|Monthly Tech-Tip |
The goblet on the left is bending, not just because the clay is somewhat unstable at the temperature being fired, but because this shape is also inherently unstable. Where extreme shapes are prone to warping, ware must be made from clays that do not vitrify (that introduces issues of strength and functionality). In this case, the clay recipe is based on a terra cotta material that matures at a very low temperature. The problem was dealt with by employing a recipe of 60:40 clay:200# kyanite.
Kyanite is a granular material used in the manufacture of ceramics and abrasives. It is notable for low thermal expansion and one-way expansion on heating.
Warping happens during the firing of ceramic ware when there is a high degree of vitrification and inadequate measures are taken during forming and firing to prevent it. Unexpected warping often happens with unstable shapes and over firing.