Wednesday, 3 August 2016

Relieving Stress at Corners

The most frequent locations of high stress in a piece is at corners or points.  The stress seems to be concentrated there and thus they become the most vulnerable parts of the piece.

Although the above image is of a plastic drawing triangle, it illustrates the point. The stresses are concentrated at the points and right angles whether inside or at the edge. The rainbow effect of some of the stress points show that those are the location of extreme stress.  If you see any of that in your glass, you need to check for compatibility and certainly anneal it again more slowly if it is compatible.  Remember though: slow annealing of incompatible glass will not enable incompatible glasses to fit together and become compatible.

Of course, the main thing that we do is to ensure the anneal is adequate to reduce the stress at these points.  It is important in a piece that has points, right angles and other abrupt changes in angle that you are more conservative in your annealing soak and cool. 

Further, if you are tack fusing, the stresses will be greater than on a full fuse. This is because the pieces of glass are not fully incorporated and tend to expand and contract independently of each other and of the main piece.  Also, the lower glass is shaded from the heat by the upper pieces on heat up. On cool down, the lower glass looses heat more slowly.  These two main effects, although there are others, require that the annealing is done much more slowly - two to four times more slowly than a piece of the same thickness.

One simple means of reducing stress before the start of the fusing process is to nip the corners off.  And slightly round the internal angles.  This requires only a very small piece to be taken from the corner or point to reduce the stress in the final piece. This is particularly important in tack fusing projects.

This nipping of the corners also removes the frequentl sharp points that some right and more acute angles develop during the cool down.  Glass, even of 6mm and more expands with the heat of the fusing.  As it cools toward the annealing temperature, it contracts.  The glass at the corners has to contract further than the edges, and so leaves a sharp point where it was unable to fully round. Removing only a small piece of glass from the corner removes enough mass to counteract this effect of contraction.