|Monthly Tech-Tip |
It is possible for glazes to be under a condition called "glaze compression". This is L4410P, a low temperature dolomite body, formulated to have the highest thermal expansion possible without the use of talc. This inside transparent glaze is G3879C (we made this as a brushing glaze, four coats are needed on this body). That glaze was formulated to have the lowest expansion possible. As the kiln cooled the body was unable to withstand the increasing compressive forces of the solidifying glaze inside, resulting in what you see here. In the days following the firing, it kept widening, branching and travelling until the mug spontaneously split in two. Another identical mug with Spectrum 700 clear inside (same outside glaze) bounced like a ball when dropped onto concrete from two feet (a subsequent ricochet right onto the handle took off a chip). But when I dropped this one the pent-up stresses within exploded it into dozens of pieces. Neither the body or glaze are at fault, it is their mismatch that causes this. The thicker the glaze the worse it is. If the outside glaze is also "pulling" (because of a higher thermal expansion than the body) the situation is even worse.
In ceramics, glazes are under compression when they have a lower thermal expansion that the body they are on. A little compression is good, alot is bad.