annealing

When a hot piece of glass cools, its surface stiffens before its interior, and the two parts end up pulling against each other. The frozen-in tug-of-war is called residual stress, and it is dangerous: a glass carrying enough locked-in tension can crack for no visible reason, sometimes hours or days later, or fly apart when scratched. Annealing is the gentle heat treatment that relaxes these stresses away, so the finished glass is calm and stable rather than a coiled spring.

The recipe follows straight from the viscosity curve. You reheat the glass to its annealing point, the temperature where the viscosity is about 10^12 Pa s. That is stiff enough to hold its shape but still soft enough that internal stress bleeds away by slow viscous flow within minutes. You hold it there long enough to relax, then cool it very slowly and evenly through the strain point (around 10^13.5 Pa s), below which the glass is rigid and any new temperature difference would freeze in fresh stress. The whole schedule, hold-then-slow-cool, is carried out in a long tunnel oven called a lehr.

Annealing is a required step for almost all glass, from cast telescope mirrors that are annealed for months, to bottles that ride an in-line lehr for an hour. One honest caution about the word: in metals, annealing softens by recrystallizing the grains, but a glass has no grains, so here annealing means only stress relief by viscous relaxation. And annealing is the deliberate opposite of tempering: annealing removes stress to make glass stable, while tempering adds a carefully engineered stress to make it strong.