|Monthly Tech-Tip |
Here is an example of how a cast porcelain piece, L3778G, can warp during the glaze firing (the one on the left is just bisque fired, the one on the right is fired to enough maturity to achieve translucency). Several factors contribute to this failure:
1. It is cast and the walls are very thin.
2. This porcelain is highly vitreous.
3. This shape has no inherent strength to resist rim warping.
Thus the following steps will help to reduce the issue: Switching to machine forming (which orients particles concentrically), reducing the feldspar in the recipe (to reduce fired maturity) or firing lower, casting thicker walls and changing to a more flared shape. There is one other option (borrowed from bone china): Fire the piece upside down on a custom alumina setter fitted to the final rim diameter - then clear glaze it at a lower temperature.
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.
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to impart a practical level of strength and durability for the intended purpose.