Raku is derived from an ancient Japanese method of firing ceramics. A technique that creates unique patterns on the glaze. The unique pattern of colours and metallic finish comes from the interplay of flames, smoke and rapid cooling. Raku is strictly decorative and may not be used as food ware. No two pieces are ever alike. Each raku potter uses the same type of firing techniques, but individual styles are vastly different and each piece is certainly one of a kind.
The Raku firing technique differs from almost all other pottery techniques. Unglazed bisqued pottery is glazed and put into a raku kiln. The kiln is custom built in most cases as it will be opened at peak heat of the firing. Unlike traditional ceramic firings in which the pieces are loaded in a cold kiln and slowly fired until the desired temperature is reached. Then the kiln is shut off and cools down until the pieces can be removed.
Most kilns are built or jerry rigged to work. By that I mean custom built to open at the high temperature needed to accomplish this method of firing pottery. The kiln is loaded and fired between 960 and 1020 C depending on the glaze, technique and application. This is the temperature point at which the glaze reaches a mature state. Experienced Raku potters are able to visually know by glaze melt that the firing is achieved. Now things get exciting.
When the firing is completed, the pieces are immediately removed from the hot kiln, with the glaze still molten. The pieces are then placed in a fire proof container. The container can be steel or concrete and should be as air tight as possible. The container is lined with your combustible materials sawdust, paper, straw, leaves, pine needles, hay, peat moss and or newspaper. When the kiln is opened an instant draw from the kiln, the thermal shock will provoke cracks which will be revealed with the smoke produced by the burning sawdust etc. Carbon is driven into the crackle patterns of the glazes created during cooling
in the can.
The can containing the hot pottery is quickly covered allowing reduction to begin. Leaving your piece in this state for up to 20 minutes will produce very different effects throughout the process. It takes some experimentation to achieve what you are hoping. Each piece is unique to the next. When the piece is removed again it can be left in the air to cool slowly which will continue to apply effects, or placed in a water bath to arrest the effects and cool.
When the pottery is cool enough to handle it is washed. If there is tough carbon you can rub softly with a very watered down grog or fine wet dry sandpaper.
Current experiences with raku have only led to more questions. How much reduction is good? What combustibles do I need to use? How much material at a time? The potters answer to all of these questions. Whatever you need to accomplish the finish you are looking for. So I continue to experiment.