There are a variety of techniques for firing ceramics and different techniques require different types of ceramic kilns. The earliest type of firing – probably discovered by ancient humans accidentally – is pit firing. This involves putting unfired, or bisque-fired pottery in a large hole in the ground, covering the pottery objects above and beneath with burning materials such as wood or coal and igniting them and leaving them to burn for hours or overnight. Near the end of the burning the pit could be covered with sand or earth to cut off oxygen and make a reducing atmosphere inside the pit. Modern wood-fired kilns are usually made of brick and sometimes contain several chambers for burning and ports for feeding in the fuel. As the wood is burned, its ash – which naturally contains silica, calcia, potash, and other minerals – is deposited on the pottery in the kiln, creating a pleasing wood-ash glaze effect. The individual burning qualities and mineral contents of different woods result in quite different effects; and the firings may require a number of days or weeks.
Another primitive firing technique is Black Firing, in which a gas ceramic kiln is heated to 1000° C (which usually takes about five hours), then the kiln is turned off; its burner port is sealed with fire clay, and a large amount of sugar is poured into the burner port ceramic pottery. The sugar quickly volatizes and impregnates the clay surface with carbon. Unglazed ceramics take on a matt-black finish; and glazed ceramics can create some interesting surface effects. Besides sugar, salt can also be added towards the end of the firing process to obtain a salt-glaze effect. As is also the case with sugar, the salt quickly volatizes, splitting into sodium and chlorine gas. The sodium combines with silica and aluminum oxides in the clay which forms an interesting glaze effect. Since the chlorine gas can turn into hydrochloric acid and exit the flue of the kiln, this technique can be quite toxic. As a result, some potters prefer using soda ash or baking soda instead of salt to achieve something of the salt-firing effect (although these substances produce weaker effects than salt does).
Another traditional firing technique uses the Anagama kiln, which consists of a long firing chamber which has a firebox at one of its ends and a flue on the other. The side of the kiln contains small ports for stacking. In medieval Japan Anagama kilns were built on slopes to achieve better updrafts. Traditionally, firing times could vary from a single day to two or three weeks. The Anagama style of firing allows for unique and surprising ash glaze effects; and the lengthy firing time is also appreciated by potters worldwide, since unlike modern electric kilns, Anagama kilns never fire exactly the same way twice. Raku is another popular Japanese firing technique, which was invented in the sixteenth century. This is another low firing technique in which bisqued ceramic vessels are quickly heated to red-hot temperatures and then removed from the ceramic pottery kiln and reduced in a combustible material such as wood shavings. While Raku is an age old Japanese tradition, it has many followers among occidental ceramic devotees.