I have read that glass is a super-cooled solution. When it is molten, the rate of cooling makes big differences in its properties. It has to be cooled very slowly.The huge panes used in modern buildings have to be tempered, or they would shatter too readily. The glass in automobile windshields is treated so that when it shatters, none of the pieces are bigger than a fingernail. The old windshields could make swords that could inflict gruesome wounds. When I was a boy testing urine specimens, the test-tubes were exasperatingly fragile. They had to be warmed up very slowly in the alcohol burner flame, or they would shatter and spill the urine. So, the invention of Pyrex glass was a very great blessing.Adding some borax made a very great improvement in both thermal and mechanical shock resistance.
And the first telescope lenses were very disappointing because of rim of color, called chromatic aberration. Many years went by before it was discovered that special optical glasses could greatly improve the quality of the image. That's why your camera lens is not just a single piece of glass, but three of four lenses of different composition nested together to avoid both spherical and chromatic aberration.
P.S. The big telescope mirrors take as much as six months to cool, to minimized internal stresses.
