Since 1993, platinum consumption for American-manufactured jewelry has increased more than 300%.  With this substantial climb in platinum jewelry demand, U.S. manufacturers have had to find ways to reduce rejects, increase efficiency and lower the cost of manufacturing platinum jewelry.  Throughout this period, new, platinum-specific manufacturing products, materials and supplies have become increasingly available to the American jewelry manufacturer.

Platinum Casting Investment Powder
One of the most critical elements in platinum jewelry casting is the investment powder used to produce a mold for molten metal.  Due to platinum's high melting and flask temperatures, an investment powder that can withstand sustained furnace temperatures up to 1800oF and molten metal temperatures in excess of 3250oF is necessary.  Phosphoric acid-bonded investment powders that have superior thermal and as-cast surface qualities are being imported to the U.S. from Japan.

Casting Machines
It has been a common misconception that vertical centrifugal casting machines were necessary when casting platinum when using a torch for melting.  This misconception is easily dispelled by using a small horizontal centrifugal machine that is made for gold or silver casting.  The crucible used must be for melting platinum and crucible adapters to fit most horizontal casting machines are readily available.

The use of a standard horizontal centrifugal casting machine is applicable in combination with oxy-hydrogen gas-fired torches.  However, induction melting is the recommended method of melting platinum.  Induction melting is favored over torch melting due to platinum's propensity to absorb gases such as hydrogen and elements such as carbon, phosphorous or silicon.  While there are many induction casting machines available from Asia and Europe, these machines are quite costly. 

There are now at least three American-made induction casting machines currently available for less than $30,000. Of special interest are the new bench-top induction casting units.  Although these units are Asian or European made, they cost around $11,000 and weigh about 75 pounds.

Zircon Oxide Paint
Whether one is using a torch or an induction casting machine, coating the platinum crucibles with a zircon oxide (ZrO) paint is recommended.  Not only will the ZrO paint minimize the possibility of cross contamination of platinum with silicon, by acting as a parting agent it will also promote longer platinum crucible life.

Gravity Paints
If you have already cast platinum jewelry items, you may have noted that the casting flask will start to slake during the cooling process after casting.  This flaking is caused by oxidation and contraction of the flask during cooling. You may not stop the contraction, but you can minimize the oxidation factor by coating the outside of the flask with graphite paint.  This graphite coating will restrict the exposure of oxygen to the heated surface of the flask and reduce the flaking.

Finishing Platinum Jewelry
Besides the casting of platinum, finishing or polishing a platinum jewelry item can be the most problematic process in platinum manufacturing.  Due the platinum's density and resistance to scratching, the finishing or polishing process from start to finish can take three times longer for platinum than for gold or silver.  As in all processes, a methodical and systematic approach and the proper materials will increase the success of the finishing or polishing operation.

Mass-Finishing
Platinum jewelry items can be mass-finished in the typical manner of gold or silver jewelry products, such as barrel or vibratory bowl tumbling.  However, new magnetic tumblers using a fine steel shot can greatly reduce the time for the burnishing cycle.

Hand-Finishing
Finishing and polishing compounds made of aluminum oxide or silicon carbide and imported from Japan are now available in the U.S. These compounds reduce the labor time for finish platinum by 10% to 15% over the common finishing compounds such as tripoli, white diamond, green compound, or red rouge.  By burnishing a porous surface, one can close minuscule cavities and achieve a high polish on a sub-standard surface.  Not only will burnishing close pits, it will also work harden the surface of platinum jewelry, allowing for a quicker finishing process.

It is important when polishing to keep the polishing buffs, brushes, wheels and laps clean and freshly charged with cutting and finishing compounds.  This allows for quicker cutting and polishing because the clean, fresh charge of cutting and polishing compounds are more abrasive when fresh.

Brazing and Welding
Although three years ago there were no tools available, this area of platinum manufacturing now has all the recommended tools and supplies available.  The tools and supplies are an alumina brazing pad, tungsten solder pick, tungsten tweezers, welding lenses that provide from #6 to #11 UV protection and zircon oxide (ZrO) paint.

Your work area should be for brazing or welding platinum only to help prevent contamination from residual metals on the workbench.  Use cadmium-free karat gold solder when joining platinum to karat gold products.  Use a tungsten solder pick when placing platinum solders on hot platinum during brazing.  Tungsten tweezers are to be used when platinum components need to be adjusted during the brazing or welding process.  Always clean platinum in an ultrasonic cleaner, using cleaning solutions to remove oil and grime prior to brazing or welding.

Be sure to polish the platinum components completely prior to joining it to karat gold.  This will allow the karat gold alone to be polished after brazing.  ZrO paint can be used as a brazing stop-off or parting agent when joining components or when joining platinum to gold.  The ZrO paint acts like yellow ochre and will not contaminate platinum at brazing or welding temperatures.

Pure Platinum Precious Metal Clay
Precious metal clay (PMC), or platinum clay, is a mixture of powdered platinum and organic binders, which was developed by Mitsubishi Materials of Japan.  When one first encounters platinum clay, they will find that it looks and feels like potter's clay.  It is soft and pliable, water-soluble and kneads just like clay.  The clay is not very dense prior to sintering; the density of the un-sintered product may be increased by rolling with a rolling pin or by pressing the PMC into a prepared mold.

There is shrinkage of 20% to 30% in size and 40% to 50% in weight.  It should be noted that the sintered PMC has a porous structure, that density is generally about 70% to 80% of an as-cast product.  However, the density after sintering is acceptable and can be improved by burnishing or compacting the surface.  Shrinkage is one of PMC's advantages, allowing one to produce fine detail to an enlarged product that would have been very labor intensive in the reduced sintered product.  After the product has been properly sintered it can be filed, drilled, sawed and polished like normal cast or fabricated platinum products. 

Platinum Electroforming
Electroforming is a metal fabrication technique used to duplicate an original model precisely by means of electro-deposition provided by an electrolytic process.  Compared with order casing, stamping and mechanical manufacturing, electroforming allows thinner and more precise fabrication.  Platinum's drawbacks have been susceptibility of electroplated films to fracture due to high levels of internal stress and unstable speeds of deposition caused by hydrogen generated during electro-deposition.  To solve these problems, new platinum compounds, additives and devices have been developed exclusively for platinum.

The Platinum Electroforming Process
In the platinum electroforming process the model can be design-cast or molded from a variety of materials.  Typical materials are fusible alloys such as brass or silver.  These models are cast in the selected metal in the normal processes required of lost wax casting.

The models are duplicated in rubber molds to provide a way for mass production casting.  Having cast the production models in the selected metal, the next procedure is to clip the sprues, leaving the vents on the model in preparation for electro-deposition.  Surface finish is crucial at this point in the process because electroplating replicates the exact surface that exists on the model.  Electroforming is not restricted to a high polish and will adhere to a matte, brush or textured surface.

The next step is to mount the models on a plating rack.  Once the product is in the electroplating bath the typical plating rate is one micron per two and a half minutes.  The deposition time is in the range of four to six hours, depending on the desired thickness.  After the products have been electroformed the internal metal core can be melted out with the remaining traces removed with nitric acid.

Heat treating the hollow platinum electroform will ensure that the electro-deposits are ductile and homogenous.  A final polish or finish can be applied at this point to produce the end product.  As stated previously, platinum electroforming can produce highly complicated, three dimensional products that are aesthetically pleasing and relatively light in weight.

Summary
As with all new processes, cost will diminish as methods and materials are made cost effective and become more readily available.  Through further experimentation, especially within the jewelry industry, these processes will increase the gross product yield and produce high-quality platinum jewelry products.

V2N5

Recent Innovations in Platinum Manufacturing
Christopher J. Cart
Platinum Guild International USA

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