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.


5 Responses to “About the Carbon”

  • Sandy Says:

    I ordered some powdered copper clay and it was quite easy to work with. I made some great things. I had also made some bronze clay pieces so I decided without putting much reasearch into it, that I would fire them together in the same container covered in carbon. I thought I would remember the time, the temp and the two firing schedules so I didn’t write it down or print it. I accidentally set the kiln to high and I forgot to leave the lid off for the first firing. Needless to say, it was a disaster. Everything either burned up or broke. It was completely careless of me but it was a valuable lesson. My PMC silver has always fired perfectly so I got a little too confident. I’ve now printed out your instructions and will follow them carefully. I’m going to take another shot at the copper this weekend. One question, aside from having to replace the carbon that turns to ash, how many times can you typically fire the bronze clay in the carbon before you have to change it? Thanks for everything!

  • Hadar Jacobson Says:

    After vacuuming the ash over and over eventually you are going to run out of carbon. Even if there is no ash, I like to refresh my carbon once in a while by adding 2 cups of new carbon to the old one.

  • Tana Says:

    I have just begun making hollow forms. I made a small white bronze hollow bead and it slumped a bit in firing. I have a larger one ready for the kiln and I really like it and I want it to turn out! 🙂 is there a special way it should be place in the carbon to avoid slumping?

  • hadar jacobson Says:

    Tana, Position it with the narrow side facing up (on its side). Good luck!

  • Tana Says:

    Thank you!

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