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FIRING DOME HEADS and FIRING SUPPORTS
By Helen Schaeffer and Teresa Desantis
Dome head dolls should be fired with the neck facing up and the head supported in a nest of porcelain prop to avoid collapsing of the pieces. Try to make sure that the head is placed in such a way that the center of gravity is central to the piece. That means the gravitational pull is evenly distributed.
There are three different “support” materials you can use in your kiln: porcelain prop, hydrated alumina and firing sand. All three have their appropriate uses.
Hydrated (or Tabular) Alumina: Alumina, when placed on a kiln shelf (about 1/8” thick), acts as a lubricant which keeps the shrinking porcelain from “grabbing” the shelf during firing and so prevents warpage and cracking. If you experience a “glazing” effect on porcelain bisque, this can be eliminated if you pre-fire the alumina by spreading it out on kiln shelves and firing it by itself to a cone 6. Reusable.
Firing Sand: Also called Silica Sand. Seeley’s Firing Sand is composed of smooth, round grains which act like tiny ball-bearings that allow the porcelain to move freely while
shrinking with no pitting or scratching. This does not have to be pre-fired. Reusable.
Both Hydrated Alumina and Firing Sand are insulating materials; if one buries part of a neck, arms, or head in the materials, the heat necessary to mature the porcelain will not get through. That buried portion of the porcelain will be underfired. A little goes a long way – a one-eighth inch coating on the kiln shelf is sufficient. A half-inch is usually all that is necessary to create a “nest” or pile to hold a dome head.
Porcelain prop: Loose prop looks much like cotton, composed of aluminosilicate fiber, which may be loosely molded to support heads and limbs during firing. Prop is also an insulating material, which means that if it is packed tightly, it will cause an underfire. Splitting of your piece occurs when one over stuffs legs and shoulder plates, not leaving enough room for the piece to shrink during firing. Blanket prop may be cut to desired shapes for firing.
If you experience any pitting (small pinholes) in your porcelain from any of these products, be sure to try one of the others to relieve your problem.
When handling any of these materials, please use gloves and a respirator appropriate for dusts (mask). You do not want to inhale any of these materials into your lungs
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