People
experience breakages of their pot and screen melts that do not seem
to have anything to do with annealing or glass sticking to the shelf.
The common suggestion is that there has been a compatibility shift
of the glass. This view is re-enforced by the opalisation of the
transparent hot colours experienced by most.
Bullseye
indicates in their glass notes that some colours are not suitable for
high temperature work. This probably applies to other fusing glasses
too. My experience leads me to believe that this compatibility shift
occurs with all the opalescent glass colours as well as the hot ones.
Further work will appear soon. is required to determine if there are any
general indicators of the kinds of glass that are likely to develop
incompatibility at high temperatures.
If
you are concerned about the lack of durability of your piece due to
possible incompatibility, you need to include tests
with the firing. To make this test, place a piece of each colour
used in the melt on a double layer of clear. If you are using a
single base piece, ensure you leave space between the colours. It is
best to place each colour on its own stack of clear. Also place a stack of clear glass as thick as your blank along side the other test pieces.
Put all those pieces somewhere within the kiln out of the way of the
area the melt will occupy and fire the lot together.
When
cool, take all the pieces from the kiln and check the test pieces for
compatibility. Do this check with a polarising filter to determine whether there is any incompatibility by looking for
the halo showing the degrees of incompatibility.
If
any or all, of the the pieces show stress, check the clear stack for
stress. If the clear also shows stress, the annealing has been
inadequate, rather than just the compatibility shift. Ideally, this
process should be conducted in every firing.
Performing
these tests will give you confidence in the durability of your piece,
as it will show the levels of stress in the finished piece.
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