When firing has been completed an important function still remains; cooling. Some special
effects require special attention on the woy down but these ore not subjects of this discussion, If
the kiln is left sealed tight, peep-holes, burner-parts and damper, until the ware has reached o temp-
erature of 200-300 degrees F. we hove nothing further to discuss, no possible harm can come to it
However, we moy hove to accomplish cooling foster and, taking the proper precautions, if can be
done. The real danger zone in fost cooling is the quartz inversion at 1060 degrees F. As each
quartz ¢rystal reaches this point it immediotely shrinks 15%, If this should happen obruptly
to all the quartz crystals on one side of a piece before the other side has even storted the piece
will certainly crock. A cooling crack is quite recognizable, its surfoces are glossy as if they were
thorough glass rather than rough cloy. (Fired ware breaks even more readily when being re-heated
through 1060 degrees F.).

Cooling moy be hostened to 300 degrees an hour down to 1100 degrees F., from there it
should be slowed to 100 degrees an hour, When 900 degrees hove been passed the cooling moy
be hastened again if there is some way to keep it uniform throughout the kiln but this is fre-
quently difficult

ATMOSPHERE

In the normal firing of an electric kiln the only goses present ore those of the surrounding
air {except far some small bits driven from the ware during the eorly stages}; of these oxygen
is the only one hoving o significant effect. |t is present in such o surplus that oll materials that
will combine with it do so. Pieces of metallic iron are converted to iron-oxide, calcium compounds
to calcium oxide and so on. Such a firing is called on “oxidizing firing." The oxide form of
minerals enter into relatively few combination changes so the most important characteristic of
oxidizing firings is STABILITY OF RESULT. Differences in degree of oxidization are insignificant
The moterials combine, melt and cool ino fairly regular manner, if firings are reasonably the same
the results will be alsa,

In gas - foil, propane, coal, wood) kilns the atmosphere is forcibly introduced; part of it is the
fuel, port the air bringing in oxygen and mast of it is a combination of the two that results from
combustion. [? more air is introduced than is needed for combustion the resultant atmosphere will
contain an unburned excess of oxygen, hence it will be an oxidizing firing and similar to that of
on electric kiln. In thot burning mechanisms ore for from perfect o “just right’ addition of oir
does not result in o nevtral tire but rather ino fluctuation from oxidizing to réducing that is called
“fhashing.”’ Flashing is usually destructive to the gloze, it causes the materials to change Bock and
forth from one chemistry to the other mony times, eoch time yielding some of its components to
the surrounding atmosphere, often leaving only a pock-marked ond cratered slag where there was
meant ta be a glaze. Occasionally such occidents are fortunate ond beautiful but usually they re-
quire o lot of rationclization fo make them anything but o disoster.

lf om oxidizing firing is intended but flashing results it is obvious that at least ports of the
kiln are not getting enough oir, The culprit here hos to be either foulty kiln design or Faulty kiln
operation. Kiln design and operation is just too large o subject to be tackled ino poper of this
sort. Probably oa “clinic’’ desling with actual kilns and practices would be much more helpful.

When insufficient air is introduced to provide the oxygen necessary for complete combustion the
unburned fuel will demand ony oxygen that it can find, Those materials that do give up their oxy-
en are soid to be “reduced” hence the term “reducing firing." Lead-oxide ts easily reduced arid
or most purposes destroyed, so we try to avoid the reduction of lead-oxide except in special cir-
cumstances. Copper-oxide is probably the most spectacular under reduction turning bright red fram
its normal bright green. The most important is certainly iron-oxide which chonges from a refoctory
material in the high oxygen form (Fe#03) to on active flux in the low oxygen form (FeO). Tron rich
bodies and glazes that mature quite handily at cone ¥ in a reducing firing moy require cone 12 to
14 under oxidizing conditions. In other words the Aigh chemical activity of reduction incré@ases the
effectiveness cf your kiln by about 4 cones, ot the some time it produces o variety of spectacular
results not available in the limited chemistry of an oxidizing Firing. To describe these differences
would require o whole paper to itself and a number of expensive micro-photographs; rather than
that | would suggest thot the readers who ore interested should moke their awn comparisons using
a good magnifying glass.

There is much uncertainty os to whot o reducing firing should be like, proctices range from “‘just
a burst of the end of the firing’ to flash the bore clay, to on owful smoking thot turns everything
dark and murky. The middle rood that is suitable for stomewore iron glazes, celeadons and cop-
per-reds is the one we will discuss here.

From light-up to 1400 degrees F. a normal oxidizing fire is best, it provides more circulation
for the rermnoval of woter and plenty of oxygen to burn out carbon ond sulphur, If all clay has been
bisqued in advance a slightly reducing Hre is quite suitable and slightly more economical.