|Monthly Tech-Tip |
This DFAC test for drying performance compares a typical white stoneware body (left) and the same body with 10% added 50-80 mesh molochite grog. The character of the crack changes somewhat, but otherwise, there is no improvement. While the grog addition reduces drying shrinkage here by 0.5-0.75% it also cuts dry strength (as a result, the crack is jagged, not a clean line). The grog vents water to the surface better, notice the soluble salts do not concentrate as much. Notice another issue: The jagged edges of the disk, it is more difficult to cut a clean line in the plastic clay.
Using a grogged body for making functional pottery is misguided. Unless very large pieces are being made it makes little sense to add the inconveniences of having a gritty material in your clay. Any normal smooth commercial pottery clay will dry without cracking if ware is dried evenly.
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.
Clay Cracking During Drying
The best way to avoid drying cracks when making ceramics or pottery is to avoid doing the things that cause it. Do not just blame the clay, anything can technically be dried.
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.
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.