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
- 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
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.