|Monthly Tech-Tip |
These DFAC testers compare the drying performance of Plainsman A2 ball clay at 10 mesh (left) and ball milled (right). This test dries a flat disk that has the center section covered to delay its progress in comparison to the outer section (thus setting up stresses). Finer particle sizes greatly increase shrinkage and this increases the number of cracks and the cracking pattern of this specimen. Notice it has also increased the amount of soluble salts that have concentrated between the two zones, more is dissolving because of the increased particle surface area.
Ball clay is the most common type of secondary clay. They are much more plastic than kaolin because
The DFAC Drying Factor test visually displays a plastic clay's response to very uneven drying. The test is extreme enough that almost all plastic clays will crack.
A method of grinding particles in ceramic powders and slurries. A porcelain, metal or rubber vessel filled with pebbles tumbles and particles are ground between colliding pebbles.
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.