About the Carbon

A strange thing has started to happen lately. Pieces of copper and bronze have been coming out of the kiln with little holes in them, especially hollow forms. I asked myself what had changed recently in the way I was firing, and the only thing I could think of is that I have been firing without a lid for the first phase of the firing. I took a good look at the carbon. I hadn’t changed it since I started firing without a lid. A large part of the granules had turned into fine powder, and some of it was white and light brown. I realized that most of the carbon had turned into ash, and it was no longer producing enough carbon dioxide to drive the oxygen away from the firing box. Too much oxygen in the box during the second phase (sintering), and inside the hollow forms, resulted in holes, where the particles of the metal could not fuse together.

I changed the carbon and the problem was solved. I still fire the first phase without a lid, though. This is how the carbon looks after the first phase of firing:

After the carbon has cooled down a little I vaccuum this ash.

So I think we can’t use the same carbon indefinitely, especially if we fire without a lid. Vaccuming the ash helps, but changing the carbon once in while is necessary. Old carbon could have been one of the culprits with pieces that did not properly sinter.

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