On 20/06/2020 18:48, Chris Hogg wrote:
On 20 Jun 2020 17:07:09 GMT, Tim Streater
wrote:

On 20 Jun 2020 at 14:41:03 BST, Chris Hogg wrote:

The stress intensity at the tip of a microcrack is very high, due to
its almost atomic scale radius; opening it up by etching increases
that radius and reduces the stress. The fine flaw produced by a
glass-cutter is in effect just a long flaw, and fine, even invisible
scratches will divert the line of fracture. Old glass, such as the
greenhouse glass I referred to in an earlier post, is full of
invisible microcracks and almost impossible to cut by conventional
methods. I used a diamond wheel tile cutter with great success when
cutting down panes of old greenhouse glass for re-purposing.

Can it be annealed by heating, and if so, at what temperature?

No idea. Annealing will relax any stresses in the glass, but I don't
know if it heals microcracks. Don't know what temperature either - I
was going to guess 600C or thereabouts, but wiki says upper 400's.
https://en.wikipedia.org/wiki/Annealing_(glass)


IIRC "flame polishing" as carried out by glassblowers aims to take the
surface just to dull red heat, which ties in with your 600C. I don't
think "annealing" would deal with the cracks.