when you sleep with clay ingot mold raw in fire it will turn into ceramic ingot mold raw
clay turning to ceramic when burned? never heard of something like this...
joke aside: it's more or less normal behavior as that is more or less exactly how you make ceramic stuff: you burn clay in fire over hours (and sleeping is a 6 hours fast forward), though if you don't do it right normally it shatters when you take it out.