A refractory powder that can be mixed with water (and gum solution) and painted on kiln shelves to prevent ware and accidental glaze drips from sticking. Porcelain clays, for example, melt enough during firing that they tend to stick onto the kiln shelf. But with a good wash there are not problems. Certain clays contain soluble salts which fire to a glaze-like sheen, these also tend to stick ware to shelves.
The foot ring on the left is plucking, the right one is not. Why?
These are translucent porcelains, they are vitreous. The firing is to cone 10. The one on the left is a cone 6 body, and, while it survives to cone 10 it does warp. But more important, it is much more vitreous (more melted). The plucking problem makes it quite difficult to get a good foot ring. The other, which has only slight plucking, is also quite vitreous (high in feldspar). The plucking problem on both can be solved by simply using a better kiln wash. What is better? More refractory, and therefore having a powdery, non-stick surface. Spend more money on your kiln wash, base it on calcined alumina or zircon.
A running glaze has stuck to a kiln shelf. Kiln wash saves the day!
This is a zircon-based kiln wash. Even though it paints on in a thin layer, there is no problem releasing the very runny glaze from the shelf.
Flaking kiln wash on a silicon carbide kiln shelf
This is a 50:50 kaolin:silica wash. While economical, this is not an ideal recipe. It gets too hard and shrinks too much during drying and firing. It is better to use a recipe that remains somewhat powdery to the touch. Pure calcined alumina with a little calcined kaolin (about 10%) gives almost zero shrinkage. Add more kaolin if it fires too powdery or less if fires too hard. Add 1% CMC gum to produce a very paintable material (it will take some time to dry).
Kiln wash that really works. How?
The shelf on the right in the traditional kaolin:silica kiln wash. Flaking constantly. Sticking on the feet of ware. A real aggravation. The one on the left: perfectly even. Yet thin. Much more refractory so it has not hardened or become brittle. Or cracked. And it paints on beautifully. The secret? Zircon. Zircopax, to be precise. Zircopax is among the most refractory materials in ceramics. We mixed it with some calcined, rather than raw kaolin. That greatly reduces drying and firing shrinkage and helps densify and stabilize the coverage (by its flat particle shape). Laguna gum solution was added to harden the dry layer and slow down the drying (their gum solution has a higher percentage of CMC than achievable using common mixing methods). Click the link below to get the recipe.
Even with good kiln wash plucking can sometimes occur
These were left by ware made from a cone 6 highly vitreous translucent porcelain. For that type of clay it would be advisable to set ware on a thin layer of silica sand or granular alumina.
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