Wednesday, 11 February 2015

Equalisation temperature

In my view a schedule has the following stages.
  • Initial rate of advance to bubble squeeze,

  • Rapid increase in temperature to target, or working temperature,

  • Quick fall to temperature equalisation (often called the annealing point),

  • Slow decrease in temperature - to keep internal stresses at a minimum - to 110C below that temperature equalisation point,

  • Faster cool to 100C or less.

Of course, some of my firings have up to 10 segments, so don't mistake the stages as equivalent to schedule segments. The following graph is a generalised version of these stages.  The times and temperatures are for illustration only.

The equalisation temperature is what is most often called the annealing point. This is a mathematically determined temperature at which the glass most quickly anneals - has stress relieved. However, the way kiln formers work, annealing does not occur at one temperature point on the controller output, because of the inherent inaccuracy of our kilns and controllers. The soak at the annealing point has the purpose to equalise the temperature throughout the glass before proceeding to the anneal cool.

There is little point in soaking above this temperature, only to have another, lower temperature soak at the published annealing point. The soak at the annealing temperature will negate any effect of a soak at a higher temperature. So, a soak above the annealing temperature will simply slow the whole cooling process.

Of course, the soak at the equalisation temperature must be long enough to get the whole substance of the glass to the same temperature. The thickness of the glass will determine the length of this equalisation soak. Fortunately Bullseye have published a table to help determine the time required.

The slow decrease in temperature is to keep all the substance of the glass to within 5C difference on the cooling. Thus, the rate of cooling is related to the thickness of the glass. It will be increasingly slower with increasing thickness. The cooling to around 110C below the equalisation temperature is all part of the annealing process. The more rapid cooling after that is to control the rate of temperature fall to avoid thermal shock.