The mirror (third largest in the world at the time) is composed of fused silica. Apparently manufactured in segments, then joined. The cooling process itself took two years. That's described in one of the linked YouTube videos in this thread: <https://news.ycombinator.com/item?id=41459171>
The result is a glass that's remarkably immune to shattering. Possibly similar to a Prince Rupert's Drop, though that's my own speculation.
Given years long cooling process (aka annealing), the mirror would be the opposite of a Prince Rupert's Drop, which is created by quenching the glass very quickly, similar to the process known as tempering.
"Possibly" is doing some heavy lifting in my comment.
The similarity is in the end result, not, obviously, the fabrication process.
The strength of the Prince Rupert's Drop comes from the differential tensions and stresses in the head-end of the drop. The tail, of course, is remarkably fragile and will lead to the explosive failure of the entire drop. The head can withstand hammer and even bullet impacts, as Destin Sandlin has demonstrated multiple times on his "Smarter Every Day" YouTube channel. I'd suspect a similar tensile arrangement in fused silica, though of course I may be wrong on that.
Isn’t the original cooling process you’re describing “annealing,” by which glasses are cooled slowly so as to allow internal tensions to equalize? A task requiring exponentially more time as the size of the slab of glass increases?
I seem to remember glassmakers using a device called a polariscope to actually see those internal stresses [0]. Which makes me wonder if there are optical considerations as well as physical ones—although I guess if the whole thing is getting silvered, probably not.
My understanding, which could be flawed, was that perfectly annealed glass resists cracking because there isn’t a clear fault line to cleave across, something akin to the way hot water in a perfectly smooth glass has trouble boiling if deprived of a nucleation site. While a Rupert’s drop achieves its stability by something more akin to the trope of a “Mexican standoff,” [1], where countervailing tensions are so densely frozen in place that a little extra tension from the outside makes little difference (but when one crack forms, all bets are off and the tensions resolve explosively).
I’m also reminded of one of my very favorite moments of video: the silvering pass during the manufacture of the Rubin Observatory’s 23.5-ton, 8.4-meter-diameter mirror; which took from 2008 until 2019 to manufacture:
Part of this that hasn't been mentioned is that the mirror is thicker than you might expect. The observatory's website says 12.5 inches (though it will vary somewhat across the curved surface).
