Sometimes it is discovered that
a kiln is firing hotter than other kilns, and you need to alter your process
temperatures from the generally presented ones.
That your kiln is firing hotter than others is when you recognise the
tack fusing profile of your tack fused piece is rounder than expected.
Altering process temperature and soak
times
There are two things you can do.
1) Reduce the time at the temperature. If the recommended schedule has the process work being done at 780°C for 15 minutes and the glass is too rounded or more like a contour fuse, you can reduce the soak time to 5 minutes, depending on how over-done the pieces are.2) If the reduction in soak at process temperature does not work, then you can begin to reduce the process temperature. Often only 5°C with a 10-minute soak is enough. For some kilns it may be as much as 20°C again with a 10-minute soak.
Remember that the speed at which you
advance to the process temperature will have an effect. The slower you go the lower the temperature
can be. The faster you go, generally the
higher the temperature needs to be. There
several factors combining to determine which is the right process temperature
and soak. Experimentation and record
keeping are required to find just the right combination.
Annealing temperatures in a “hot”
kiln
If your kiln fires hot, you do not
need to alter the annealing soak temperature.
I have seen the recommendation that when you need to reduce the process
temperature you also need to reduce the annealing temperature by the same
amount. This is not so for several
reasons.
The first is that reducing the
temperature of the annealing soak runs the risk of trying to anneal below the
acceptable range. These are
a few paragraphs to explain.
Annealing occurs over a range. The annealing point is the temperature at
which annealing can most quickly occur. But
there is a range during which annealing can occur. It is generally around 43°C either side of
the annealing point.
If you follow the recommendations to
anneal in the lower end of the annealing range, it is possible that you will
start the annealing soak at too low a temperature by reducing the annealing
soak temperature in line with the reduction of the top temperature.
The second is that the temperature
measurement is of the air, not the glass.
On cooling, the glass is hotter than the air temperature in the
kiln. The recommendations for the
annealing temperature take that into account.
So, reducing the temperature risks straying outside the annealing range.
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You should note that if you are using
the Bullseye recommendations to do the anneal soak at 482°C, you already are in
the lower end of the annealing range.
The average annealing point of Bullseye remains at 516°C. This new
recommendation for the annealing soak is 34°C below the annealing point and any
reduction of more than 9°C will put your anneal soak outside the annealing
range, meaning that your anneal will be inadequate, no matter how long you soak
there.
The third element relates to the
annealing range. The anneal soak can
occur anywhere within that range. But the practical measure is to soak at, or
below, the annealing point. If your kiln
fires hot, you do not need to alter the annealing soak temperature. It will not matter if the glass is in fact
hotter at the annealing soak than in some other kilns.
It does not matter, because the soak
at the annealing point, or lower in the range, is to equalise the temperature throughout the
glass piece. The annealing point is not some magic number or temperature that
sees to producing a sound piece of glass.
The soak at annealing point is to equalise the temperature to + or - 5°C
within the glass. This is referred to by
the technically minded as Delta T = 5°C, or in symbols as Δ T = 5°C. Bullseye has published a table that gives practical information on the length of soak required for this temperature equalisation for different
thicknesses.
Once the temperature is equalised
within these limits, you can begin the anneal cool. This is an essential part of annealing and is
designed to maintain the equality of temperature differentials during the
cooling. The rate of cooling is directly
related to the length of the temperature equalisation soak required for the
piece which in turn is related to the thickness of the piece. This forms the fourth reason that
starting the anneal soak slightly higher than recommendations, will not affect
the annealing process adversely. The first slow cool is essential to achieving a sound piece as it maintains this small differential in temperature during the early part of the cooling into the brittle phase of the glass.
The reasons annealing temperatures do
not need to be altered if you kiln fires hot or cool are related to:
·
annealing range
·
air temperature measurements
·
rate of the anneal cool
More detailed information is available in the e-book: Low Temperature Kilnforming.
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