Most
introductory kilns are now being supplied with pre-set schedules.
This can make moving on to the schedules you need for the new work
you are doing appear to be difficult.
The
first thing is to get the print-out of the pre-programmed schedules
and determine what each stage of the programme is designed to
achieve. If you compare the programme temperatures with a
description of what is happening with the glass at that temperature,
you will be going a significant distance to making your own schedule
with an understanding of what you will be achieving with each stage
of your purpose made schedule. A very good guide to what is
happening to glass at various temperatures is this note from
Bullseye. This
also has the advantage of telling you what happens with different
thicknesses of glass.
Next
compare the pre-programmed schedules with those printed on the
manufacturer's website, for example:
So,
now you know what temperatures you are trying to achieve, how fast
should you go to get to that temperature? I have developed a
guideline that
the initial rate of advance should be no more than twice the rate of
your initial cooling rate for the final piece. This means that you
start planning the schedule from the annealing portion of the full schedule. If you will have a final flat thickness of 6mm, the annealing
rate will be around 80ºC, so the initial heat up rate could be about
160ºC. This is a conservative rate, and experience will guide you
to how much quicker you can heat up the glass. This initial heating
phase can be all the way up to the bubble squeeze/ slumping
temperature, but must be to a temperature at least 40ºC above the
annealing point.
There
are at least three elements that will reduce this initial rate to
less than this general guidance: Thicker pieces need more care. The
more layers, the more difficult it is to get the heat to the bottom
layer, so slower rates of advance are needed. The greater the
unevenness in thickness, the slower the rate of advance.
There
are, of course many other variables relating to the kiln, some of
which are:
Side
or top elements
Distance
to the elements – side or top
Distance
to the sides of the kiln
Placement
in the kiln – e.g.,floor or shelf and how high
Nature
of the firing surface – e.g., ceramic, fibre board, fibre paper
Placing
in relation to the hot and cool spots in the kiln
How the glass is supported - especially on a slump or drape
At
the initial stages of learning about fusing schedules, you need to
make notes of all these things (and the results) on your firing
records so that you can refer back to get guidance on what rates of
advance are acceptable for any given firing.
Part 2
Part 2
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