A glaze and the clay under it are one object after the firing, and they cool together. If the glaze wants to shrink more than the body, it cracks into a craze; if it wants to shrink less, it can flake off. That relationship, called fit, decides whether a pot lasts, and it has nothing to do with colour.
What fit means
Every material has a rate of thermal expansion. Glaze and body cool from over a thousand degrees to room temperature together, and if their rates differ enough the glaze ends up in tension or compression. A glaze in slight compression is ideal: it is stronger, and it seals the pot. A glaze in tension crazes, sometimes immediately and sometimes months later in a kitchen cupboard.
Crazing, and why it matters beyond looks
Craze lines are a network of fine cracks in the glaze surface. Some potters use them deliberately. On functional ware they are a problem: the cracks hold water and staining, and they weaken the pot noticeably. If a mug crazes, the glaze does not fit the body, and the answer is to change one of the two rather than to fire differently.
Shivering, the rarer and worse fault
When a glaze shrinks less than the body it ends up in too much compression and flakes off, usually at rims and edges. Slivers of glaze coming away from an edge is shivering, and it makes ware unusable rather than merely imperfect. It is less common than crazing and needs the same kind of fix: a different glaze, or a different body.
Getting fit right without chemistry
The practical route is to use glazes sold for your body's family and firing range, and to test. Buy the glaze, dip a tile of your own clay, fire it in your own kiln, then boil the tile and plunge it into cold water: a glaze that is going to craze will usually show it after that. It is a crude test and it catches most problems before a batch of work does.
Questions people ask about clay glaze
Why is my glaze crazing?
The glaze is shrinking more than the clay as they cool, so it is in tension. Change the glaze or the body; firing differently rarely fixes it.
Is crazed pottery safe to use?
It holds water and staining in the cracks and is weaker. For functional ware it is a fault to correct rather than a look to accept.
How do I test glaze fit?
Fire a tile of your own body, then boil it and plunge it into cold water. A poor fit usually shows as crazing straight afterwards.