There are frequent suggestions that holds in the rise of temperature for glass are required. Various justifications are given. A few notes before getting to the explanation of why they are uncessary.
A note is required about the softening point sometimes called the upper strain point. There is a reasonable amount of discussion about the
lower strain point. So much that it is
often simply referred to as the strain point.
Below the lower strain point, the glass becomes so stiff and brittle
that no further annealing can occur.
Thermal shock can happen though, so the cooling needs to be controlled.
There also is an upper point at which the
behaviour of the glass is different. Above this
temperature, no annealing can occur either, because the glass has become
plastic and the molecules randomly arranged. It is only just pliable, of course,
but its molecules are no longer strongly bound to one another. This is the temperature at which much of slumping is done.
It is disputed whether such a point exists. Still, in practical terms it is where the
glass becomes so plastic that it cannot be temperature shocked. The temperature of this “point” is
approximately 45°C above the annealing point, rather than the temperature
equalisation soak.
Note that the temperature at which Bullseye recommends
that the annealing soak should occur is a temperature equalisation point, which is about
33°C below the glass transition temperature - the point at which glass can be most quickly annealed. The average glass transition point for Bullseye is 516°C. Most other
fusing glasses use the glass transition (Tg) point as the annealing temperature for the soak. They or you could employ the Bullseye
technique on thicker slabs of the glass by setting the temperature equalisation
point 33°C below the annealing point, and soaking for the same kinds of time
used in the Bullseye chart for annealing thick slabs. In fact, this is what Wissmach has recently done with its W90 and W96 fusing glass ranges. They now recommend 482C (900F) as the anneal soak temperature.
Now to the point of the post.
The soaks that are often put into schedules on the rise
in temperature are justified as allowing the glass to equalise in
temperature. Glass in its brittle phase is an excellent insulator. This means that heat does not travel quickly through the glass. Consequently glass behaves best with
steady and even rises in temperature (and correspondingly on the reduction in
temperature). Rapid rates risk breaking the glass on the temperature rise, no matter how many or how long the holds are.
This means a slower rate of advance will
accomplish the heating of the glass in the same amount of time, and in a safer
manner, than rapid rises with short soaks/dwells/holds. The slower rate of temperature increase allows the glass to absorb and distribute the heat more evenly. This slow heating is most obviously required in tack fusing where there are different thicknesses of glass.
This means that it is possible for thin areas of glass to heat up much more quickly than glass covered by different thicknesses of glass. It also applies to strongly contrasting colours such as black and white, because they absorb the heat differently - black more quickly than white.
There are, of course, circumstances where soaks at intervals are required – usually because of mould characteristics, in slumping, and in pate de verre.
Sometimes people add a soak at the annealing temperature
on the way up in their schedules. This
is unnecessary. If the glass has
survived up to this point without breaking, it is highly unlikely it will break
with a further increase in the rate of advance unless it is very fast. The temperature after all, is above the strain point meaning the glass is no longer in the brittle phase.
Many people add a soak at around 540°C (ca. 1000°F) into their
schedule on the increase in temperature, before their rapid rate of advance to
the top temperature. The choice of this temperature relates to the lower strain point. This also is
unnecessary, except possibly for very thick pieces. By this time the glass has reached its plastic stage and if it
hasn’t broken by then, it won’t with a rapid rise in temperature either.
Further information is available in the ebook Low Temperature Kiln Forming.
Soaks at various temperatures during the advance to the
upper strain points of glass are not necessary.
What is necessary is a knowledge of when the glass becomes plastic in
its behaviour, and an understanding of how soaks can overcome characteristics
of moulds, or how to achieve specific results and appearances of the glass.
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