|Monthly Tech-Tip |
These are made from a 50:50 mix of bentonite and ball clay! The drying shrinkage is 14%, more than double that of normal pottery clay. These should have cracked into many small pieces. Yet notice that the handle joins with the walls are flawless, not even a hairline crack (admittedly the base has cracked a little). Remember that the better the mixing and wedging, the smaller the piece, the thinner the walls, the more even wall thickness, the better the joins, the fewer the sharp contour changes, the more even the water content is throughout the piece (during the entire drying cycle) and the damper the climate the more successful drying will be. What did it take to dry these in our arid climate? One month under cloth and plastic, changing the cloth every couple of days. Implementing these same principles on a normal clay body will assure drying success.
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.
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.
Bentonite can make a clay body instantly plastic, only 2-3% can have a big effect. It also suspends slurries so they don't settle out and slows down drying.
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.
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.