing. Nickel white gold is more difficult to set; palladium and silver are much softer and easier to finish.
Not all white gold is cast. Problems also arise in wrought product, which actually is a much larger volume product. In addition to greater difficulty in manufacturing which increase manufacturing cost, Nickel white gold is subject to what is called Stress Corrosion Cracking. Stress corrosion cracking is a metallurgical issue caused by weak grain boundaries that occur in wrought product that is hard and under stress, usually seen as broken or cracked prongs. The grain boundaries under stress are corroded easily by many chemicals including household variety.
It would appear that the simple solution is to use high silver master alloys to manufacture white gold and rhodium plate properly. There are some issues that would need addressing to accomplish this. These issues would be:
- Re-plating after sizing and repair
- Masking off parts with the associated labor
- Pen plating
These issues are not minimal. All sizing and repair would require new plating, as the old plate will be worn from heating. This would require all repair shops to be able to plate these parts. Masking parts is time consuming and not always possible. Pen plating works well for a flash plate but concerns of wear should be noted. Pen plating should only be used on areas of the part that does not touch the skin.
- At the retail level does the retailer know the potential allergic reaction of nickel alloys?
- What education to the consumer is necessary?
- Should the consumer have a choice and should the consumer be educated?
- Would there be a potential for liability from nickel allergy reactions if the industry knows the potential?
- Should Platinum be the metal of choice for die struck settings?
- What happens if Nickel is banned in the U.S. jewelry market?
With the increasing popularity of "White" jewelry, Platinum, Sterling Silver and White gold, I believe the standard practice of using Nickel white gold needs to be reviewed. First is the potential safety issue with allergic reaction. Second is the difficulty and increased costs to using Nickel or Palladium as alloying elements. Third is the stress corrosion cracking problems with die struck findings.
Currently, some individual manufacturers are finding that simple, safe master alloys of the silver variety provide ease of manufacture, cost effectiveness and immunity to allergic reaction of the nickel variety. Without intervention by the government, the jewelry industry needs to adapt the most practical approach to providing the consumer with a safe and quality product. Communication between manufacturers and retailers should take place to decide these issues. Although not without some problems the simplest solution is to use high silver alloys with proper Rhodium plating. This has already proven to be the most productive, cost-effective solution for mass production of high quality jewelry.
Hopefully this paper stimulates the industry to address a problematic situation.
September 20, 2000 Patent Pending
Precious Metal Spot Prices
|Gold||2019-04-23 2:41 PM|
|Silver||2019-04-23 2:41 PM|
|Platinum||2019-04-23 2:41 PM|
|Palladium||2019-04-23 2:41 PM|
Prices shown are for reference only. Please contact our trading desk for accurate, up-to-the-minute pricing.