Polarised light
filters are used to detect stress in a non-destructive testing method in
kilnforming. The use of the filters is
described in this blog. To produce
consistent reliable results, there are certain conditions.
The light source
needs to be diffused in such a way that it is even across the viewing
area. An intense, single point light makes
it difficult to determine the relative intensity of apparent stress. Another
tip is that you can use your phone or tablet as a source of diffused light and
as the bottom filter. It emits polarised
light, meaning only a top filter is needed.
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| Stress halos from broken and fused bottles |
It is important that the glass being tested is of the same
temperature throughout to get a meaningful result. This was emphasised to me when I was running
a series of tests. I got in a hurry to test for stress to be able to start the
next trial quickly. I began to notice inconsistencies
in the amount of stress I recorded for results of the series of tests. Going back to the stressed test pieces,
showed different stress levels when they were cold from when they were warm.
The conclusion is that the glass to be tested for stress
must be the same temperature throughout.
Even if it is only slightly warm, the apparent stress will be
exaggerated. It may be that the testing
can only be done 24 hours after removed from the kiln.
Stress will be more
evident at points and corners. The light
will be brighter at highly stressed points, and even at extreme stress exhibit
a rainbow effect. More generalised
stress is evident in a lighter halo.
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| Stress points in a drawing square illustrating the concentrated stress at corners |
It is much more
difficult to check for stress in opaque areas of a piece. If there are transparent areas, the stress will
show there, although the stress may originate in the opaque ones. To be aware
of potential stress in the combination of opaque glass, strip tests must be conducted
on samples of the glasses.
Remember to include
an annealing test too, as the stress test does not distinguish the type of
stress. If the annealing test shows
stress, the annealing was inadequate. It is of course, possible that the glass
is stressed because of incompatibility. But the only way to determine that is to fire
another test with a longer soak at annealing.
