March 1998


Jurgen Maerz answers common platinum bench questions

Q. Casters are using the 950/50 platinum/cobalt alloy. What should I know about that?

A. Platinum in its pure form is very soft and needs to be alloyed. These platinum alloys have different advantages and are used for specific purposes. The flow properties of the 950/50 platinum/cobalt (Pt/Co) mixtures are best suited for casting. With this alloy, very fine pieces of excellent quality can be cast and considerable detail can be achieved.

Platinum/cobalt has an annealed hardness (HV) of 130 and when cold-worked achieves a hardness (HV) of 270. It polishes well and holds its luster for a long time. When welded with a torch, Pt/Co may oxidize slightly. This can be removed by firecoating the object after the initial welding and reheating it to about 1,000°F. Pickle in a 10% solution of nitric acid. Some light oxidation can be rubbed off with a pencil eraser. The oxidation will not reappear during wear.

It's not possible to torch-weld different platinum alloys together as they have different melting points. It's always safe to solder them together using 1500 or 1700 solder. As a point of interest, Pt/Co is slightly magnetic. Also be sure to wear No. 5 or No. 6 welding goggles to protect your eyes.

Q. When is it proper to solder platinum and when should I weld?

A. Soldering is usually required when you're working on a multimetal piece, such as 18k and platinum combination. The metal with the lowest melting point dictates the method used. When installing minor components to platinum, such as findings, soldering is preferred. When you are sizing a ring and need to use solder, use a 1700 seamless solder to do the task. Lower temperature solders tend to show a seam.

Sizing a platinum ring is normally done with the welding method, as long as there are no stones nearby and the ring is made of platinum. The exception here is platinum/cobalt (Pt/Co), as cobalt oxidizes. To size a Pt/Co ring, the 1700 seamless solder is recommended.

Copyright © 1998 by Bond Communications.


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