I have seen
many schedules with initial rates of advance interrupted by soaks. These kinds of schedules that are written
something like this:
250°C/450°F to 200°C/482°F, soak for 10 (or 20 or 30) minutes
250°C/450°F to 500°C/933°F, soak for 10 (or 20 or 30) minutes
300°C/540°F to 595°C/1100°F, soak for 10 (or 20 or 30) minutes
300°C/540°F to 677°C/1250°F, soak for 10 (or 20 or 30) minutes
330°C/600°F to working temperature (1450°, 1500° etc.)
When I have
asked, I’m usually told that these are catch up pauses to allow all the glass
to have an even temperature. There are
occasions when that may be a good idea, but I will come to those later. For normal fusing, draping and slumping these
soaks are not needed.
To understand
why, needs a little information on the characteristics of glass. Glass is a good insulator, and therefore a poor transmitter of heat. Glass behaves better with a moderate
steady input of heat to ensure it is distributed evenly throughout the glass. To advance the temperature quickly during the
initial heat up stages where the glass is brittle risks thermal shock.
The soaks at
intervals do not protect against a too rapid increase in temperature. It is the rate of heat input that causes
thermal shock. Rapid heat inputs cause
uneven temperatures through and across the glass. When these temperatures are more than 5°C different across the glass, stress is not relieved. As
the temperature differential increases, so does the stress until the glass is
not strong enough to contain those stresses and breaks. At higher temperatures these stresses do not
exist as the glass is less viscous.
If, as is
common and illustrated in the schedule above, you advance at the same rate on
both sides of the soak, the soak really does not serve any purpose – other than
to make writing schedules more complicated.
If the glass survived the rate of heat input between the soaks, it will
survive without the soaks.
But you may
wish to be a little more careful. The same heating effect can be achieved by
slowing the rate of advance. Just consider
the time used in the soak and then slow the rate by the appropriate
amount. Take the example above using 30-minute
soaks:
250°C/450°F to 200°C/482°F, soak for 30 minutes
250°C/450°F to 500°C/933°F, soak for 30 minutes
This part of the schedule will take three hours. You can achieve the same heat work by going
at 167°C/300°F per hour to 500°C/933°F.
This will add the heat to the glass in a steady manner and the result
will be rather like the hare and tortoise.
If you have to pause periodically because you have gone too quickly, you
can reach the same end point by steady but slower input of heat without the
pauses.
But, you may
argue, “the periodic soaks on the way up have always worked for me.” As you work with thicker than 6mm glass, this
“quick heat, soak; quick heat, soak” cycle will not continue to work. Each layer insulates the lower layer from the
heat above. As the number of layers
increase, the greater the risk of thermal shock. Enough time needs to be given
for the heat to gradually penetrate from the top to the bottom layer and across
the whole area in a steady manner.
To be safest
in the initial rate of advance, you should put heat into the glass in a
moderate, controlled fashion. This means
a steady input of heat with no quick changes in temperature. How do you calculate that rate? Contrary as it may seem, start by writing out
your cooling phases of the schedule. The
cooling rate to room temperature is the safe cooling rate for the final and now
thicker piece. If that final cool rate
is 300°C/540°F, the appropriate heat up rate is one third of that or 100°C/180°F.
This “one third speed” rate of advance will allow the heat to penetrate the layers in an even
manner during the brittle phase of the glass.
This rate needs to be maintained until the upper end of the annealing
range is passed. This is normally around
55°C/100°F above the annealing point.
Then you can
begin to write the rate of advance portion of your schedule. It could be something like:
100°C/180°F to 540°C, no soak
225°C/405°F to bubble squeeze, soak
330°C/600°F to working temperature, soak 10 minutes
Proceed to cool segments
I like
simple schedules, so I normally stick to one rate of advance all the way to the
bubble squeeze. This could be at the
softening point of the glass or start at 50°C below with a one hour rise to the
softening point with a 30-minute soak there before proceeding more quickly to
the working temperature.
Exceptions.
I did say I
would come back to an exception about soaks on the first ramp rates segment of
the schedules. When the glass is
supported – usually in a drape – with a lot of the glass unsupported you do
need to have soaks. The kind of
suspension is when draping over a cylinder or doing a handkerchief drop. This is where a small portion of the glass is
supported by a point or a long line while the rest of the glass is suspended in
the air. It also occurs when supported by
steel or thick ceramic.
The soaks
are not to equalise the temperature in the glass primarily. They are to equalise the temperature between
the supports and the glass. A thick
ceramic form supporting glass takes longer to heat up than the glass. The steel of a cocktail shaker takes the heat
away from the glass as it heats faster.
The second
element in this may already be obvious. The
glass in the air on a ceramic mould can heat faster than that on the
mould. The glass on a steel mould can
heat faster over the steel than the suspended glass. Both these cases mean that you need to be
careful with the temperature rises.
Now,
according to my arguments above, you should be able to slow the rate of advance
enough to avoid breakage. However, my experience
has shown that periodic soaks in combination with gradual increases in the
rates of advance are important, because it is more successful.
An example
of my rates of advance for 6mm glass supported on a steel cylinder is:
100°C/180°F to 100°C/212°F, soak 20 minutes
125°C/225°F to 200°C/392°F, soak 20 minutes
150°C/270°F to 400°C/753°F, soak 20 minutes
200°C/360°F to draping temperature
Call me
inconsistent, but this has proved to be more effective than dramatically
slowing the rates of advance. This exception
does not apply to slumps where the glass is supported all around by the edge of
a circular or oval mould, or where it is supported at the corners of a
rectangular or square one.
In both these cases, these are about the materials holding or contained in the glass, rather than the glass itself.
Revised 23.2.25
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