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.