The Basics of Sintering Carbon-Fired Clay

With the arrival of metal clay that needs to be fired in carbon came a whole set of challenges—determining the proper firing schedule and producing sintered pieces being the biggest. Quirky shrinkage can be another.

It’s helpful to have a basic understanding of what is going on in the sintering process.

For this article I talked with Bill Struve, inventor of BRONZClay and COPPRClay, and owner of Metal Adventures, manufacturer of those brands. Bill shed some light on this complex topic. I’m going to summarize what he said.

Sintering happens when the small metal particles in the clay get hot enough for the metal atoms of touching particles to begin to diffuse into each other. At first, there are a lot of connected open spaces between these particles. This is why, in the early stages of sintering, the piece is weaker. As sintering progesses, the particles coalesce more and the spaces get smaller. The metal gets less porous and stronger, until the piece is fully sintered. So, the sintered state ranges from a beginning stage to complete. Depending upon what you are making, and/or what you are co-firing, you may need or want to accept the sintering at an earlier stage or take it all the way to fully sintered.

Sintered metal powder.

Sintered metal powder. (courtesy of European Space Agency)

The sintering process is also the cause of shrinkage. Shrinkage for carbon-fired clays can be a bit mysterious. A piece can shrink at different rates in different directions. The forces on the clay that cause shrinkage during firing are not very strong. Because of this, if there is any resistance on the clay, it will inhibit shrinkage in that direction. But, as we know, due to the sintering process the clay must shrink. What will happen is there will be more shrinkage in the directions with lesser resistance.

Carbon can hold back the shrinkage of firing clay, providing the resistance we are talking about. The lesson here is to pay close attention to the position of pieces in the firing chamber, as it has an effect on the shrinkage. Also, carbon resisting the shrinkage can distort the piece. This is why we cover openwork pieces to keep the carbon out of the openings.

Thin pieces sinter practically all at once. But for thick pieces it takes time for the heat to penetrate. The outside will be sintering at a faster rate than the inside. It is even possible to have a piece that has a sintered shell on the outside while the inside is remains a powder. This occurs when the ramp is faster than the heat can penetrate the thickness. That is why it is wise to slow the ramp when firing thick pieces. It gives the heat time to gradually soak into the piece.

Knowing this little bit about the science of what’s going on when you fire these types of clays can help troubleshoot some of the problems we encounter.

Next time I’ll write about test strips, a good tool for troubleshooting.


About Evelyn

I'm a metal clay artist who is learning something new every day. My goal in my work is to make things that are well crafted and distinctively from my own hands, like the work of a good tailor. My jewelry can be found at

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