|Monthly Tech-Tip |
This clay is used by traditional potters in the Mazatlan, Sinaloa, Mexico area. This DFAC test shows a very wide main crack and many edge cracks. These combine to indicate high shrinkage (a product of very high plasticity). Although the clay has some coarser grains that help channel water out, this is a very poor showing for drying performance, no large-scale manufacturer could tolerate this. That being said the Mexican potters use it with success! How? By adapting their drying procedures. The high dry strength of this material is also a factor to help prevent cracking. Almost any clay can be dried if it is done evenly. And any can be cracked if done unevenly enough. This DFAC test is also a good indicator of the amount of soluble salts present, in this case almost none.
Soluble sulfates in clay produce efflorescence, an unsightly scum that mars the fired surface of structural and functional ceramic products.
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.