Casting Moissanite in Place

Can moissanite, a precisely calibrated hand-cut jewel, be successfully cast in place in platinum and karat gold alloys? Ongoing research being conducted by industry veteran and head caster Tony Milad of Hoover & Strong, Richmond, Va., has shown that the procedure can be done successfully using karat gold alloys. Procedures using platinum alloys are showing promise.

Because moissanite is known for superior heat resistance, it’s a prime candidate for in-place casting. A comparison study to test heat resistance was conducted using faceted samples of diamond and moissanite. Both were placed in a high-temperature oven and the temperature was increased while a flow of oxygen was introduced. As the oven reached 2,012ºF, the diamond caught fire and began to vaporize. The oven was turned off and the diamond continued to burn until it was completely vaporized. The moissanite jewel remained intact.

An article in the Winter 1997 issue of Gems & Gemology reports that moissanite is stable in air up to about 3,092ºF and is inert (not readily reactive with other elements) to well over 1,832ºF except around certain substances including some molten metals.* Moissanite-in-place casting requires a general understanding of temperature relationships, as shown in the chart below:

When analyzing the numbers on the chart, you will see the range of casting temperatures for 950 PlatOro require heat slightly above the known stability temperature for moissanite. The chart also shows the casting temperatures required for 14k Royal yellow and Pall white gold alloys used are well below moissanite’s stability range.

Moissanite-in-place casting procedures. Charles & Colvard Ltd. provided a selection of moissanite jewels—4 mm to 6 mm in size—for the moissanite-in-place casting procedures research. Hoover & Strong provided 950 PlatOro as well as 14k Pall white and Royal yellow casting alloy. Tony Milad, Hoover & Strong’s casting director, cast the trees using the following methods.

• Casting with PlatOro. Hoover & Strong uses 950 PlatOro alloy exclusively for all platinum cast products and for its contract casting service when platinum is requested. For this process, Milad used standard platinum casting investment. The flasks were invested and stood for about eight hours prior to burnout. The burnout cycle is programmed for approximately 12 hours. The first two hours of the cycle did not exceed 165ºF, and the highest temperature of the burnout cycle did not exceed 1,150ºF.

The finished cast flasks were placed on firebrick and cooled for a minimum of two hours. After reaching room temperature, the investment was removed from the flask by hammering and power washing. The tree was then placed in a de-investing solution and power-washed again for final investment removal.

• Casting with 14k Pall white and Royal gold. For the research project, Hoover & Strong selected its most popular 14k yellow and white alloys. Wax rings preset with moissanite were sprued on both buttons and trees. The burnout cycle was programmed for eight hours, and the oven/flask temperature immediately prior to casting was about 1,130ºF.

When doing torch casting, the method of melting the metal is important. Here, the pre-flux-glazed crucible was preheated for eight to 10 minutes, after which the 14k palladium white gold was placed in the crucible. The metal was melted using a heat transfer method—i.e., the torch flame is directed on the crucible and not directly over the alloy. A generous amount of powdered borax flux was sprinkled over the metal as it reached its melt/flow temperatures, and the flask was placed in the casting machine while the final melting took place. Then the arm of the casting unit was released and the metal cast. The flask was allowed to cool at room temperature for two hours prior to de-investing.

Flasks containing larger trees were cast using a vacuum centrifugal induction-melt casting machine. Kerr Satin Cast 20 investment was used, and the burnout cycle was programmed for eight hours. The highest temperature during the burnout for moissanite-in-place was 1,150ºF. The 14k palladium white gold was placed in the machine’s crucible, and the melting temperature was set so the alloy would not exceed 2,065ºF when melting. The oven/flask temperature immediately prior to casting was 1,130ºF.

950 PlatOro results. After the flasks were sufficiently cooled and de-invested, initial inspection of the moissanite revealed that it remained brilliant and appeared undamaged through the casting process. The rings were clipped from the buttons and the moissanite removed from the mountings.

Stewart Grice, Hoover & Strong’s chief metallurgist and director of mill products, prepared and inspected a cross-section of the platinum prongs that held the moissanite in place for casting. Using a metallurgical microscope, Grice observed a darkened area on the prong at the contact location between the moissanite and molten platinum.

Grice considers a eutectic between materials as the possible reason for the jewel damage. (A “eutectic” is the lowest possible melting point of an alloy or solution, usually below that of any of the alloy’s elemental components.) Further analysis and research reveals that the eutectic is between silicon from the moissanite (silicon carbide) and the molten platinum. When contact occurs at temperatures required for casting, the moissanite decomposes into the individual elements of silicon and carbon. The black areas identified are most likely elemental carbon or a high carbon-silicon compound.

“The elemental silicon will ‘leach’ into the platinum at the point of contact and readily form an inter-metallic, probably Pt-Si,” Grice explains. “This, combined with the excess silicon, will form the eutectic. Platinum and silicon form three eutectics, with the highest melting point of all three being 1,796ºF, well below the temperature of the interface during casting. In this case, not only are the jewels damaged, but the presence of the platinum-silicon inter-metallic will make the prongs brittle.”

Milad and Grice are conducting additional research into eliminating the damage that occurs when casting moissanite in place in platinum.

14k Pall white and Royal gold results. All 14k palladium white and royal yellow gold moissanite-in-place cast samples were examined under magnification by Milad, Grice, and Mann. No damage was observed on the unmounted jewels.

The success of in-place casting of moissanite in karat gold presents retailers with new possibilities for the design and production of original jewelry featuring these jewels.

For additional information related to the design and working characteristics of moissanite at the bench, visit www.visualcominc.com or contact Mark B. Mann at (406) 961-4426, (800) 210-4367, ext. 251, or e-mail: mark@visualcominc.com.

References:

* Gems & Gemology, Winter 1997; Nassau, McClure, Elen, and Shigley

† Hoover & Strong, 2002/2004 catalog

Charles & Colvard Ltd., Jewel Properties

¶ Tony Milad, Hoover & Strong, moissanite-in-place casting research

§ Vines, R.F., The Platinum Metals and Their Alloys, The International Nickel Company Inc., New York. Book reference provided by Stewart Grice.

Mark B. Mann is co-founder and partner of Visual Communications Inc., a company that provides technical content; professional images; product testing; research and development; and presentation and publication of shop, bench, and service department emerg ing technologies and best practices. Tony Milad is head caster and Stewart Grice is metallurgist and mill products director for Hoover & Strong, Richmond, Va.

This article about moissanite-in-place casting has been underwritten by Charles & Colvard Ltd. and is being presented as a service to JCK’s readers, courtesy of Mark B. Mann and Charles & Colvard, Ltd.

Copyright 2004 – Visual Communications LLC

Alloy/Material Temperature
Melt/flow temperature of 950 PlatOro (Hoover & Strong’s platinum alloy used for moissanite-in-place casting) 2,895ºF / 2,965ºF† (casting temperatures range from 3,090ºF / 3,240ºF)
Melt/flow temperature of 14k Pall white (palladium white gold) 2,030ºF / 2,165ºF† (casting temperatures range from 2,290ºF to 2,380ºF)
Melt/flow temperature of 14k Royal yellow gold 1,530ºF / 1,600ºF† (casting temperatures range from 1,725ºF to 1,815ºF)
Known stability of moissanite in air 3,092ºF*
Highest temperature flask reaches during oven burnout prior to casting 1,150ºF