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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.
Yes, it sounds as if the heads were probably near th firebox in the Alpine.
Any gas kiln is much hotter near the firebox in the early stage of the fire.
The cones or pyrometer may have been reading a correct temperature but the
temp near the flames probably raced out of control soon after the kiln was
turned on. The explosions happened in the earliest stages, the color
developed later, from the reduction of the iron in the clay. Iron in an
oxidation atmosphere is stable but when subjected to an environment of
carbon monoxide, it is forced to revert to (reduced) to its metalic state.
In that state, it becomes a more active flux and numerous chemical reactions
are set into motion by the iron in the clay. However none of these involve
exploding clay heads.
On the construction of the clay heads, if you must make them thicker then
3/4" to 1" in wall depth, the old trick is to take a needle tool or thin
wire such as a paper clip and perferate the thick wall with tiney holes,
then smooth over the surface of the form. This way, little paths have been
creted in the thickness to let the moisture move out of the deeper areas of
the clay wall in the earlier stages of the preheat and fire.
Better luck next time. Perhaps you need to take direct responsibility for
the firing of your student's work. That way, you will be more cautious when
you load the thicker greenware in the center of the kiln away from the
fireboxes and in the early stages of preheat. Perhaps you will want to run
the piolet lights only, a hair dryer, or small portable tourch into the
fireports for 4 to 6 hours before turning on the larger gas burners.