Firing quickly is often our desire,
in spite of the mantra of the experienced – slow and low. How to do this safely
– without fractures or bubbles – is the requirement.
Firing quickly on smaller things
(say, up to 100 mm) is not normally a problem.
Difficulties can arise due to the kind of layup, but usually the mass is
not great enough to be thermally shocked, nor the size enough to trap air that
would cause bubbles.
Firing quickly for larger pieces is
where difficulties arise. These relate to the initial rate of advance to the
softening point, the bubble squeeze, advance to top temperature, and annealing.
Advance to
softening point
The first place this occurs is to the
upper strain or softening point. This is
the range where the glass is solid and does not transmit heat well, leading to
the risk of thermal shock. You need to
find a rate of advance that is a little slower than that which would cause the
glass to break. My guidance is to use no
more than three times the annealing rate for the glass of that thickness to
reach the softening point. This temperature
is approximately 40C above the annealing point. The glass is certainly plastic
above that temperature, so the rate of advance can be faster.
Bubble squeeze
Strategies from the softening point
to bubble squeeze vary. You can go
quickly, say 1.5 times the previous rate of advance, to the bubble squeeze heat
soak of around 30 minutes. The other is
to go quickly to 50C below that temperature and advance at 50C per hour to the
bubble squeeze heat soak. This is often
used on more complex and thicker lay ups. There are numerous variations upon
these two strategies depending on the circumstances.
Top temperature
Ways to get to top temperature from
the bubble squeeze vary, but as fast as possible risks bubbles due to excessive
softening the surface, over firing due to the controller not shutting off
quickly enough, and a lack of control of the surface texture. Twice the initial rate of advance is quick
enough, but still allowing the controller to shut off when the top temperature
is being reached.
The soak at top temperature does not need
to be more than 10 minutes. If you can
achieve the desired results in less that time, you should consider reducing
your top temperature.
Annealing cool
A soak at the bottom end of the
annealing range will reduce the anneal cooling time. The lower temperature of the annealing range
is about 40C below the annealing point, so to be safe the annealing soak can be
set to be 30C below the annealing temperature.
This reduces the range of temperature over which the slow anneal cool
takes place.
The initial anneal cool should be to
55C below the soak, the second stage of the anneal cool to 110C below the soak
can be at twice the initial anneal cool rate.
The rate of cool can be increased to 370C, where for pieces of 9mm or
less, the kiln can be turned off.
However, you need to think carefully
about firing quickly. When realistically
will you be able or actually need to take the piece out of the kiln? If it is the next day or after work, then a
slower firing reduces the risks of rapid firing and still enables you to take
the piece out when needed.