Refers to a phenomenon where a plastic clay develops cracks and splits open to relieve the self-support stresses it is subjected to. The process often occurs over a period of minutes. This typically happens on the bellies of thrown vases, rims of overhung bowls, on applied handles. This phenomenon is most pronounced in bodies having a particle size distribution exhibiting large percentages in the plus 100 mesh size range and where straight water is being applied to or left on the surface (the particles provide discontinuities in the surface where the water can enter). For example, if handles are being hand pulled from fresh clay and then applied immediately to the ware, the combination of water left on the surface and the weight of the handle itself will start at split at the point where the surface is being stretched the most. While meticulous avoidance of leaving water on any stressed surface can alleviate the problem on the bellies and necks of vessels, for handles it is generally better to pull handles first, and set them out to harden a little before applying them (using slip, not water).
An example of how water can start a split in a plastic clay. This complete process occurred in about 1 minute.
Sieve Analysis Dry
Sieve Analysis 35-325 Wet
SIEV particle size distribution test to evaluate and compare the particle size profiles of ceramic material powders
Plasticity (in ceramics) is a property exhibited by soft clay. Force exerted effects a change in shape and the clay exhibits no tendency to return to the old shape. Elasticity is the opposite.
|By Tony Hansen
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