The question on whether incompatible glass can be included in a piece gets a range of positive and negative responses.
The real answer, as indicated by the diversity
of responses, depends on where you start, and what assumptions are being
made. However, responses such as "Less
than 10% of area is ok" are not helpful because they take no account of
the conditions.
Degree of compatibility
How incompatible are the two glasses? The greater the difference, the less can be
used. If you have two test pieces of glass that show a little stress upon viewing with a set of polarised filters,
you can attempt to combine a greater area than if the test pieces show significant stress.
Mass
The
relative mass of the two glasses are important.
Thin Bullseye confetti placed sparingly across an Oceanside glass of 6mm
thickness and 300mm diameter will usually survive, although there will be some
stress visible through polarising filters.
If you are placing a large or thick piece on the disc, you will have
much more trouble.
Placing
The
placing of the incompatible glass has an effect too. The nearer the incompatible glass pieces are
to the edge, the more likely a fracture is to develop.
Shapes
The
fourth consideration is the shape of both the base and the added incompatible
glass. A circular base can contain more
stress than a rectangular one. An
angular incompatible inclusion will show greater stress than a circular one.
With
included incompatible glass you are asking the main piece of glass to contain
the stresses. The factors affecting the
ability of the base glass to contain the stress are:
The degree of difference in stress between
the pieces
the mass of glass applied to the base
the shapes of the base and the inclusions
where the incompatible glass is placed.
These
all affect how well the main or base glass can contain the stress. If the piece is at all important to you, do
not include incompatible glass at all.
If it is really important, test all the glass you will be using.
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