Glaze fit is how well a glaze matches the clay underneath in how much each expands and contracts with heat. A good fit leaves the glaze under slight compression; a poor fit causes crazing or shivering.
What Is Glaze Fit?
Glazes and clays expand as they heat and contract as they cool, each by its own amount, known as its coefficient of thermal expansion. Ideally the glaze contracts a little less than the clay, so as the piece cools the clay squeezes the glaze gently, which makes the surface stronger.
If the glaze contracts too much, it ends up in tension and cracks into a web: crazing. If it contracts far too little, it is squeezed so hard it flakes off at edges: shivering. Digitalfire describes both as the same problem in opposite directions, fixed by adjusting the glaze chemistry or choosing a glaze that suits the body.
Because fit depends on both materials, a glaze that fits one clay can craze on another. Test new combinations with a thermal shock test, such as boiling water then ice water, before using them on tableware. Glaze makers often list clays their glazes have been tested on, which is a helpful starting point.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.