Crazing takes the form of fine cracks spread over the glaze. They can be so fine they're often hard to detect, especially on coloured glazes. Crazing causes moisture to enter the clay body making it porous and unhygienic.
The fundamental cause for crazing is the difference in contraction between the glaze and clay body
Causes for crazing;
Underfiring- the most common cause for crazing. The ware needs to be fired above the minimum recommended for the body. Most potters biscuit fire around 1000°C but for buff or earthenware bodies need to be fired to 1100°C in order to mature body and glaze. Refiring the crazed ware can often clear the problem provided the glaze can withstand the higher temperature.
Insufficient heatwork-Over firing or firing too quickly can cause crazing. A firing cycle of 100-120°C per hour is the norm for studio potters.
Low expansion clay body- use of a low expanision body can craze a normal balenced glaze e.g. stoneware/ porcelain/ raku bodies fired at earthenware temperatures will probably craze the glaze. If the body cannot be fired higher consider an addition of flint or cristobalite to the body
High expansion glaze- if the thermal expansion of a glaze is too high crazing is inevitable. The addition of silca (flint or quartz) can reduce this
Glazing too thickly- the thinner the glaze coat the better the craze resistance.
Use of unmatched engobes or slip- crazing may appear over the areas of decoration
Addition of metal oxides to the glaze will increase the thermal expansion to avoid this you may need to fire higher or soak longer
Heat shock-ideally you should wait until the ware load is room temperature before opening the kiln. Certainly, kilns should not be opened while the kiln is over 200°C