Wow, Tuesday already. I think time goes faster when you’re sick and not able to pay attention to the world.
Anyhoo, this week’s search is… “what happens to clay when it is overfired”.
My answer: NOTHING GOOD. WHY DO YOU EVEN HAVE TO KNOW? WHAT HORRIBLE THINGS ARE YOU PLANNING TO INFLICT ON YOUR POOR HELPLESS LOYAL KILN, YOU MONSTER?!?
The more technical answer: it melts. Most clays are a mix of actual clays (such as kaolin, bentonite, or ball clays) and a few other minerals (feldspars, talc, various grogs, colourants like iron oxide, etc.). Commercial clays are very carefully formulated so that they have an ideal firing temperature. The “maturing” or “vitrifying” temperature is the point at which the clay reaches its optimum strength and compactness. Below this temperature, it will be porous and fragile. Above it, the clay can warp, bloat, collapse, or even turn into a sad, difficult-to-clean puddle on your kiln shelf. Unless you have some sort of project about the agony of creation, or the warped mind of the artist, or just plain disappointment, I recommend you avoid overfiring your clay.