Casting can be done with several alloy combinations, of which platinum/cobalt (Pt/Co) and platinum/iridium (Pt/Ir) are the most common. In the Pt/Co alloy, the cobalt acts as a grain refiner and gives this alloy the ability to fill very fine detail. Casting employs the lost wax process and can be accomplished with either the torch or an induction machine. Other alloy combinations can also be cast.
The extremely high melting point of platinum makes it possible to do bi-metal casting, where another metal is cast onto a finished platinum piece, thus creating colour contrast.
When casting platinum with a torch, hydrogen/oxygen is the preferred fuel combination for the melt. Acetylene is not useful for melting platinum as it expels carbon in the flame and carbon will contaminate the metal through absorption. While it is very hot and melts the metal rapidly, platinum will become brittle if acetylene is being used. Propane/oxygen can be employed successfully. It is also important to get the proper torch for this job.
Due to the characteristics of platinum, the high temperatures and the variety of applications, there is a substantial learning curve to become successful with induction casting platinum. It is recommended that small manufacturers send casting-to-casting houses.
Modern laser equipment has proven to be very helpful in removing defects such as excess porosity on a platinum casting. It also allows the repairing or welding close to stones, without causing damage to them. With a laser welder, repairs on vintage jewellery can be performed that are not possible with conventional methods.
~Jurgen
Learn more about working on Platinum Jewelry
at the Bench Jewelers Conference
April 23 - 25 ~ St. Louis, MO
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