An important property of the fired glaze is its degree of expansion when subjected to heat.
- If too much expansion is developed then crazing of the glaze on the body may result.
- If insufficient expansion is developed then peeling may occur.
The image below shows what can happen when the glaze does not expand sufficiently to fit the clay body.
In this case, the glaze peeled from the body during firing and had such low expansion that it actually cracked the piece. This can be a common outcome with cylinder shaped vessels when a glaze is in a high state of compression.
The term 'glaze fit' is often used to refer to the difference between the expansion of the glaze and that of the body on which it is applied. A good glaze fit is one in which there is no risk of crazing or peeling.
When a glaze suits the body on to which it is applied, the glazed article will be stronger than the same article unglazed. For this to happen the expansion of the glaze needs to be similar or slightly lower than the body to which it is applied.