Platinum (chemical symbol Pt) is a very heavy precious metal, silver-white in color. It is the best known and most widely used of the six platinum group metals, which include iridium, osmium, palladium, rhodium and ruthenium. Platinum has a high melting point -3223°F/1773°C- and a very high specific gravity of 21.46. One cubic foot of platinum weighs about 21 times as much as a cubic foot of water.
Large deposits of platinum were first recognized in South America during the 16th century Spanish conquest. The Spaniards termed the metal “platino” because of its resemblance to silver (“plata”). They held the metal in low regard, because its high density permitted its use in the counterfeiting of gold.
The earliest workings of platinum were heavy river sands, where the
metal had been concentrated by gravity. Since then, productive deposits have been found in
igneous rock. First workings occurred in Colombia and Brazil. In the early 1800s,
additional platinum group metals were discovered. Although the usage of platinum has
increased considerably over the years, sources of supply remain very few. The two principal producers today are the Soviet Union
and South Africa, with additional yields coming from Canada, Colombia and the U.S.
Platinum is a popular metal for use in the jewelry industry because of its many desirable properties. It is soft, ductile and easily worked. It can be hammered into very thin sheet or drawn to fine wire. It alloys readily with other platinum group metals, and with gold and silver. It is an excellent metal for gem settings, with the ability to hold stones firmly in place. And it is highly resistant to tarnish and corrosion from the atmosphere.
In its pure state platinum is soft, like pure gold. To add hardness, small amounts of iridium are added. The form of platinum generally used in the jewelry industry consists of an alloy of 90% pure platinum and 10% iridium. With its excellent properties of strength, tarnish-resistance and workability, platinum lends itself to the fabrication of delicately designed jewelry items and gem settings.
Platinum is used in many other industries besides jewelry. The platinum group metals are used by the electrical industry for resistors and contacts, by the chemical industry for crucibles and electrodes, and by the glass industry to make dies for fiber glass. A particularly heavy consumer is the automobile industry, where platinum is used in catalytic converters that control automotive emissions.
In certain respects, platinum is a simpler metal to cast than gold. Gold comes in a wide variety of alloys and colors, while platinum has no variation in color and is almost always used in the standard alloy formulation of 90% platinum/ 10% iridium. The iridium does not burn off during the casting process. And clean scrap can be re-used without special preparation.
Platinum casting follows the same sequence of basic steps as gold casting-waxing, investing, burnout, depositing molten metal into the flask, and cleaning the investment. But because of platinum’s unique properties, there are differences in the way these procedures are performed, and these differences are important for successful casting.
Waxes: The same type of waxes used for gold casting are used for platinum. Platinum, however, soli- almost immediately in the casting process, so that the long trees commonly used in gold casting are not practical. With platinum, the caster sprues right off the button, and uses a much smaller flask. To compensate for platinum’s faster solidification, additional sprues are sometimes necessary.
Investment: Because platinum melts at nearly twice the temperature of gold, a different type of investment must be used. The typical plaster-of-paris gold investment would crack under the high temperatures required by platinum. Commercially available platinum investments contain hardening agents to strengthen the mixture and increase its heat retention. Mixing is done more slowly, with powder added in small amounts. And to facilitate drying out, absorbent material is applied to the insides of the flask before the investment is added.
Burnout: Platinum investment burnout requires a special type of oven designed to accommodate the high melting point of platinum. With the investment in place, the oven is brought to temperature slowly, in increments of 200°F. If heated too rapidly, the flask will crack. Burnout temperature is between 1500°F and 1900°F, depending on the article being cast. Heavier articles may be done at cooler temperatures, and more delicate items, requiring flow, at hotter temperatures.
Casting: Platinum must be hand melted with a torch because of the high temperatures involved. (Conventional electrical or high frequency melts used for gold are too large and impractical for these temperatures.) Torches especially made for platinum melting are available from suppliers. They use a mixture of natural gas and oxygen. Casting is performed in a vertically mounted centrifugal casting machine, equipped with a spring-loaded lever that propels the molten metal into the flask before it can solidify. Successful casting depends on high heat, close timing and high-speed acceleration.
Cleaning: Because the investment used for platinum is harder than that used for gold or silver, a different after cast cleaning process is required. After the flask is broken out, the investment adheres to the castings. The invested platinum button must be placed in a strong acid solution for approximately 12 hours. If the buttons are not completely clean after this time, the acid bath should be changed and the buttons immersed again. Careful handling is required, with rubber gloves and apron used to protect the operator.
Handy & Harman manufactures a range of platinum iridium and platinum ruthenium products. Like Handy & Harman’s gold and silver alloys, these products are manufactured under strict quality control procedures. Compositions and properties do not vary from order to order.