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The 12.5 inch diameter porthole glass finally arrived today. It's surprising just how big that diameter is when experienced rather than imagined. It's also disconcerting just how thin 0.75 inches is for a piece of glass that size. The glass looks nice, as it should, and arrived intact - not so much as a chip on the edge.
In looking it over I noticed a bit of lettering near the edge, "HEAT TREATED." This is either very good or very bad. "Heat treated" can mean two things in the case of glass. It could mean that the glass was annealed, which means that it was heated, or at least cooled, slowly and that there is very little stress in the glass. That means it can be worked on (ground) without problems.
Or it could mean that the glass is tempered, which means it was cooled fast to induce stress. This is intended to make it more difficult to break from an impact and when it is damaged bad enough to break, it breaks into lots of tiny pieces rather than a few big ones. Unfortunately it also means that the trying to grind it could shatter it into bits. Also, with the internal stress, it'd be useless as a mirror as the polished figure would never be right even if it held together.
There is a test, which I plan to do sometime before trying anything else with the porthole. Polarized light (such as from a laptop screen) is shone through the glass and then viewed through a polarizing filter (polarizing sunglasses will do). As the filter is turned, a stress-free disk should dim evenly as the polarizations cross. A stressed disk will not dim evenly.
Considering the abuse a porthole is expected to take, I fear that the disk is tempered rather than annealed. In that case, I don't know that I have any use for the glass. Even as a tool piece, it would be risking it shattering partway though the grinding process. It could be annealed and the tempering removed, but that would take several hours, I expect, in a kiln with good temperature control - something to which I don't happen to have ready access. It'd likely be less hassle to just get a different piece of glass. Time, and polarized light, will tell.
