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.
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
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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Firing and Raku