February 1998


Part I: Alloys and Housekeeping

by Jurgen J. Maerz, Platinum Guild International-USA

Let me paint a picture of platinum, how it is being used, what its features and benefits are, and what a jeweler new to platinum needs to know to convey the message to consumers. Platinum, designation Pt, has a density of 21.45 and a melting point of 3,214°F. Fine gold, meanwhile, has a density of 19.32 and a melting point of 1,948°F. A comparison of these figures makes it clear that different rules apply when working with platinum than when working with gold.

As with most pure metals, platinum must be alloyed to make it suitable for jewelry. For this process, the fact that platinum readily absorbs other metals is a good feature.

Platinum's purity is measured in parts per 1,000. The internationally preferred purity is set at 950 parts per 1,000 of pure platinum, with the remaining 50 being other metals, picked according to the features they lend to the mix. For example:

  • Platinum alloyed with cobalt (Pt/Co) will make a very good casting alloy. It's wet and capable of filling great detail during lost-wax process casting.
  • Platinum alloyed with ruthenium (Pt/Ru) is a good alloy for use in machining and lathe operations.
  • Platinum alloyed with iridium (Pt/Ir) has great features in fabricating and all-purpose work.

Some other specialty platinum alloys are used all over the world, from platinum/gold alloys to platinum copper alloys, to platinum alloyed with tungsten. But most likely in your store you will see the Pt900 alloy, a mix of 900 parts of platinum with 100 parts of iridium or the Pt950 alloy with cobalt or ruthenium added.

Don't let these alloys confuse you. It's not too much different from alloys used in gold manufacturing, where the variety is also great (10k, 12k, 14k and 18k in yellow, white, pink, green and rose, to mention a few).

Because of the higher temperatures used to weld, braze or solder platinum, different rules apply than those you may have used with gold. That other metals readily melt into platinum is great for alloying, but can be a big problem when it happens unintentionally. This is called contamination and happens when metal particles are left on a file, a rolling mill or a draw plate and attach themselves to the platinum surface. When you anneal the platinum, these particles will melt into it and contaminate the metal.

Good housekeeping will prevent this. Dedicate files to be used only with platinum. Clean the rollers of the rolling mill frequently and place the platinum into a warm 10% solution of nitric acid before applying heat to it.

When welding platinum, no flux is used and fire-coats aren't necessary; the piece still comes out clean and shiny because this metal doesn't oxidize. There is no fire scale, so flux is not wanted. In fact, when you stick the solder in place using flux, you may do yourself a disservice. At high temperatures, the flux will enter the grain boundaries of the platinum and it may crack.

Only castings made of platinum/ cobalt mix will oxidize slightly. Here you may use a fire coat after you have done the soldering or brazing. In the 10% solution of nitric acid, the coloring will disappear. Just heat the piece until it is orange, then quench it in the pickle. It is easy to tell platinum/ cobalt alloy from others, as it is slightly magnetic.

Next month: Safety, Repairs & Finishing

Jurgen J. Maerz is manager of technical education for Platinum Guild International-USA. He'll answer your technical questions, day or night, at PGI's platinum hotline (714) 760-8882. He is only the fourth person in the United States to earn the Jewelers of America certification as JA Certified Master Bench Jeweler.

Copyright © 1998 by Bond Communications.


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