Ceramicware Defects Handbook/Crack

Cracks

Defects description

Cracks are usually caused by stress/strain in the clay material either due to the design of ware itself OR types of clay used. Cracks in clay are considered to be naturally occurring and its causes are often too broad and overtly complex in its nature. It is highly possible that the ceramic ware may be strained during the processing but are able to withhold the strain until changes in clay body causing the drastic failure. When searching for remedies, it should be understood that the objective is not to totally eliminate strain-free pots (due to the impossibility of this tasks as each ware have different shapes and sizes) but rather minimizes the magnitude of the problems and minimizes the effect due to natural variation in recipe ingredients. Solutions:

Try to minimises the differences of section thickness to prevent uneven drying which causes the strain as thinner sections shrink earlier and faster to compared to thicker sections. The strain will be stored permanently once it is dried
If the cracks are appearing at same places, it is advised to use paraffin. The paraffin will make the "invisible" crack line appearing for few seconds and the crack lines can be pegging to close the crack.
Cut, trimmed and fettled areas should be wiped with sponges to close up the minute cracks as it will act as stress manipulators causing the cracks to deepen and open out during firing.

There are a few types of cracks namely:

Edge cracks
Surface cracks
Handle cracks
Biscuit cracks

Edge cracks

Causes:

Too fast drying
If appeared even before drying occurs, the raw materials have excess addition of filler, grog or sand or reclaimed clays.
Uneven cross-section thickness
Readsorption due to dried clays exposed to high humid temperature. If clays readsorp the water vapour, the slight expansion will take place. Usually, ceramic ware with low clay contents may not enough strength to withstand the moisture expansion causing fine cracks that are visible after biscuit firing.
Clay is too dry when cut or fettled upon. The clay is much weaker when too dry and can have minute cracks in trimmed surfaces. Smoothing the surface by wet sponging will remove the problems
AFter glost firing, the kiln is immediately open of its door without proper cooling down to more ambient temperature. This is called thermoshock as the just fired ceramicware is exposed to relatively cooler air. The kiln door should be opened between 80 C - 150 C to minimizes the crack.

Surface cracks

If excess water formed at the base of the ceramic wares, it will form S crack on the bottom of the wares. Use the suitable sponge to remove excess water after jiggering may help
If the straight crack appears at the base of the jiggering ware, perhaps it is caused by uneven drying

Biscuit cracks

Rapid firing /Uneven firing will cause uneven shrinkage. If the top of ceramic wares reaches high temperature quicker than the bottom of the wares, it will cause the cracking due to uneven firing.
Cracks can be repaired by using stopping without the need of re-firing. Stopping is a filler made from pre-fired grounded materials mixed with special gums and glazes.

Handle cracks

Descriptions:
Cracks occurring at joint may be due to imperfect joining or using too much/little slips. It also occurs when the handle "spring" aways from its main body due to internal stress during drying and/or firing. It is usually caused by different "clay hardness" between handles and the main body. Therefore, during drying/firing, one part may shrink faster compared to others, giving it increased stress and eventually cracking.

Solutions:

It is also generally found better to mix a bit of Epsom salt (magnesium sulfate) into the slips as Epsom salts will increase the plasticity of slips. It is also a flocculant, meaning the clay particles will attract each other.
Ideally, the handle clay and main body should be of same type of clay and ideally in same conditions to reduce the risks of differences shrinkage causing cracking
If handle section is too wet, it may cause the handle to become weak causing cracks