PLASTICITY is the ability of clay to hold the shape without breaking apart or cracking while being formed. It is the unique and valuable quality of clay.
The keys factors that affect the plasticity of clay:
Shape of Particles - large (flat) surface area are more plastic than smaller ones.
Size of Particle – finer particles are more plastic than large ones.
Attraction between Particles – electrolyte and deflocculates such as soda ash, sodium silicate good for slip casting, but will weaken the attraction between particles that cause difficult for throwing and hand building.
Organic Matter Content - small amounts such as fabric or paper lumps are advantage, but too much will increase the shrinkage and cause rough textures.
Water Content - clay too dry will not from well and crack easily, clay too wet will feel stickily and slumps easily.
Wedging Preparation – kneading the clay not only help to remove air pockets, but also make the moisture/water content of clay more even to work with.