Specify the % water content. This is normally based on plastic length, thus to calculate:
( plastic size - dry size ) / plastic size * 100
It is easiest to make bars with a 100mm mark. Then the shrinkage is just 100 minus the dry measurement.
For a comprehensive test to measure shrinkage and porosity, see the SHAB test.
Specify the % shrinkage from wet
The Physics of Clay Bodies
Learn to test your clay bodies and recording the results in an organized way and understanding the purpose of each test and how to relate its results to changes that need to be made in process and recipe.
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
Tests conducted on bodies made from materials, as opposed to the materials themselves.