CASTING GEMSTONES SET IN WAX WITH WHITE GOLD

Casting gemstones in place with nickel white gold alloys is becoming more popular. United #900, #925, #940 and UW1 white alloys have been successfully used for casting gem stones in place. The #925 alloy is the most popular alloy for this purpose.

Palladium white gold alloys have too high a melting point and casting temperature to be successfully used for casting gem stones in place. Palladium white gold alloys will chill rapidly and diamonds will pull the heat out of the metal quickly causing fill problems on the castings.

Good sprue systems are essential for obtaining a good casting with white gold alloys. Additional sprues may be required to shoulder areas to prevent shrink porosity in stone set areas.

Smaller trees are usually more successful for vacuum casting gemstones in place with white gold. Some manufacturers prefer to use centrifugal casting for gem set white gold castings due to the faster chill rate of white gold alloys.

Using stone casting investment or proprietary additives to the investment will allow higher burn out temperatures and flask temperatures for casting gemstones in place with white gold.

Most stone casting investments will have a top burn out temperature of 621° C/ 1150° F and will allow a higher flask temperature for casting. Be mindful of the quality of the gemstones being used, lower temperatures may be needed for poorer quality stones.

Nickel white gold alloys are very sensitive to carbon residues left in the flask after burn out and nickel has a great affinity for sulfur that may be picked up from the calcium sulfate bonded investment. The cleanest burn out possible is important for casting nickel white gold alloys.

Steam de-waxing is helpful to remove the bulk of the wax before burn out. Allow flasks to set for 2 hours after investing before putting them in a preheated steam de-waxer. Steam de-wax flasks for 1 hour maximum and transfer them immediately to a preheated oven set at 250 to 300 degrees F. Steam dewaxing will remove about 70% of the wax. Avoid long steam dewax cycles or inadequate investment set up time as this can cause investment breakdown during the steam de-wax process.

Regular casting temperatures may be used when the special stone casting investments are used.

Flasks are allowed to air cool after casting and are broken out with a hammer. Air-cooling will make the white gold castings quite hard. Quenching flasks can cause the gem stones to break from thermal shock.

The white gold scrap from casting gemstones in place should be kept separate and will need refining sooner than metal used for regular casting. White gold alloys have about 1/3 the reusability of yellow gold alloys.

A 75% fresh to 25% scrap ratio is advisable. The scrap from gem stone in wax casting will need to be refined at more frequent intervals.

Models used to make molds for stone in place casting should be designed to avoid restriction in metal flow. Helper sprues may be required on some designs. Remember that diamonds and other gemstones are good conductors of heat causing molten metal to chill during the cast.