Building the Smelter
Used base of four cinder blocks, 10" cardboard construction tube as form.
Placed fire bricks on end around tube in two layers (put one fire brick on its
side on two half-bricks as a tap arch). Pulled out tube to put a 1-2" layer
of cob (clay + sand + straw) as a base, replaced tube and put a thick layer
of cob on outside of fire bricks, and a thin layer in tap arch. Did not have
ceramic tuyere, so left a hole to place it later. Let Dry. About a week later,
installed tuyere and sealed with more cob. (Tuyere was 1" diameter cylindrical
kiln support tube from Pottery Supply House)
Furnace was a cylinder, 10" inside diameter, 20" tall (inside), wall
thickness about 4." Inside end of tuyere was near center of cylinder, 7"
from base layer of cob. On outside wall, tuyere was 12" above cinder block
Blower was connected to tuyere by a galvanized pipe T, with a screw cap on one end of T that could be removed to look into tuyere.
Cob was made from local clay, which was dug, allowed to dry, then mixed with water. (More about cob.) After stirring a few times over a couple days, the clay was allowed to settle and the excess water was poured off. Clay was mixed with local sand (screened through 1/4" hardware cloth) and dry straw. Mixing was done by feet on a lumber tarp.
Furnace cracked around tuyere where it was patched in, but we did not have significant gasses escaping here. Including tuyere in initial furnace build would be preferable, but the late addition of tuyere does not seem to have been a problem.
Notes after smelt: Furnace size is about right. Might be better a bit wider and taller, but not be a lot. Tuyere placement seemed good. Thicker cob layer at bottom might be good. Slope tops of walls to inside, so any charcoal that misses the furnace will tend to fall back in rather than out. I have seen other smelters that were tapered (narrower at top). This might help contain heat, and allow charcoal to settle. In our smelter, the charcoal settled pretty well, but sometimes a small cavity would develop and the charcoal would abruptly settle an inch or two.
Next: Operating the smelter
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