Troubleshooting Copper

This posting is in response to a question that I have been asked by many people. There seems to be some misunderstanding about the firing schedule of copper, and I would like to clarify.

The problem that people are having is that although they get fully sintered pieces of bronze, they are often able to break the copper pieces. There is no powder at the cross section, but the pieces are not very strong.

Here is my answer: copper, in order to be as strong as the bronze, needs to be fired at higher temperatures. If you look at my instruction manual for Quick-fire Copper and Bronze, you will see that if you fire copper alone you can go as high as 1800°F/980°C. In fact, I fire copper and steel clay in the same batch.

Therefore, there is no point in testing the strength of copper if it was fired at the sintering temperature of bronze. The binder may be gone, but the metal has not reached its highest density and the copper has not shrunk all the way. It’s exactly the same as when we torch-fire silver clay: you have probably noticed that if you re-fire it for repair, it shrinks more. That’s because it hadn’t reached its highest density the first time around.

So what do we do? When I have just copper to fire, I fire it by itself at 1800°F/980°C. But what if I have mixed pieces?

Luckily for us, mixed pieces are stronger even when they are fired at the sintering temperature of bronze. Even though the copper may be under-fired, the bronze gives the piece its strength.

Under-fired is not necessarily a bad thing. When we add silver to fired copper and torch-fire it, it’s because we want it under-fired. We don’t want it to shrink too much and crack. And the fact that it is under-fired is not important, since it is not a structural part of the piece.

If you want to make mixed pieces even stronger, make sure that the structural part of the piece is made out of bronze. For example, inlay copper in bronze rather than bronze in copper.

I must say, though, that I don’t usually find it necessary to worry about this. Most of the time mixed pieces, if fired properly, are strong enough for normal wear and tear. This is especially true for hollow pieces, since hollow pieces in general are stronger than flat ones.


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