Due to the huge increase in customer demand for platinum jewelry, more and more small shops are needing to add platinum jewelry design and manufacture to their in-house services in order to stay viable. Completing jobs on time and under budget with minimal expense and with the least amount of down time has always been a hurdle for the smaller shops. Adding platinum production comes with its own set of additional hurdles … or so I was led to believe.

The biggest misperception about platinum is that it is difficult to work with. Platinum is not a difficult metal to work with, it just has different properties than other metals. Once platinum's unique properties are properly understood and allowed for, many find that working in platinum is actually easier than other metals. And most important for any size shop, while platinum is an expensive metal, it is also a forgiving metal. Unlike gold alloys, if your casting fails, the platinum can be re-cast over and over again. I believe that every goldsmith can learn to work with platinum.

Misperception #1:

Specialized Equipment is Needed
Because of platinum's unique properties many assume that they will need to buy special equipment to cast with it. This is only minimally true. I discovered that many of the tools I already had in my own shop were more than sufficient for casting platinum.

Casting Machines/Models
The vertical, horizontal and the induction casting machine models are all acceptable for casting platinum.
I use a Neycraft horizontal centrifugal spin caster in my shop.  My Neycraft casting machine has held up for at least 600 platinum castings to date, and I see no end in sight.

Crucibles
There are special crucibles on the market designed to withstand the high temperatures (3250°F) required to melt platinum without breaking down as readily as crucibles designed for other metals. The standard platinum casting crucible is the large Wesgo A type fused alumina crucible from Rio Grande.

Misperception #2:

Platinum is Difficult To Cast
Platinum's density is what gives it its satisfying weight, but what also makes it flow less easily and solidify very quickly. In casting, this poses problems that are easily overcome by preparing waxes properly. As with other metals the correct placement of sprues is critical to making a successful casting.

Preparing Waxes
As with other metals, before investing check waxes for problems: make sure that the piece being cast has an even wax thickness throughout the piece to ensure correct solidification (cooling down) of the metal. Check for imperfections, smooth surfaces and for the placement of sprues or gates. Larger gates are required for platinum than for gold, and the use of extra gates are strongly recommended as platinum solidifies in approximately three seconds.
Investments and Flasks

All platinum investments have specific mixing instructions and must be followed to the letter. This will ensure full set-up of investment and a smooth surface on the casting. Once over the initial shock of the length of time it takes to mix platinum investments, one realizes that the procedure is just like mixing regular casting investment for other metals.

Platinum investments take anywhere from 12 to 16 hours to set-up and harden. During this time period they need to be checked every three to four hours and shaken down if necessary to make sure that the investment has not climbed up the sides of the absorbent paper cylinder. The burnout cycle ramping the investment up to 1600°F takes around five to eight hours. The entire investing process ties up the small shop jeweler for almost a 24 hour period.

Jeweler's Choice
The investment I use, Jeweler's Choice, was created originally for the dental industry by a company called, Precident Alloys/Talladium. What can I say other than Jeweler's Choice is FAST. It sets up in only 15 minutes and burns out in 45 minutes ready for casting!

Jeweler's Choice is so strong it can withstand the high temperatures and impact of the molten platinum without using a metal flask. Mixing the investment can be done in a special mixer made for the investment called the Whipmix machine. The Whipmix mixes the investment and vacuums it at the same time taking only 1.5 minutes. It is then poured into the flask as it is vibrated on an attachment on the side of the mixing machine.

Based on a 90 gram flask, the Jeweler's Choice invested flask sets-up in only 15 minutes. Once set, the top of the investment must be scraped to enable air to escape out through the top of the flask. The flask is then placed in the kiln and burnt out for only 45 minutes evacuating the remaining wax, cintering the investment and hardening it.

Just prior to casting the flask is removed from the oven and set in the cradle of the casting machine centrifuge and the platinum alloy is placed in the crucible.  The flask will rest for seven minutes to lower the flask to around 875°F before casting.

As with any investment procedure, a dust mask is recommended to protect the lungs.

It's Time To Cast Platinum Alloys
The choice of casting alloy varies obviously depending on one's budget and the desired use of the metal. The standard alloy mix used to be a 10% Iridium/90% Platinum. Now several alloys are available and becoming standards.

Gas, Torches and Pressure
As in most small shops I use a propane and oxygen gas mixture. I prefer propane as it is excellent for its low carbon content making it less likely to contaminate the molten platinum when using an oxidizing flame which must be used to achieve a fast melt.

 The torch I use is the Meco Weld Master for its versatility, heavy body and long neck. I use it with a large rose bud tip #2.  This torch can deliver the pressure and the heat needed to melt 1½ ounces of platinum in approximately 20 seconds.

The right pressure mix is very important no matter what gas mixture is being used.

I recommend that beginners take an ounce of platinum and place it in the crucible of their own machines and practice melting the metal striving to achieve the fastest melt while timing oneself.

Melting Technique
The flame to use is a hot oxidizing flame with the full amount of gas being used and all the oxygen that can be forced into the tip without the torch going out. The torch tip should be placed very close to the metal (about 2 to 3 centimeters), and the flame should be played directly on the metal. The metal will begin to melt on the surface and become fully molten within seconds. At this time the flame is used to force the metal up and down in the crucible. This should be done at least twice. When the metal is completely melted it should not stick to the bottom of the crucible and the whole mass of metal will be forced up into it. The flame is actually being used to mix the metal ensuring complete fluidity. It is crucial at this point to remove the torch while simultaneously releasing the machine. If the metal is kept at a molten state for too long it will begin to absorb gas which can cause gas porosity and brittleness.

Eye Protection!
Platinum melts above 3250°F; the high white radiation of the molten metal is very damaging to the eyes. To protect the eyes when casting one should have flip-type welding goggles with a #6 top plate and a #5 inner plate.

Investment Breakout
After casting the flask is left to set for about five minutes. Using a heat resistant glove the flask is picked up and the investment is hit with a hammer or other striking tool to break the investment away from the cast piece of jewelry.
 

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Platinum Casting in a Small Shop
Edward J. Friedman
The Buehn Company

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