What
does the nature of the fracture tell about the reason for the break?
- incompatibility
- annealing
- adhesion
- splits
- lamination
Incompatibility
Fractures
that follow the outline of a glass are normally indicators of
incompatibility. The
fracture starts at the incompatible glass and then - usually – goes
directly to the nearest edge. Occasionally, the stress is not so
great, so it only breaks around the offending glass without
proceeding to the edge.
Annealing
A
sinuous break – often with a hook at the edge – across the whole
of the piece is generally an indication of one caused by an annealing
stress. Inadequate annealing builds up stress within the glass that
breaks through the whole piece in a lazy “S” pattern, rather than
a straight line or following outlines of glass pieces.
Adhesion
Another
kind of fracture occurs that is most often seen in ceramics. It is a
kind of crazing that leaves the glass in granules. I call these
adhesion fractures. This is indicative of the glass having stuck to
the surface it is resting upon. This can be ceramic, steel or any
other rigid refractory material. This comes from inadequate amounts
of separator, often at high temperatures.
Split
Sometimes
during slumps the piece can develop a tear or split in the lower
surface without the upper breaking. This kind of split comes from
heating the top of the glass more rapidly than the heat can penetrate
the whole thickness. The weight of the relatively plastic upper
surface overcomes the resistance of the lower surface by splitting it
on the bottom face.
Lamination
Occasionally,
a break will have both of the characteristics of incompatibility and
annealing stress. The break is relatively straight and goes through
differing colours rather than skirting them. This seems to happen
most often on tack fused pieces and so is likely to be inadequate
annealing. The annealing requirements of tack fused glass are much
greater than flat fused glass, as the pieces are to some extent still
reacting separately. If the whole piece is not given enough
time for each piece to settle with the others they will contain unrelieved annealing
stresses, which may have be too great to be held within the whole.
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