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.
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