Questions such as “How compatible are Wissmach
W90 and Bullseye?” are asked from time to time.
This does show some awareness that Bullseye may not be Coe 90 and that
CoE does not equal compatibility. The
same question may be asked about whether Youghiogheny Y96, Wissmach W96 and
Oceanside are compatible with each other.
What is CoE
It is important to know what CoE
means before the question can be answered. It is a measure of average expansion from 20°C
to 300°C. This is suitable for
crystalline materials as their low temperature expansion rates can be projected
onto the behaviour of the material until near molten temperatures. However, it is not suitable for non-crystalline
materials, such as plastics or glass, as their behaviour is much more unpredictable
as the temperature rises. Measurementsof CoE have been made of glass at the glass transition temperatures which show
at least seven times greater expansion near the annealing temperature than at 300°C.
An extended essay on compatibility
written by Lani Mcgregor is here,
Compatibility Tests
The degree of compatibility is uncertain between different manufacturers. Each
manufacturer will take their own way toward balancing the viscosity with the
CoE. While they can say their glass has
similar characteristics to another manufacturer’s glass, they cannot guarantee
compatibility.
When using glass from different
manufacturers together, the best advice is to test the glasses yourself for
compatibility. Do this before you commit to the project. Bullseye
notes how they do their stress tests on the education section. I have been unable to ascertain how other manufacturers test for compatibility within their range of fusing glasses. Another simple method of testing for stress
is here.
There are reports that W90 and
Bullseye work together and others that say they don’t. There are those that say the 96 CoEs work
with Oceanside, and those who say they don’t. Testing for yourself is the only way to know what works.
Scale
It seems that combining different
manufacturers’ glasses may work at smaller scales, but less well at
larger. Since very few people test for compatibility before, or after,
when combining different manufacturer's glasses, they don't know whether their
pieces are showing signs of stress. Just because the pieces do not break
immediately does not mean they are compatible or stress free.
Size, Shape and Quantity
You should also note that the
relative sizes and shapes of the combined glasses effect the survivability (rather
than compatibility) of the piece.
Shape
The shape of the main piece has an
effect. Circular or broad ovals can
contain the stress much more easily than a long rectangle or a wedge-shaped
piece.
The same applies to the pieces
added. Pointed pieces concentrate the stress more than rectangular ones. The stress from
circular additions are easier than rectangles for the base piece to hold.
Placing
Where you place the additions is
important. Anything placed near the edge
of the base is more likely to cause enough stress that it can not be contained
and so the piece breaks.
Mass
How much of another manufacturers’
glass are you putting on the base? The
bigger the area or the thicker the piece(s) the less well the base will be able
to hold the stress before breaking.
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