Drying Crack
When stresses are present within a drying ceramic item cracks can appear to relieve them. These stresses appear when a ceramic item does not dry evenly. If one part dries ahead of another it also shrinks ahead of the other. Thus when the latter part needs to shrink and dry a crack can appear to relieve the stress. Small drying cracks will normally grow during firing, especially if significant firing shrinkage occurs. Different types of clays (e.g. kaolin, ball clay, bentonite) have different characteristics (e.g. shrinkage rates, shrinkage curves, drying speed, ability to withstand the stress, dry strength, ability to terminate micro-cracks at pores and particles, etc). A common type of crack is called the 'S-crack', it is a signal that ware is being dried unevenly, contours are too angular or thickness too uneven, stress areas have surface imperfections that can foster cracking that might not otherwise occur, or the type of clay being used is too plastic. Fast even drying will get better results than slow uneven drying. Out Bound Links
Pictures
An example of an S-crack in the bottom of a porcelain mug.
DFAC dried disk showing soluble salts and drying performance crack typical of a plastic pottery clay body.
Example of how simple placement of mugs during drying can prevent cracking of handles
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