The goal of fire polishing
The goal of fire polishing is to heat the skin of the glass enough to smooth it while minimizing the heat that penetrates the glass and allows it to flow/deform.
Fire polishing a thick piece of glass is difficult. That’s because the force of gravity on the significant mass means it takes less heating (softening) of the interior to make it deform. Most thick pieces, when fire polished, come out bulging around the edge.
When fire polishing thick glass, slowly heat it to the annealing temperature and hold it for as long as you would anneal it. This ensures that the glass has lost its brittleness and will not thermal shock in the next step, which is to heat as quickly as possible to the fire polish temperture where no hold is required. The faster the kiln can heat between about 900F and 1375F, the less heat that will penetrate the glass and the less it will deform. This is one of the few places that 120V kilns are usually at a disadvantage. Cool the kiln at full speed (or faster by opening it to vent heat) back to the anneal hold temperature. Anneal and cool as required.
Helios Kiln Glass Studio