|Monthly Tech-Tip |
Drying disks used for the DFAC test are 12cm in diameter and 5mm thick (wet). A crack pattern develops in almost all common pottery clays as they shrink during drying. This happens because the center portion is covered and stays soft while the perimeter dries hard. This sets up a tug-of-war with the later-drying inner section pulling at the outer rigid perimeter and forcing a crack (starting from the center). If the clay has high plasticity and dry strength it can pull so hard from the center that cracks appear at the outer dried edge to relieve the tension. Or, it can create cracks that run parallel to the outer edge but at the boundary between the inner and outer sections. The nature, number and width of the cracks are interpreted to produce a drying factor that can be recorded.
Drying Ceramics Without Cracks
Anything ceramic ware can be dried if it is done slowly and evenly enough. To dry faster optimize the body recipe, ware cross section, drying process and develop a good test to rate drying performance.
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.
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.
In ceramics, drying performance is very important to optimizing production. More plastic clays shrink more and crack more, but they are also better to work with.