|Monthly Tech-Tip |
Failure is often not something specific. The greater the wall thickness variation the more difficult it is to dry and fire it evenly. The stronger the rim the better it will withstand stresses. The finer particled and more porcelaneous the body the more difficult drying and firing will be. After throwing, the longer a piece is left rim up in air the greater the moisture content difference will be between it and the base. The earlier a bowl can be trimmed the better the success. The less a soft or leather hard bowl is handled such that it goes out-of-shape, the better. The slower and more even the drying the better the success. The better the air flow and slower the heat-up through water-smoking in the kiln, the better. The more that heat can reach all sides of the piece the more evenly it will heat up during firing. The less the point of contact between the base of the bowl and the kiln shelf, the less the shelf will heat-sink it and increase temperature gradients within it.
The difficulty of crying large plates increases dramatically by size and thickness. The better the mixing and wedging, the smaller the piece, the thinner the walls, the more even the wall thickness, the less porcelaneous the clay, the lower the drying shrinkage, the higher the dry strength and the more even the drying the fewer cracks there will be. The poorer the mixing and wedging, the larger the piece, the thicker the walls, the more uneven the cross-section, the more porcelaneous and smooth the clay, the higher its drying shrinkage, the lower its dry strength and the more uneven the drying the more cracks there will be. Do not dismiss the possibility that it could take a month or more to dry this type of piece.
During drying clays and porcelains shrink as they become rigid. When this occurs unevenly, cracks are the result.