thanks - I also thought of 'firing glass' when I first looked at it, but assumed possibly it wasn't old enough, although no idea as to when such things went out of fashion. I've handled a few masonic glasses, and from memory they seem always to be engraved with a Lodge No. and, of course, the square and compass - so this one probably not seen masonic use.
Re-thinking, this could be older than first thought, and you're probably right that it's a firing glass............ I always associate such glasses with late C18 and early C19......... but not later. May well be wrong on these dates, and possibly firing glasses continued for longer than I thought.