Before an object is glazed, it is often bisque fired. The purpose of bisque
firing is to render the object more durable, while leaving it porous enough
to absorb glaze. Bisque firing occurs at a temperature (minimum 500-600°C)
below that of glaze firing, and must begin slowly to prevent the object
from cracking or exploding.
After an object has been formed and glazed, it
is fired in a process known as glaze firing. Glaze firing - which occurs
at a higher temperature than bisque firing - fuses the glaze to together
in combination with the surface of the clay body. The atmosphere within
the kiln has a dramatic effect on the final appearance of the fired object.
Glaze firing can be either oxidizing or reducing, depending on the presence
of absence of oxygen within the kiln.
Oxidation firing usually takes place in
an electric kiln, where oxygen is present in the atmosphere. Oxidation
is necessary for the development of certain colours (such as greens from
copper) and is often associated with clean, carefully controlled, bright
colours and surfaces.
Reduction firing usually takes place in a fuel-burning kiln, where the
kiln atmosphere is starved of oxygen. In reduction firing, unstable gases
combine with oxygen in the metals of the clay body and glazes, reducing
them to a lower oxide and altering their colour. Reduction firing is useful
in developing certain colours (such as reds from copper) and effects,
such as the characteristic speckles caused by iron pyrites in clay.
Pit firing is the most primitive method of firing,
consisting of a hole in the ground, lined with combustible materials.
The pottery is placed on top of that, then covered with some shards or
broken pieces of pottery and more combusitble materials, then lit.
Raku firing is a process developed by early Japanese potters, in which
the object is placed in a hot kiln, fast fired until the glaze melts,
and then removed from the kiln while still red hot. Often, Raku firing
is combined with post-firing reduction, where the still hot object is
placed into a covered container with combustibles such as leaves or sawdust.
The Raku method is popular both for its speed, and for the wide range
of colour and surface effects of which it is capable.
Despite being labor intensive and often unpredictable, wood firing has
become increasingly popular among contemporary potters, as the effect
of wood ash, flash markings, and the unique aerial environment of a wood-fired
kiln produces surface effects that are otherwise unattainable.