What is the meaning of the term "glaze fit" ?

Unit 7 Topic 15
Using Unity Formula

A glaze is said to "fit" a particular clay body when that glaze does not craze (crack) after firing.
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Crazing occurs when a glaze contracts, in the cooling cycle, more than the clay body after the form has been fired.

If, on the other hand, the clay body contracts more than the glaze then the glaze will not craze. In this case the glaze is said to be "in compression" and it will have greater strength than if the glaze has crazed.

Crazing sometimes takes years to emerge particularly in the case of low or mid range firing where the clay body can absorb moisture over time thus swelling minutely but sufficiently to place stress on the glaze.

Usually, however, a glaze prone to crazing will begin to crack quite soon after the firing. Crazing can sometimes be heard occurring as a "pinging" sound in the cooling kiln or soon after the form has been taken from the kiln.

Rapid cooling of a form after it has been fired may encourage crazing as the glaze will cool more rapidly than the clay body even though eventually the glaze may be in compression.
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Here is a glaze which has a fine pattern of crazing which suggests that the glaze is a long way away from fitting the body on which it was fired.

This glaze displays a much coarser pattern of crazing (stained with ink). This suggests that the glaze is much closer to fitting on the body. With some adjustments the crazing may be eradicated.
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