"THERMAL SHOCK: Shock pertaining to, or
caused by heat. Thermal cracking occurs in
glass as a result of too rapid heating or
cooling below the strain point temper-
ature of the glass."
One of the most
dramatic examples of thermal shock is in a
kiln. If the temperature is raised quicker
than the glass can tolerate (as the size of
the glass increases so does the potential for
thermal shock), the piece of glass will
literally blow apart in several pieces. [I
know this from experience!]
What if you don't use a kiln in your glasswork. Should you be aware of thermal shock Yes. First, there are stresses in glass when it is manufactured. It's a ' smart idea to keep this in mind when handling full sheets of glass. The smaller the piece of glass, the less stress on the glass.
Some examples of thermal shock are:
1) Unvented lamps - the heat of the light
bulb will cause the glass to expand, then
con- tract when it cools. Venting allows the
heat to dissipate and avoid breaking glass.
2) Bring glass into your warm house after
being stored in the cold garage, the re-
verse in the summertime, or even transporting
the glass home from the glass store. The
glass needs to acclimate before cutting.
Erratic breaking usually results otherwise.
3) Another example is after having a
finished project hanging in your window. It
may be cool over by the window, but check to
see if the sunshine hasn't created heat in
your stained glass project.
If you are
planning to move it, take to a sale or to
Aunt Martha's for her birthday, wrap it up in
a towel or newspapers before going out in the
cold.
One of my biggest lessons I learned about thermal shock was a miniature quilt, 97 pieces fused into one piece, then blown up into many pieces during slumping. I had spent many hours cutting and fusing this piece. When I put it in the kiln to bend/slump it into the shape to look as if it was draped over a bed, I learned about what can happen when you are slumping a piece of glass inwhich the glass extends out past the mold and the heat is on both sides. As I said, I speak from experience!
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