Every few months someone sends a photograph of a mug with a fine web of lines across the glaze, worried it is about to fall apart. It is not. The lines are called crazing and they are extremely common on handmade stoneware.
What is happening
Glaze and clay are two different materials fired together. As they cool, both contract — and they almost never contract by exactly the same amount.
If the glaze contracts more than the body underneath it, it ends up stretched slightly too tight across the surface and relieves that tension by crazing. The lines are in the glaze layer only. The pot underneath is untouched.
Does it matter?
For our work, no. Because we fire at a very high temperature the clay body is sealed solid, meaning it is effectively non-porous on its own. Liquid cannot travel through a craze line into the body, because the body will not take it.
On low-fired earthenware the answer would be different — there the body is porous and crazing does become a hygiene question. That is not what these pieces are.
Will it get worse?
It will develop slowly over years, usually accelerated by the dishwasher, and then stop. Tea and coffee stain the lines faintly darker over time, which is why crazing becomes more visible on a well-used mug.
Some potters spend years eliminating it. We adjusted our glaze to reduce it, then stopped chasing it. On a matte oatmeal surface a fine craze reads as age rather than damage, and most people who own these pieces for a decade end up preferring them that way.
When to actually worry
Crazing runs in a fine, even, shallow web. If you see a single line that runs through the rim and continues down the wall, feel it catch under a fingernail, or hear a dull thud instead of a ring when you tap the piece — that is a crack in the body, not crazing. Send us a photograph and we will replace it.