PLASTICITY is the ability of clay to hold the shape without breaking apart or cracking while being formed. It is a unique and valuable quality of clay.
The key factors that affect the plasticity of clay:
Shape of Particles - large (flat) surface area is more plastic than smaller ones.
Size of Particle – finer particles is more plastic than large ones.
The attraction between Particles – electrolyte and deflocculate such as soda ash, sodium silicate good for slip casting, but will weaken the attraction between particles that cause difficulty for throwing and hand-building.
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 helps to remove air pockets but also makes the moisture/water content of clay evenly to work with.
Moreover, by adding Organic Matter Content to Clay - small amounts such as nylon fibers or paper lumps are an advantage (for hand building and construction bonding), but too much will increase the shrinkage and cause rough textures.