- It keeps pots from sticking to the shelves during firings. This isn't usually an issue with low fire pots, but with mid range and high fire pots our clay bodies get close enough to their melting points that the clay becomes just a bit tacky at peak temperature and will stick to the shelves a little bit. When you take them out of the kiln, little pieces of the pots will get plucked off where they have stuck to the shelf.
- It provides a protective layer in case a glaze runs off a pot and onto the shelf during a firing. While it won't always keep the glaze from sticking to the shelf, it does keep it from soaking in as much, and makes it easier to chisel the glaze off.
Two things are needed to make a good kiln wash: refractory material and some clay. Most people use alumina hydrate (or oxide) or flint (silica) or both for the refractory material. I like to use both. I find that alumina works better, but it is much more expensive that flint. The combination is a nice mix of functionality and affordability. The clay is necessary to hold the alumina and silica together and make it easier to brush onto the shelves. For the clay portion of the mix I use EPK (Edgar Plastic Kaolin), just like everybody else, although any kaolin will work. Kaolin is fairly refractory for a clay, and it has no iron impurities that could affect the performance of the wash in reduction firings.
My wash recipe started out as equal parts alumina hydrate, flint and EPK, but I found that it was flaking off after a couple of firings. This is a major problem because those flakes could fall off the shelf and land on a pot. The reason it was flaking was that is was shrinking too much. As it shrunk it pulled loose from the shelf. Specifically, the kaolin was shrinking too much.
So how do we get the kaolin to shrink less? Calcine it! Calcining is simply firing the raw material before using it in the mix, therefore pre-shrinking it. I just put a bunch of EPK in a bowl and run it through a bisque firing to accomplish this. You can buy calcined kaolin, but it costs a lot more.
I mixed up a test batch using calcined kaolin, and it just wouldn't apply to the shelves right. Once the kaolin is calcined, it doesn't behave like regular kaolin. So I tried half and half, which worked great. It applies nicely, and lasts many firings. My final recipe:
Alumina Hydrate 33%
Calcined Kaolin 17%
Add water and mix up to a pancake batter consistency. Apply 3 coats to kiln shelf.