The glass firing process explained

During the firing process the glass undergoes considerable change. It can be either solid or fluid. With changes of temperature there will be changes in viscosity: the higher the temperature the less viscous the glass and the more easily it will flow, slump, sag and change shape.

Glass can tolerate more rapid changes of temperature when in the fluid state than it can when in the solid state where it is prone to thermal shock.

Thermal shock

Thermal shock is caused by excessive stress within the glass in the solid state, due to too rapid or uneven heating or cooling. In between the solid and the fluid states there’s a stage where the glass is neither one nor the other. The glass is in a state of transition. This ‘transition zone’ can be defined as: “An arbitrary zone in which, when the glass is being heated the temperature changes: from that where the glass can be damaged by thermal shock to that where rapid temperature change can do it no harm.” Of course, the reverse applies when the glass is being cooled.

Kiln temperature vs glass temperature

There are two “temperatures” encountered in kilnforming. One is the ‘glass temperature’ based on viscosity values and published in data sheets by the glass maker. The other is the ‘indicated kiln temperature’ for the particular kiln in which the glass is being fired. The two will rarely be the same.

Hot air rises and, as a thermocouple will generally be positioned higher in a kiln than the glass, then the indicated temperature will usually be higher than that of the glass itself.

Should it be at the same level as the glass it will protrude such a short distance from the wall that it can be partly shrouded, still giving readings different to the glass temperature. It takes time for the heat to flow within  or through a piece of glass so there can be differences in temperature between the top and bottom of a sheet in a Top-hat or top loader kiln; or between edges and centre in a front loader..

Its coolest point when heating must be ‘the point most remote from the source of heat’. It’s the point which must be considered when ‘taking the glass to the top of the transition zone’, or to the soak temperature. Similarly when cooling, it’s hottest point must be ‘the point least able to lose heat’.

This can be the underside of a slab of glass on a hearth, or various other situations.

It may be said of a particular kiln that it ‘fires hot’ or ‘fires cool’ in comparison to another kiln. This really means that the glass must be fired at a higher or lower indicated temperature in one kiln than in another to achieve similar results. Reasons for this include:

• Difference in position of the thermocouple relative to the glass
• Instrument errors (rare)
  
• Difference in effectiveness of kiln insulation
• Difference in power density

There’s more on these matters in the relevant sections, but it comes down to ‘knowing one’s own kiln’.

Viscosity

In theory, significant points during the kilnforming process are defined in relation to the viscosity of the glass. The unit of measurement of viscosity is the “poise”. Because it is usually used to define the fluidity of liquids, the poise is a small unit. It takes vast numbers to define the viscosity of a near solid material such as hot glass, hence its statement in powers of 10.

A data sheet supplied by a glass maker will define some or all of the following points in degrees C or F. It will actually be stating the calculated or measured temperature at which the glass will have a particular viscosity.

Table 21A-1

Other terms such as Annealing point, Annealing temperature, Upper annealing point and Forming range appear in the technical literature but serve mainly to confuse the newcomer. Their values are rarely stated in makers literature.

Whilst the viscosity values above apply to ALL glass, temperature values for those viscosity points will be valid only for a SPECIFIC glass formulation. Thus, the strain point for various glasses used by kilnformers can differ by up to about 30°C (55°F).

Transition zone

Is not the same as ‘transformation range’ often seen in literature. The latter is a technical term linked to specific viscosity values for glass, and stated as specific temperatures for the softening and strain points for a particular glass formulation. Kilnformers are unable to measure glass viscosity and generally lack precise knowledge of the actual temperature of the glass. It follows that knowledge of the precise temperatures for the various points, as supplied by the glass maker, can be only a starting point in working out a firing pattern.  

Transition zone is used here to help in understanding the firing process and is somewhat ‘rubbery’; like much of kilnforming.

The limits of the transition zone can be loosely based on the ‘annealing point’ and ‘strain point’ values provided by the glass maker, but with modification to meet the unique conditions in the kiln in which the glass is being fired.

When heating, about 20°C (35°F) or so above strain point should generally be suitable as the top limit of stage 1, the initial heating stage; where it is likely that the coolest parts of the glass will be in a fluid state.

When cooling, about 10 to 20°C (20 to 35°F) below strain point should be OK as the bottom of stage 4; where it is likely that the hottest parts of the glass will be in a solid state.

Kiln heating & cooling curves

Before delving into the firing pattern we should first consider the heating and cooling curve of an uncontrolled kiln, shown in Fig 1

Whilst the rates at which individual kilns can heat and cool vary widely, depending on the power input and the effectiveness of the insulation, the temperature profile will be generally similar to that shown: varying only in the time to reach soak and the time to cool.

Fig 1

These curves bear absolutely no resemblance to the desired glass temperature profile shown in Figure 2. They clearly illustrate the need for some form of controller to properly fire glass.

Fig 2

Kiln controls

Kiln instrumentation can vary from a temperature switch in a pottery kiln providing only ON-OFF control through to an extremely complex programmable controller allowing up to 100 steps to a pattern.

The majority of instruments introduced in recent times provide storage for between four and nine programmable patterns, each pattern made up of between seven and 10 steps.

One step can raise the temperature, hold the temperature constant or allow the kiln to cool down, as shown in Fig 3.

In controllers used in Australian made kilns these steps are linked together to create a firing pattern roughly conforming to the five stage firing curve seen in Fig 2

However in kilns designed primarily for firing pottery, the heating is halted at various points, sometimes for many hours, so these controllers evolved with a ‘hold’ period tacked onto the end of each of the steps, as in Fig 4, below.

For more, see 21B. Steps in the Firing Process.