|Monthly Tech-Tip |
Pure kaolins are clay. It seems logical that "pure clay" is plastic. However most kaolins are not plastic (compared to a typical clay for throwing or modelling). This is because they have a comparatively large particle size (compared to ball clays, bentonites, etc). This small bowl was thrown from #6 Tile kaolin. It is, by far, the most plastic kaolin available in North America. It's throwing properties are so good that one might be misled into thinking it should be possible to make pottery from it. Unfortunately, if it was survive drying without cracks, it would not make it through firing without this happening. This was fired, unglazed, to cone 6. Pure kaolin particles are flat and the throwing process lines them up concentric to centre. So shrinkage is greater across than along them. A filler is needed to separate the kaolin particles. All pure kaolins are also refractory, so even if this bowl had not cracked, the porosity of this piece is very high, completely impractical for functional ware (it needs a flux like feldspar to develop fired maturity).
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.