Plainsman Clays publish dry and fired shrinkage data for their clay bodies. Dry shrinkage is, of course, the shrinkage from wet to dry. Fired shrinkage is not, however, the total from wet to fired. Rather it is the shrinkage from dry to fired. And you cannot just add the dry and fired numbers together to get the total because the fired shrinkage value is based on the dry length, not the original (in this example, 6.25 dry shrinkage plus 6.66 fired equals 12.9 whereas the actual total shrinkage is 12.5). It is not a huge difference but this is the way to calculate it correctly if you only have drying and fired shrinkage. Thanks to Tom Hittie for deriving this for us.
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
Clays used in ceramics shrink when they dry because of particle packing that occurs as inter-particle water evaporates. Excessive or uneven shrinkage causes cracks.