The warm glass firing process

As glass is being heated it becomes softer and more able to flow. For convenience, it can be considered to go from being solid to being fluid.

There are five stages in the firing of glass, as shown below.

Stage 1 heats the glass through its ‘solid’ state at a rate slow enough to avoid thermal shock and into the fluid state where heating rates are no longer restricted.

Stage 2 the ‘getting the work done’ stage. Heats the glass to a ‘soak’ temperature and holds it there for as long as is necessary to allow the desired work to be done. The soak temperature can vary widely, depending on what is being done to the glass, ranging from around 600°C (1110°F) if painting to over 800°C (1470°F) if fusing.
The time can also vary considerably, from minutes for firing a glass paint to hours for a thick fuse.

Stage 3 cools the kiln down to the annealing stage. This varies somewhat but is generally in the 560°C to 530°C (1040°F to 990°F) range.

Stage 4 takes the glass through the annealing process until it has cooled to the point where it is no longer fluid but is again in the solid state.

Stage 5 Cools the glass down to room temperature.

These ‘stages’ are not the same as the actual ‘steps’ in the pattern used to fire a particular piece of glass. The temperatures, duration of the soak period and the rate of temperature change can all vary with the type and thickness of the glass, as well as with the size of the piece and even with the type of kiln being used.