Stress Build Up

Does annealing stress in one firing cause breaks in the next firing?

Stress does not cumulatively build up from one firing to the next. Once the glass has achieved the strain point temperature, the relief of stress begins and is complete by the softening point (usually the slump temperature).

So why does the glass sometimes break on the second firing?

Picture credit:  Emma Lee

There are three general causes of breaks of fused pieces on subsequent firing:

Too rapid initial ramp rates.

Inadequate annealing.

Incompatibility. If there is incompatibility, no amount of extended annealing will cure it. Some of the causes may be:

• cumulative compatibility shift due to extended time at high temperature

• long soaks during multiple firings.

• Normally, hot colours are most subject to these.

How is it possible to be sure the fired piece does not have excessive stress before the next firing?

Stress testing with polarised filters of the cooled glass will indicate the amount and sometimes the cause of stress. This test should be conducted on each piece after it has cooled and before firing again. If there is a large amount of stress, the piece must be warmed up slowly to avoid adding additional stress during the heat up for a more adequate anneal.

This heat-up needs to be a single slow ramp to the strain point.  After the strain point is reached, there is much less chance of a break and the ramp rate can be increased.   It isadvisable to go at least 55C/100F above the annealing temperature and soak for at least 10 minutes before proceeding down to the anneal. The annealing for this new firing needs to be scheduled to be much longer and the cool much slower than the first firing of the piece. The additional length may be for one layer thicker for full fused pieces, but for at least two times thicker for tack fused pieces. Guidance for calculating the thickness for the annealing time and cooling rates are available in this blog post.