What does the nature of the fracture tell about the reason for the break?
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.
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.
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.
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.
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.