Breaks during slumping sometimes occur. What can be done?
Cause of Break
The first element in assessing the piece is to determine why it broke.
Should it be Repaired?
The second element is whether it should be repaired or re-used.
Is it worth the effort of repairing? This will be about the importance and the
time and effort you have already put into the piece.
Can it be repaired?
This is a third element of assessment. If the break resulted
from incompatibility, any attempt at refusing will also break for the same
reason. If inadequate annealing caused the break, it may be possible.
It is sometimes suggested that those pieces which fit
together exactly, should be fused together flat and re-slumped. This ignores
the fact that the glass will have stretched or deformed from the flat piece it
once was.
- · This re-fusing may be successful for shallow and simple slumps. But the piece will not be corrected by fusing the broken pieces from deep or complex slumps as a result of the stretching and thinning or thickening in the slumping process.
- · The glass pieces will have an imperfect join when flattened because of deformations from the changes during the slumping.
- · If the base is a single layer, the separate pieces will pull apart during the re-fusing process due to the lack of volume.
- · The fusing process will make a tack fuse much flatter than originally intended. A contour fuse - at minimum - will be required to join the pieces.
For all these reasons, any flattening, fusing and then attempting a slump again is unlikely to be successful.
Fusing in the mould
In recognition of these problems about flattening,
re-fusing, and slumping again some people suggest mending by firing in the
mould. This would get over the difficulty of changes of shape. However, the
required contour or full fuse will leave marking on the back and may lead to
thickening at the bottom. It is also hard on your ceramic moulds if you fire
quickly.
Changing the Shape
If it is desired to flatten an unbroken slumped piece for
use in a mould of a different shape without much change in tack profile
dimensions, there are two things to do. The maximum temperature to be used to
get the glass flat and retain the degree of tack is the sharp tack - or
lamination - range. It will require a significantly long soak at top
temperature - hours.
This long soak time is a consequence of the effects of
weight and span. The effective weight is less at the unsupported edges than at
an unsupported centre. The slumped piece has most of its weight on the shelf
now. This makes the flattening have to use a higher temperature or a longer
soak. The effective span and weight at the edge is almost zero. This requires
long soaks and frequent observation to know when the flattening is complete. Both
these effects make the flattening of a piece without altering the profile a
lengthy process.
More information is
available in the ebook: Further information is
available in the ebook: Low Temperature Kiln Forming.
Repairing a broken slumped piece of glass requires knowing
why it broke, can it be repaired, is it worth repairing. Difficulties related
to the changed shape, temperature to fuse, and changes in tack profile.
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