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Every solid has a thermal expansion, that is, an amount by which is expands and contracts on heating. If the thermal expansion of a glaze does not match the body it is on, then the glaze either cracks (when it is under contraction) or chips off when under compression.
The compression occurs while the piece is cooling in the kiln. When the glaze solidifies it acquires its solid characteristics. This might happen at 1500F, for example. As the piece continues to cool in the kiln it contracts. If the body is contracting more than the glaze then the glaze is being put under compression. Some compression is actually desirable and strengthens the glaze-body combination. However too much compression puts the piece under internal stresses seeking an opportunity to relieve themselves. When the difference is severe the piece will not survive cooling in the kiln without fracturing. When individual shards of such pieces are dropped onto a cement floor, for example, they will literally explode into hundreds of tiny pieces. In less severe mismatches, glazes will flake off areas where they wrap around contours (e.g. the lips of mugs), this is known as shivering. This can be serious if it occurs while a piece is being used and someone ingests a micro-flake of glaze (having razor sharp edges).
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By Tony Hansen