It is possible to break the glass by heating it up too fast during the initial temperature rise. How fast you can increase the temperature is dependent on how even
the heat is within your kiln and the profile of the glass. Any suggestions have to be tested
within your own kiln and setup rather than relying exclusively on
others' experience. Some of the considerations relating to the kiln
are given in this blog about initial rates of advance.
So with those precautions, I put
forward a suggestion based on my experience and information gleaned
from the Bullseye site, education section and from Graham
Stone's work. These lead me to suggest that the
initial rate of advance can safely be the same as the second cooling segment as listed in the Bullseye chart Annealing Thick Slabs (Celsius and Fahrenheit). This ramp rate applies up to the softening
point of the glass.
Experiments have shown that an evenly thick piece of glass 6mm thick cooled at 150ºC/270ºF per hour during the second cooling segment - can also be fired up at the same rate. And by extension:
- A 12mm thick piece could be taken up at 99ºC/178ºF per hour
- A 19mm piece could have an initial rate of advance of 45ºC/81ºF per hour
- A 25mm thick piece of glass could be taken up at 27ºC/49ºF per hour.
These rates depend on a number of
factors:
- how the glass is supported,
- the nature of the shelf,
- the composition of the mould, and
- the kiln characteristics as well as
- the colour combinations and
- whether the piece is tack fused or full fused.
Slower rates of advance are
indicated if
- the kiln is side fired or has cool spots.
- the shelf has not supported on 25mm/1" kiln posts.
- the piece is tack fused, you need to slow the ramp rate by half.
- there are strongly contrasting colours next to each other
Remember that these numbers can only be
used as a guide in conducting you own experiments.
More information is given in the eBook Low Temperature Kilnforming available from
and Etsy
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