CASTING WITH NICKEL WHITE GOLD ALLOYS

The biggest problem encountered in casting nickel white gold alloys is that it’s not yellow gold. The yellow gold alloys are much easier to cast, very forgiving and have a much greater reusability. The nickel used to bleach the yellow color of gold in white gold alloys is very sensitive to sulfur in the calcium sulfate bonded investments, carbon from crucibles, unburned residues in the flask and oxide build up in the metal when reused. White gold alloys have about 1/3 the reusability of yellow gold alloys and will need refining at more frequent intervals. White gold alloys are not as fluid as yellow gold and will chill faster when being cast. Marginal sprue systems on castings that come out O.K. in yellow gold will be a problem in white gold. White gold alloys will solidify faster and more shrinkage porosity will be experienced in poorly sprued white gold castings. Nickel white gold alloys will be harder than yellow gold and will cause a lot of whining in the setting department by setters accustomed to setting yellow gold. The following information may be helpful in making good quality, porosity free white gold castings.

Alloy Selection: There are two types of deoxidizers normally used in nickel white gold casting alloys, Silicon and Boron deoxidizers. Each has its benefits and drawbacks. Silicon deoxidized white gold alloys will have the best reusability but under certain conditions can form silicide hard spots in the castings. Boron deoxidized white gold alloys will give excellent results on the first cast but the reusability will be rather limited. United # 910, #915, # 920, # 930, # 940 , # 950 and UW series alloys are silicon deoxidized nickel white alloys. United # 925 and # 900 are boron deoxidized nickel white alloys. The nickel content in white gold alloys will affect the whiteness and hardness of the finished goods. The high nickel content alloys will have the whitest color but will be much harder, prone to fire cracking and have a higher melting point. The most popular white alloys have a moderate nickel content that provides an acceptable white color, good working characteristics and good castability. United # 970 alloy has a lower nickel content and contains silver making it easy to cast but has an off white color in 10 and 14 karat gold. United # 980 alloy is a silver base white alloy that is nickel free and quite soft, making it suitable for 10 karat but has a slight greenish tone in 14 karat. About 95% of white gold castings are Rhodium plated after finishing to provide a platinum like color to the finished goods. United is always working on improved formulations for white gold casting.

Check your spruing: The first rule of casting is "Always sprue to the heaviest area of the casting". Much of the shrinkage porosity experienced in white gold casting is due to incorrect sprue placement and size. The heaviest areas of the casting will solidify last and if sprues are not placed in this area to provide a fresh supply of molten metal, shrinkage will occur in this area. Marginal sprue systems are a big problem in casting white gold alloys. You may be able to get away with some marginal spruing on yellow gold castings but the nickel white gold alloys will solidify faster and you’ll have shrinkage porosity in white gold castings. The shrinkage problems most often seen are in rings with heavy tops and shoulder areas that were sprued at the shank.

Waxes used for wax patterns: Be aware of the type of wax being used in the wax patterns being invested for white gold castings. The regular injection waxes will burn out better than plasto-waxes or carving waxes. The plasto-wax and carving waxes will contain polymers and /or resins that will require longer burn out times to completely destroy all carbon residues during the burn out cycle. Adjust the top end burn out time to the type of wax being used.

Investing the wax patterns: Regular investment and investing procedures are used for nickel white gold alloys. Be sure to follow your investment manufacturers instructions as to water to powder ratios, mixing times and set up times.

Wax burn out: Most of the gas porosity problems in nickel white gold castings are directly related to incomplete burn outs or insufficient air flow during the burn out process. Carbon residues left in the mold cavities and / or a reducing (oxygen poor) atmosphere in the oven during burn out can cause decomposition of the calcium sulfate binder in the investment. The decomposition of the calcium sulfate binder in the investment will release sulfur compounds in the mold cavities. When the hot molten metal comes in contact with the sulfur compounds, sulfur dioxide gas is formed. The molten metal will absorb large quantities of the sulfur dioxide gas and cause porosity in the nickel white gold castings. The nickel white gold alloys have a great affinity for sulfur dioxide gas and the porosity problems will be much worse than in yellow gold alloys. An adequate amount of time at the top end (1350 deg.F / 732 deg. C) of the burn out cycle is required to burn out carbon residues left from the wax. A good air flow is required to completely burn out carbon residues and prevent sulfur gas reactions in the investment. Keep in mind that a burn out oven is an incinerator not a kiln and an adequate air supply and proper exhaust is required for complete combustion. More burn out problems are experienced in electric ovens due to the inadequate air supply during the burn out process.

Melting & Casting: If you are mixing your own gold with one of United’s white gold alloys, the metal must be pre-alloyed before casting. Pre-alloying temperatures will be much hotter than the recommended casting temperature and if the metal is cast at the pre-alloying temperature, it will be much too hot for casting. The usual method for pre-alloying is to place the alloy in the bottom of the crucible, add a small amount of boric acid flux and place the gold on top then proceed with the melt. If torch melting is done a reducing flame should be used, in open melting a reducing or inert gas cover is helpful to prevent oxidation. The metal should be thoroughly melted and mixed well, adding a small amount of boric acid flux to keep the metal clean and poured into water to make casting shot. Flux can not be used in some bottom pour melters. The pre-alloyed white gold casting shot should be dried before using. The metal can now be melted for casting. Instruction sheets are available for all United white gold alloys with pre-alloying and casting temperatures. Boric acid flux is helpful in open melting equipment during the melt.

A protective gas cover or reducing torch flame should be used for melting and casting. A slightly higher flask temperature should be used for white gold usually 50 to 100 degrees F hotter than yellow gold.

Quench Time: An 8 to 10 minute quench time is recommended for 4" X 8" nickel white gold flasks. Shorter times may be used on small flasks. Avoid long cooling periods inside the investment as this will result in harder castings. If very heavy castings are made, a 15 minute quench is advisable.

Reuse of scrap: A 70% fresh, 30% scrap is the recommended ratio for nickel white gold alloys. Be sure all investment residue is removed from scrap before re-melting to prevent sulfur contamination of the metal. If sprue buttons are heavily oxidized, cut them off and put them in your refining scrap, this will avoid putting oxidized metal into the cast. If excessive casting defects, porosity or inclusions are seen in the castings, scrap the metal and start fresh. Heavy oxide build up in the metal will inhibit the fluidity of the metal and cause more shrinkage porosity and fill problems. Remember nickel white gold alloys are not forgiving and the reuse will be limited.