More good info;
Instructions from the Alabama Belle with a death wish, said to fire the shell-tempered pottery to at least a cherry-red temperature and hold it there for two hours. What temperature is cherry red? All pottery turns “red hot” when being fired. I guessed she meant “earthenware temperature” and placed the appropriate temperature cones in my kiln.
The Maya tripod pot fired beautifully. There was not a flaw. The pot pinged like the finest porcelain. It glistened like a Corvette with candy-apple enamel on it. As soon as the pot cooled enough to pull out of the kiln, I photographed it, and sent an email to the tribe’s Mikko (Principal Chief) to brag about the gift that was coming his way.
The next afternoon, I happened to glance at the pot. Hairline cracks were forming that gave it the appearance of raku pottery. I didn’t understand how cracks could form 24 hours after coming out of the kiln, but the pattern made the pot look even more exotic.
While I was cooking breakfast the next morning the pot’s handles fell off. By noon the handles were two piles of ceramic pebbles. Throughout the day, I could hear pop, pop, pop, as more tiny shards exploded from the vessel. By noon of the third day after firing, the beautiful two gallon Maya tripod cooking pot was a pile of polychrome pebbles. It was an artistic disaster.
Further research in a book on Britannic-Roman shell-tempered pottery revealed the source of the disaster. Shell-tempered pottery can only be fired to the point when it just begins to glow red. If it is fired the translucent, orange-red glow of stoneware and porcelain ceramics, the calcite changes into an unstable chemical similar to Portland cement. After it cools, the unstable calcium compound will absorb moisture from the air and revert to hydrated lime. At this point, nothing bonds the clay particles together, so they disintegrate.
There were enough crushed, hydrated freshwater mussel shells to make a big batch of pottery, if I mixed them with limestone sand. The chemical reaction was the same for limestone as shells. The new pieces of pottery were fired to just the right temperature. They came out of the kiln perfectly and did not disintegrate afterward. However, making shell-tempered pottery was not worth the effort and risk.
