Techniques on how to tests for clay bodies

Before a new clay or clay body is used, a few simple tests should be made to determine its various characteristics.

PLASTICITY
Plasticity is essential to any clay body. A standard simple test for is to loop a pencil size roll of clay around your finger. If the coil cracks excessively, the clay is probably not very plastic. The ultimate test, however is to work with the clay to determine if it throws without sagging and joins without cracking.
Wedging, a process by which the clay is kneaded by hand to remove air pockets, has considerable effect in increasing plasticity.
Water of plasticity refers to the amount of water need to bring a dry powdered clay into a plastic state. The finer the particle size , the more plastic a clay will be and more of water it will absorb.

POROSITY
The porosity of a fired clay body is directly related to the hardness and vitrification of the clay. To make a porosity test, weigh an unglazed fired clay sample. After an overnight soak in water, wipe the clean of surface water, and weigh it a seconds time. The percentage gain in weight will be the porosity of the clay body. A higher firing will reduce porosity.

SHRINKAGE
Shrinkage of the clay body occurs first as the clay form dries in the air and then again as the form is bisque and glazed fired. The more plastic clay will always shrink the most.
Clay shrinkage rates generally range from 5 to 12 percent in the drying stage, with an additional 8 to 12 percent shrinkage during firings.