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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 (given frits we have). 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.
The milk was applied to inside-glazed L210 terracotta ware (fired to cone 06) that I had preheated to 250F. This has not gone on as thick as usual so it appears it might be best to dip the pieces into milk and then pat them with a milk-damp sponge to break all the bubbles, remove the drips and even out the coverage.
Since these are glazed only on the inside I had to be cautious to avoid glaze compression issues. And tell users not to put these in the dishwasher (or if they do tell them how to restore the surface in their own oven).
Why? Glaze fit. Do these yourself and they might end up being glaze compression demonstration pieces. These are available on Aliexpress (as Drip Pottery, Drippy Pottery or Goopy Glazes) and they are made by a manufacturer that has close control of body maturity (and thus strength) and the capability to tune the thermal expansion fit of glaze-on-body. Glaze fit has to be better than normal because of the absence of an outside glaze. Too low an expansion and the compression (outward pressure) will fracture body (especially for thin-walled pieces). Too high and it will craze. And the glaze is thick, it will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They likely deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. And applied the glaze to preheated ware. If done right these pieces are a visual and technical achievement. However hobbyists, for example, often just brush multiple layers of commercial glaze that only by accident fits the body they are using. No wonder their pieces often end up as time bombs or crazed bacteria farms.
Glossary |
Glaze Compression
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. |
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