It is sometimes difficult to determine what the cause of
any cracks might be. There are a variety
of possibilities with pot melts and other high temperature processes.
Surface of slumped melt |
Cracks only on the top of a piece indicate a stress
problem. Yes, there may have been a shift in compatibility, due to long soaks
at high temperature. It would be a small shift though, or the cracks would have
progressed to be more obvious.
Possibilities of healing the cracks
relate to the kind of stress. If the stress is from incompatibilities, there is
no means of healing the cracks. Further
firing may worsen the problem.
If the stress cracks are due to the
annealing being inadequate, a very slow rise in temperature to about 40°C above
the annealing point before going to a full fuse is required. To heal the crack,
you will then need to go to full fuse temperature. This may require dams to reduce the expansion
of the piece, if that is critical. Then follow with an annealing that has a
longer soak and slower anneal cool than previously used.
Slumping will not help. Yes, the
compression may bring the open cracks together, but temperatures are not high
enough to heal (if possible) any cracks or imperfections.
The pattern of splits on the bottom of the slumped piece |
In this case splits developed on the
bottom during the slumping. The splits on the bottom - if not due to
incompatibilities - are usually due to a too rapid rate of advance in
temperature in the early stage of the heat up.
If it is thought that the cracks
occurred as a mistaken combination of, say Bullseye and Oceanside, the stress
would have been great enough to break the piece completely. There is too
great a mismatch of these two glasses to co-exist in one piece. Of course, if only one or a few pieces were
mixed in, this kind of small crack could occur, but it will normally be around
a particular colour.
It is possible that different
manufacturers’ glasses were used in this piece. The differences in
compatibility can produce mild stress within a piece that do not break
immediately. In high temperature process
like this, the incompatibilities will be exaggerated more than in thinner
pieces fired at lower temperatures.
More detailed information is available in the e-book: Low Temperature Kilnforming.
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