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Kaolinite, the basic building block of clay is Al203-2Si02-2H20 that is the
small hexogonal crystal which can be found in dry or wet clay.
Then, to make the clay plastic and workable, water of plasticity is added.
After the clay form is constructed (not more then 3/4" to 1" thick in wall
depth), it should be dry to the touch, and no longer humid and cool. At
that point, there is still the same amount of water in the clay work as
there is in the humidity of the air around it, plus the water which is still
there, held chemically in the kaolinite crystals.
Now, to make a long story shorter, water, when it changes to steam, is
acconpanied by a tremendous increase in volume. When the kiln is fired too
fast, the water is still in the wall of the clay form, between the crystals
and in the chemical composition of the crystal. ..... Heat the kiln too
fast and you have an explosion (perhps a neat science experiment but not too
good if you are an art teacher).
We usually let the kiln preheat with one or two elements on low for an
afternoon and evening before the day of the final firing. In the afternoon
preheat, we keep the plugs out of the spy holes and the lid propped with a
stilt or brick ( As a general rule, over 50 times the interior volume of the
kiln will move out of the kiln in the form of water vapor and other gasses
in the early stages of preheating). We close the lid and put a few plugs in
the spyholes for the evening stage of the preheat.
If the kiln is a dull black red or hotter, in the morning when you come in,
you are past the watersmoking, quartz and crystobalite stages of the firing
so you can move the heat up fairly rapidely. We usually set all the
switches on low for an hour or two, then put on a couple (bottom ) switches
to medium for another hour or two. We keep walking up the switches in the
mornig after the preheat. By noon, they should all be set to high and by 3
or 4 the Dawson Kilnsitter should have switched the thing off. Do not leave
school without knowing that the kiln is really off and it is then allowed to
cool for the rest of the day, the evening and part of the next day.
Hope this helps........
> My Introduction to Art students made clay sculptures this
>spring. Our school has another art teacher who has the kilns in her
>room. She fired the clay for us.
> ALL the clay heads in the Alpine kiln blew up! The majority of
>the heads in the electric kiln were fine. The color of the heads in the
>Alpine kiln was sort of gold, even brownish; the color of the fired clay
>heads in the electric kiln was basically white.
> A few weeks before the other teacher fired the example clay head
>I made; it came out perfectly. I used the same clay on the head I
>made; the walls of the clay head were about the same as those of most of
>the students. Three years ago my students did this project; most of the
>heads were fired successfully; I also had a similar experience several
>years earlier--they came out ok.
> Would you assume that the clay was fired to too hot a
>temperature in the Alpine? That the clay was fired too quickly--not
>candled...? I know it could not have entirely been because of the
>thickness of the walls of the clay, or mine would have blown up, too. I
>would like to avoid this problem in the future.
> The other teacher says that the problem was because they were
>all too thick; some of them probably were too thick.
> Any help would be appreciated.