Types of Firing in Kilns – Sewing Machines Los Angeles

There are a number of different ceramic firing techniques, some of which are quite familiar and others which are rather recondite. In this article we will take a brief survey of some of the kiln firing techniques which are commonly available to studio potters.

Black firing is a primitive technique which is still being used at the present time. In this technique a gas brick kiln is heated to approximately 1000 degrees Celsius, and then large amounts of sugar are added to it. After the requisite temperature is achieved (which usually takes about five hours), the gas is shut off and the flue and any cracks are then sealed. Sugar is inserted in the burner port, and then it volatizes and impregnates the clay body with carbon, which produces an attractive black matte surface. The kiln is allowed to cool slowly and can be opened the following day. It is also possible to use glazes in connection with black firing, which can produce rather interesting effects.

Raku firing originated in Japan in the sixteenth century and is used extensively in bowls used in traditional tea ceremonies. It is a low fire technique in which bisqued ceramics are heated quickly to a red-hot temperature, and then removed from the kiln and reduced with paper, wood shavings, or similar material. Raku-ware is decorated with low fire glazes which often contain lots of frit. The clay used in raku firing, like kiln silver clay, contains a lot of grog so that the work can withstand high temperature fluctuations. Because raku is a battle with the elements of smoke and fire, it is an exciting technique which has become popular world-wide. Salt firing entails the introduction of salt into the kiln towards the end of the firing process, in order to obtain a salt glaze effect. The intense heat in the combustion chamber volatilizes the salt, and the sodium chloride molecule is split into sodium and chlorine ions. The chlorine combines with moisture in the kiln to form hydrochloric acid, which is vented from the kiln through the flue. The sodium fraction combines with silica oxide and aluminum oxide in the clay body, which creates a glaze on the exposed surface of the clay. Because the vented hydrochloric acid is highly toxic, salt firing must be done in the open with adequate masks fitted with gas filters. Soda firing is quite similar to salt firing, except here soda ash (sodium carbonate) or baking soda (sodium bicarbonate) provide the necessary sodium for the glazing process. The effect of soda firing is not as strong as that of salt firing, but it generates carbon as a waste product rather than hydrochloric acid, so it is safer and more environmentally friendly.

The foregoing types of firings have all been species of combustion firings, which require a fuel such as gas to create the required heat inside the kiln chamber. By contrast, electric kilns are heated with electricity running through coils built in to the kiln walls. Electric kilns are mostly used for earthenware or mid-range firing. It is possible to reach stoneware temperatures in electric kilns, but the coils deteriorate considerably faster although they can be replaced. Because no combustion takes place in electric firings, they can only be used for oxidation firings. Reduction can be accomplished by introducing materials such as wood into the chamber of the AMACO kilns towards the end of the firing; however this also reduces the life of the coils.