|Monthly Tech-Tip |
The brick-halves on the left cracked in two during drying, the crack opened at the center. I dried six of them and all cracked in the same way. The one-inch-slices were cut laterally from an extruded slug of clay and sun-dried. The radial pattern of the laminations are clearly visible on the break. These laminations are "a weakness" formed-into this extruded and unwedged clay, they would, of course, extend to fired integrity, weakening the piece. The halves on the right are from a brick that I made by first wedging (kneading) the clay, then forming and cutting it to size. It was likewise sun-dried. But did not crack. I broke it (with difficulty), notice the break followed the stresses of the breaking process, not internal lines of weakness.
The process of mixing a plastic clay by hand before forming it. Similar to kneading of bread dough, it is considered an essential step by most potters.
Laminations because of improper pugging of a clay body will cause separations and drying cracks in the ware.
The practice of removing air from clay as it is pugged. Deaired clay has better forming properties and produces a smoother fired surface.