Crazing is a network of fine cracks in a fired glaze, caused when the glaze contracts more than the clay underneath as it cools. It can appear straight out of the kiln or develop weeks or months later.
What Is Crazing?
Crazing is a glaze fit problem. Glazes and clays each expand and contract by different amounts with temperature, and if the glaze shrinks more than the clay on cooling it ends up stretched, so it cracks to relieve the tension. Digitalfire notes that slow cooling or refiring only delays crazing; the lasting fix is a glaze that suits the clay.
Delayed crazing often happens on porous or underfired bodies, which slowly absorb moisture and expand. Thermal shock in use, such as boiling water poured into a cold mug, can reveal a poor fit that looked fine at first.
On decorative work, crazing can be a deliberate effect, called crackle, sometimes rubbed with ink to show the lines. On tableware it is a fault: the cracks can harbour bacteria, let water reach the clay and weaken the piece, so glaze makers advise against crazed glazes on food surfaces. A quick check is to fill a glazed piece with boiling water, then plunge it into ice water, and look for new cracks.
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.