Shrinking From Wet to Bone Dry
When a clay is wet and very pliable, it contains a great deal of water. The clay particles ride within the water, which is what makes clay plastic, or easily workable. As the clay drys the water evaporates, escaping from those spaces in between particles. The particles move closer together, resulting in the entire pot shrinking.
How much the clay shrinks depends on the characteristics of the clay. Highly plastic wet clays have a very fine particle size and will shrink more. On the other hand, clays with large particles will shrink less. Also, clay bodies that include non-plastic additives, such as grog or sand, will shrink less.
Shrinkage due to drying is generally between 4% and 10%.
Shrinkage Due to Vitrification
When clay is fired at a high enough temperature, it begins to gradually vitrify. This process of melting and fusing also compacts the clay body. The clay shrinks as the particle sizes slowly decrease as they fuse. In addition, the particles also compress into a tighter, more dense configuration within the glassy material that fills up all the nooks and crannies.
The amount of shrinkage due to vitrification is very dependent on which type of clay is involved. Refractory clay bodies may have a very low degree of shrinkage at this stage, while highly vitreous clay bodies such as a high-fire porcelain may shrink up to 8%.
