Crazing

Crazing shows up as a web of thin cracks across a glaze's surface, sometimes visible immediately out of the kiln and sometimes appearing days, weeks, or even years later as a piece goes through ordinary temperature changes in use. It happens because glaze and clay are different materials fused together, and each has its own natural rate of expanding when heated and contracting when cooled; when the glaze contracts more than the clay body underneath it as the kiln cools, the glaze layer is placed under tension until it finally gives way in a pattern of cracks.

Crazing is a real functional issue on top of a cosmetic one, since a crazed glaze on a piece that holds food or liquid can allow moisture to seep into the tiny cracks and into the clay body underneath if that body is not fully vitrified, which can eventually lead to staining, odor, or bacterial growth in the cracks over years of use. Purely decorative crackle glazes are the deliberate exception, where the same physical mechanism is intentionally exaggerated and often accented with ink or stain rubbed into the cracks for effect.

Fixing unwanted crazing usually means adjusting the fit between clay and glaze rather than blaming either material in isolation, options include switching to a glaze formulated for that specific clay body, adjusting the firing schedule since faster cooling tends to worsen crazing, or in some cases changing the clay body to one with a closer expansion match to a glaze the potter wants to keep using. Testing any new glaze and clay combination on a spare piece first, then checking it again after it has cooled fully and sat for a few days, catches most crazing problems before they show up on a finished batch.