Learning to fire raku well is an exercise in patience, control, timing, and, observation. The results depend not only on the course of the firing in the kiln, but especially on the timing of the smoking as the piece cools. Some of the important factors are the temperature of the pot as it comes out of the kiln, how long it takes to get the piece in to smoke, the weight of the piece, the size of the container and the combustible material used for smoking. There are many special effects glazes each of which can vary quite a bit depending on the firing. The three main types of raku glazes commonly used are crackle glazes, lusters, and copper matts.
Lustres need a very heavy reduction while the glaze is still hot, so it's best to move them as quickly as possible from the kiln to the smoking container. A small container holds less oxygen, so using the smallest container possible will help to obtain a heavy reduction. Because the piece comes in contact with the combustibles while the glaze is still soft, lustres usually show some scarring in the glaze surface.
For best results with crackle glazes, after pulling the piece out of the kiln, cool the pot for a minute before putting it in to smoke. A crackle pot crackles because the glaze (the outer layer) cools before the clay itself. The glaze shrinks before the clay gets a chance to, so it cracks. Blowing on the piece before smoking it will help to promote a good crackle. Red or brown bodies will fire to a pink color with a clear crackle glaze. If you want a white pot, use a white clay body. Opaque white crackle glazes can be made, but they don't usually crackle as well.
Copper matts are the most difficult raku glazes to fire consistently. To get the rainbow colors characteristic of copper matts, the glaze must first be reduced to metallic copper and then re-oxidized slightly. The final effect is similar to the halo created by heating a piece of copper with a torch. There are many different schemes for firing copper matts. The most common is to pull the piece from the kiln hot, smoke it as usual to reduce the copper, then let it re-oxidize while it cools in the container. When removed from the container, the piece is usually quenched in water to stop the re-oxidation.
Raku Safety
There is clearly no more dangerous activity available in our studio than raku firing. The major dangers include burns, fire hazards, and smoke. It's imperative to be prepared and alert. Everyone involved in firing raku should be properly clothed. This includes long sleeves and pants, shoes, insulating gloves, and a hat.
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Guru Spotlight |
Patricia Walters-Fischer |