If
you do not want to go to that detailed effort for a one-off process,
you can adopt the shotgun annealing approach. This does require some
observation of the glass, of course.
You
need to observe when the glass has reached the temperature for the
process you are performing. This will enable you to compare the
behaviour of this unknown glass with what you normally use. This
will give some idea of the relative annealing temperature to use. If a higher temperature is required for this glass than your normal glass, a higher annealing point can be assumed. The difference in top temperature can be added to the annealing point of your known glass. If the top temperature is lower, you subtract the difference from the known glass' annealing point.
Set
the annealing temperature to be 10C to 20C above the predicted
annealing temperature and soak there for 30 to 60 minutes. This will
help ensure the glass is all at the same temperature throughout. Set
the annealing cool to be at about 30C per hour for pieces up to 6mm
for the first 55C. The next segment should be about twice that to
110C below your chosen annealing temperature. The final segment can
be around 150C per hour to 100C. For thicker glass, the annealing cool should be proportionately slower.
This
may seem an excessive, overly cautious process, but as you get to know the characteristics of the glass, you will be able to alter the schedule. This is a conservative and safe
process to ensure your glass is well annealed. And to be certain, you should check the cooled glass with polarised light filters.
amended 22.12.18
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