Firing the Kiln
Building the kiln.
First firing with the medieval kiln, Oct. 2015.
Pottery to be fired is packed into the kiln. For this first firing, I only have enough pieces to make a single layer. Pots should be covered with broken shards from previous firings, but for this time I used mostly large pieces of broken floor tiles.
The tricky part of firing is to warm the pots up slowly. If they are heated too fast, residual water in the clay will turn to steam, breaking the pots. Pots can also break from thermal stress, if one part heats up faster than another part. I first tried building a small fire on top of the fire bars, but this did not work very well. The fire bars allowed lots of air in, and as soon as the first died down a bit, the embers fell through the bars, so the fire tended to be either large, or to go out. While trying to get this to work, I heard a couple pots explode, accompanied by fragments falling down into the fire. After some experimentation, I discovered that it was much better to build a fire *under* the fire bars (left picture below). With less airflow, and embers remaining in place, it was much easier to keep a small fire going. I did not hear any more explosions after this. After keeping the small fire going for a couple hours, and once things seemed to have warmed up, I started placing small pieces of wood on top of the fire bars (central picture below) and gradually worked up to larger pieces of wood (right picture below).
The kiln didn't draw too well, I think I may have blocked off too many of the air passages through the firing chamber. Even so, the firing chamber reached red heat.
After keeping the firing chamber at red hot for a couple hours, I let the fire die out and let the kiln cool overnight.
The white items are made of a commecial clay, and most of them came out just fine (a couple cracked due to too-rapid heating). Most of the red clay items were made from clay I dug and processed myself. Some of them came out okay, but many of the red clay items shrank, cracked, and became very brittle. Not sure what I can do about this, it made just be poor clay. Overall a sucessful run and I know some things I can do better next time.
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