Building the Pottery Kiln
The site I selected for the kiln was one that gets pretty wet in spring and fall, so I made sure the kiln would be up out of the water. I started by leveling the area and spreading a couple inches of drain gravel, well tamped down and leveled. I then placed a layer of flat cinder blocks and filled the cracks with gravel.
The cinder blocks were covered with a layer of cob, the first two wooden forms were placed on the cob, cob was built up around the forms, and lengths of rebar were placed over the forms (the rebar will be the fire bars in the finished kiln).
The next forms were placed on top of the rebar. This will be the fire box. These forms were made from cardboard tubes, with a wood tray underneath and a sheet of wood covering the back. Cob was packed against the forms a bit more than halfway up the tubes. At this point I started using wooden "arches" (one is visible as the back of the righthand picture below). Cob was packed around the arch and then the arch was removed, leaving a gap for the hot air to enter the firing chamber. It might have been best to make enough arches to leave them all in place while I packed the cob, but I didn't want to make that many, so I made six and "leapfrogged" three along each of the cardboard tubes. I also added some wedges in the center so there would not be such a thick section without hot air (not sure if this was really necessary).
Here are some more pictures showing the use of the wooden arches to leave slots in the clay. The surface being formed will be the bottom of the firing chamber.
Now a wooden box (firing chamber) was placed on top of the slots, and cob was layered around the outside of the box.
After giving the cob a few weeks to dry, I started a fire in each cardboard tube and burned out the wood and cardboard forms. Once the fire got going, it burned pretty strongly for a bit, so I was glad I have a piece of sheet metal to place over the top to keep flames and sparks from shooting out too much.
Finished kiln, ready for use!
What would I do differently - The kiln works fine! However, if I was going to build another one, I would do a few things differently. I would make the slots between the firebox and the firing chamber narrower. Also, I put the end slots right against the wall to avoid cold areas, but this makes it more difficult to load the firing chamber. Next time I would leave a small "shelf" along each side of the firing chamber. I would also probably make the firing chamber a little smaller. I made this one about 2' x 2' which doesn't seem all that large, but if you're making smallish items it takes a lot of items to even make a good layer for firing. I think about 18-20 inches on a side might be better, at least for the quanity of items I usually make.
Firing the kiln
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