This paper will describe in moderate detail the equipment, tools and methods appropriate to casting platinum on a minimal budget and work area. For those of us who cast in gold, platinum casting work is reasonably familiar. We have the familiar elements of waxing, investing, an oven burnout and a hydrogen torch melt.  Then an upright "Ferris wheel" centrifugal type of casting.  We even enjoy the same sequence of inspections for quality control; model, mold, wax, tree, casting and prepolished castings all need careful attention before stones are set or more work is performed.

The first unfamiliar procedures relate to the latest generation of platinum investments. These investments all come with specific instructions that must be adhered to in every detail. The new powders need acid in addition to water, to make a mild acid solution. These new investments all require the use of a porous paper sprue base so that the solution can drain from the invested flask. One must sprue as appropriate to a high temperature fast freezing metal like platinum. This means while you are new to casting platinum, keep the design simple and as you design, keep spruing needs in mind. The mixed, vacuumed investment is poured into the flask that is then again subjected to vacuum.

All of us who cast gold are accustomed to working in the 1700oF-2100oF range.  Platinum melts in the mid 3500oF range, depending on exactly what alloy you prefer. A small amount of molten platinum usually escapes from the crucible flask/ seam during casting. To protect your workers from this hazard, a safety shield must be installed around the perimeter of the swing arm/crucible assembly. Also, use casting gloves and proper protective clothing.

The high heat brings up the need for a very special crucible to melt the platinum in. The most common, the WESGO crucible, is silica bound by flux. Unlike gold casting, this crucible barely tolerates the melt temp of platinum. If the metal is held molten too long, the crucible actually melts and can make Pt /Si compounds that do cause casting flaws. A fast oxidizing flame from a hydrogen torch is the best heat source. The oxidizing flame prevents Hydrogen/Pt reactions that would otherwise cause brittleness.

The very nature of platinum is that it freezes solid very, very fast. For this reason we suggest two people be on hand to cast platinum. One will melt the Pt while the other stands ready to get the flask out of the oven and place it carefully in the casting machine. Some molten platinum will usually escape during casting.  If there is an unexpected problem with the flask, all the molten platinum can escape at great hazard and expense. 

After the flask is removed, the cooling period before you hammer out the flask will be under one hour. Hammer the flask gently from the button side until the investment breaks up and the casting falls from the rest. Rinse in cold water. Remove as much investment as possible by hand with small tools. 

When all investment is removed from the tree clip and carefully inspect your castings. Do all sprue grinding, filing or sanding at a bench set aside for platinum. Gather all platinum dust and scrap frequently and set it aside in a safe location to accumulate to a quantity large enough to refine efficiently.

A final note for those who implement this method... Every supplier worth the money will assist you in working with their product. From the equipment and tools, through the investing, and the clean up, environmental supplies, and the rest. Call these folks when you have any question on the use of their product.

V5N9

A Look at the Low Cost and Methods of Hydrogen Torch/
Upright Spin Casting of Platinum Alloys
Daniel Ballard
Precious Metals West

This is an abbreviated version of the original work. For full technical details, please consult the original paper.