The center portion was covered and so it lagged behind during drying, setting up stresses that caused the disk to crack. This test is such that most pottery clays will exhibit a crack. The severity of the crack becomes a way to compare drying performances. Notice the test also shows soluble salts concentrating around the outer perimeter, they migrated there from the center section because it was not exposed to the air.
|Hazards||The Use of Barium in Clay Bodies|
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.
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface.
During drying clays and porcelains shrink as they become rigid. When this occurs unevenly, cracks are the result.