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Kiln Wash

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.

When a shelf is properly protected by kiln wash pure molten glaze can run onto it during firing and simply be lifted off when the kiln is unloaded. If the kiln wash works really well, lifting it off will only take upper layers of the refractory wash powder, leaving some still on the shelf. Wash should be reapplied to areas when it has flaked of or is better thin.

It is important for the kiln wash powder to be as refractory as possible. Non-refractory washes harden into flakes that stick to the bottoms of ware (because they have lost their powdery character). These chips can be very difficult to remove. A very high percentage of calcined alumina should be used. Even better, zircon (although more expensive). Since these powders are very fine, they create a good slurry. Gum solution should be employed to slow the drying and impart painting properties. The gum also produces a dense and even laydown that hardens well (a raw clay is not even needed, or wanted). However a calcined kaolin addition is good, not only to dilute the zircon or alumina, but also to condition the laydown (its flat particle shape helps keep the wash in place after firing).

Common recipes contain high percentages of kaolin (50% is common). This percentage is creating a clay body like material (with plenty of shrinkage). Blending raw and calcined kaolin can cut the shrinkage, but kaolin is not refractory enough to produce the powdery character a kiln was should have. A small amount of feldspar can be found in some recipes. This is strange, perhaps it is used to de-powder the fired layer. Sometimes kiln washes are deflocculated. This is also not necessary, it makes a wash that already dries much too fast on brushing dry even faster.

Good quality kiln wash can be used to line crucibles so that molten materials will shed off or can be removed as an ingot after the kiln cools.

The foot ring on the left is plucking, the right one is not. Why?

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!

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

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?

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

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.

Out Bound Links

In Bound Links

  • (Glossary) Water Solubility

    A host of water soluble materials are available to source most of the important oxides needed in ceramic glazes. However such materials cannot normally be used in glazes. Why? Because glazes are suspensions of particulate materials, not solutions of soluble materials. Such suspensions have a far low...

  • (Glossary) Plucking

    Plucking refers to the chipping away of small fragments of the base of a ceramic vessel because the piece sticks to the kiln shelf during firing. The stuck piece either pops off during kiln cooling (due to the difference in thermal expansion with the shelf) or it needs to be broken off. There are se...

By Tony Hansen

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