FIRING

Throughout the 6,000 year history of ceramics a host of methods have been used to fire ware;
from simple piles of burning brush to the sophisticated gas and electric commercial kilns of today,
Artists firings employ practices from almost the entire range. For our purposes all of these
methods may be divided into two fundamental classes:

(a) Ms which the fuel is burned in the presence of the ware, heating it with the gases of com-

ustion, :

(b) where energy is used to heat some other object which in tum radiates heat to the ware.
For our discussions here | will refer to only the commonest forms of these two classes, (a) the
studio gas kiln, and (b) the usual studio electric kiln.

It is usually thought that the prime purpose of a kiln is to heat ware to a given temperature,
this is quite evident from the way people talk about their kilns, their firings and their results. But
this is far from so; firing does mot consist of bringing were to a certain temperature but rather of
bringing it to a certain condition. Do | hear the reader saying ‘‘Doesn‘t this amount to the same
thing?’ The answer is emphatically “No! “Why? Because it would be ignoring the effect of
TIME. None of the changes taking place in the kiln is immediate—eaoch requires a certain time
at a specific temperature for its development. The fingl result depends upon the degree of deve-
lopment of each stage of the firing, if the stages ore taken at different rates in different firings the
results cannot be anything but different. So in discussing firings it is misleading to speak about
“temperature,” it must be the “time-temperature relationship.’ Temperature sets the stage but it
takes time to develop the play.

“Time-temperature’’ leads to what Seger referred to as “’heat-work." To provide a measure for
the hect-work done in the kiln he developed the pyrometric cone, it is still the best heot-work
measure that we have for work done with pottery bodies and glazes but it is still far from removing
firing responsibility from the neck of the potter. For exemple, how often have you been dismayed
at failing to reproduce a result even though you have fired the same materials to the same cone?
Don’t tell me; | know.

But that isn't all; to time-tempercture we must add “atmosphere,” In electric kilns the atmas-
phere is constantly fixed at “oxidizing'’ but with gos kilns it is whatever one makes it, even dif-
ferent from place to place within the kiln. Changes in atmosphere are important because they
moke profound chemical changes in some ceramic materials. So the big three variables in firing
are TIME, TEMPERATURE and ATMOSPHERE: it is about these thet | am writing Temperature
alone requires very little discussion but the time-temperature relationship deserves a great deal more
than we can give it here. The effects of different atmospheres are familiar enough but their con-
trol in the gas kiln are not and will require all the time we can spare.

Many practitioners learn to run their kilns efficiently and successfully but there is very little
understanding of the firing process itself. Little wonder, the important parts of this process take
place under conditions thot ore so extreme, so alien to us that we connot observe directly whot
is going on; nearly all observations consist of comparing what the object was like before it was
fired to whot it is like after and trying to relate this to what we think was going on in the kiln,
Pretty vague ot the best, If this set of notes succeeds in clarifying the main sequence of events,
even a little bit, | will be quite satisfied.

Every kiln should have its little clay kiln-god that stands just above the door in the smoke and
the heat, the kiln should be morked with the appropriate mystic symbols and a prayer-wheel should
turn in honor of the ancestral masters of our craft, The burner himself should wear his cap back-
wards during the firing and drink nothing but mead. The forces of MISFORTUNE and EVIL have
aryl ee disposed of and the firing may proceed, governed by REASON and the LAWS OF

ATURE.

The first thing to be understood is that there is nothing strange or mysterious about the things
that hoppen to your work during firing, it is oll familiar chemistry, the same kind of chemistry that
takes place every day in every kitchen. The only real difference is that the moterials used are
so stable that they have to be pushed to extreme temperatures to get anything at al! to happen. Once
these higher temperatures have been reached al] the usual things happen, acids ond alkalies com-
bine to form more stable compounds, solids dissolve in fluids, gases ore given off causing bubbles,
$uper-soturations recrystalize on cooling {just like fudge) solids melt and run then cool and harden.
it is just a motter of getting used to thinking in terms of such high temperatures and much longer
reaction times. Simply stated ceramic products are mixtures of materials that will have certain
chemical reactions under specific conditions af temperature. If there is something wrong with the
result it means that there is something wrong with the material mixture, the temperature ES
vided or the time allowed for the temperature to do its work. Frequently all three. In the follow-
ing we will assume that the moterials, the clays and the glazes are satisfactory and the problem
is to apply the right temperature for the right times under the right conditions of atmosphere.