Ceramic Firing Techniques

his week we continue our survey of ceramic firing techniques with a description of wood firing, salt & soda/vapor firings and electric firings. Next feature -- the difference between oxidation and reduction firings.

Wood Firing
Woodfiring has an age-old tradition in ceramics. The very first ceramics ever fired were done (probably accidentally) in camp fires and rudimentary kilns thousands of years ago. Under woodfiring we now understand firing a brick kiln with wood as fuel. This necessitates a a certain kiln construction type, sometimes with several burning chambers and stoking ports for feeding in the wood. As the wood burns, ash is created which deposits itself on pots in the kiln, creating a natural 'wood ash' glaze. These natural glazes are made from silica, potash, calcia and other various ingredients that form naturally in the burnt wood. Special knowledge of woodfiring is necessary -which wood burns best? What different types of wood will result in special ash glaze effects? Where in the kiln should pots be placed for certain effects? (This is an individual matter for each and every kiln and firing cycle). How long will a firing take? (It may actually be a matter of days or even weeks.) What firing cycles are necessary? If you want to know more, check out these tips on how to build a woodfiring kiln.

Salt Firing
Salt firing refers not so much to the fuel used to fire the kiln, but to the introduction of salt towards the end of a firing to get a salt glaze effect. Usually done in large wood or gas kilns, salt is introduced into the mature kiln chamber by the pound at the end of a firing. Due to the intense heat, the salt volatilizes and the sodium chloride splits into sodium and chlorine gas. The chlorine combines with moisture to form hydrochloric acid, escapes into the kiln atmosphere and exits via the flue, while the sodium combines with aluminum oxide and silica oxide in the clay, forming a glaze on any exposed surface of the work. Due to the escaping hydrochloric acid, which is highly toxic, the utmost care must be taken and a good mask with a gas filter (a dust filter is not good enough!) must be worn. Of course such firings must be done in the open, and not in residential areas. Due to the environmentally unfriendly nature of the salt firing, some people prefer soda firing.

Next Page> Soda Firing & Electric Firing

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