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The video MIRROR LAB: Preparing for Casting GMT's Mirror 6 shows the unpacking several hundred boxes full of broken glass from OHARA, packed in styrofoam.

The glass chunks are a fairly uniform size of perhaps 15 cm, and of compact shape; there are no long slivers or sheets; they are all very roughly cuboids.

This seems to be deliberate. It could have been ground to a finer, pourable size, or even smaller for filling the nooks and crannies of the support web at the bottom of the mold, or in much larger sheets or bars. But these roughly block shaped pieces are stacked like bricks by hand, one at a time, into the mold.

Why is this so?

screen shot from MIRROR LAB: Preparing for Casting GMT's Mirror 6

more screen shots (in case video disappears) click for full size:

screen shot from MIRROR LAB: Preparing for Casting GMT's Mirror 6 screen shot from MIRROR LAB: Preparing for Casting GMT's Mirror 6 screen shot from MIRROR LAB: Preparing for Casting GMT's Mirror 6 screen shot from MIRROR LAB: Preparing for Casting GMT's Mirror 6 screen shot from MIRROR LAB: Preparing for Casting GMT's Mirror 6 screen shot from MIRROR LAB: Preparing for Casting GMT's Mirror 6

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    $\begingroup$ I expect it is easier to clean chunks than powders. Those Styrofoam peanuts look like trouble to me, but maybe there's no static interaction with the glass. The next video shows the spinning melt, annealing and removing of mold pieces: youtube.com/watch?v=c-lBKuHqHk0 From light reflection, it looks like the glass surface is far from smooth. $\endgroup$ – Wayfaring Stranger Mar 6 at 16:06
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    $\begingroup$ @WayfaringStranger oh I didn't think to check for more videos, thanks* $\endgroup$ – uhoh Mar 6 at 21:14
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    $\begingroup$ It's a very interesting series. Thanks for pointing it out. I still remember how they shipped the Hale (200") telescope mirror by rail, cross-country in a metal container,and found bullet scars on they outside container when the mirror reached California. $\endgroup$ – Wayfaring Stranger Mar 6 at 21:38
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    $\begingroup$ According to the aforementioned follow-up video, the glass is Ohara E6 Borosilicate, perhaps the clearest optical glass in the world. Perhaps these are the clearest sections of whatever process is used to make it, and melting pourable-sized chunks of glass means more air bubbles to remove. $\endgroup$ – notovny Mar 7 at 1:56
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    $\begingroup$ The optical qualities are irrelevant, but the homogeneity is important for uniform expansion & contraction due to temperature. Also, we want the surface to be as amorphous and uniform as possible so that it can take a high polish, and so that the aluminium coating is also as uniform as possible. $\endgroup$ – PM 2Ring Mar 10 at 0:18
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The material used for the mirror is indeed E6 Borosilicate manufactured by Ohara corporation as pointed out by @notovny. The fabrication process of a borosilicate glass honeycomb mirror has remained almost unchanged since the first one, an 8.4m LBT mirror, was cast in 1997 (here is the article of the casting). Quoting the article:

E6 is a good match to our process because it is manufactured in appropriate volumes with good quality control, and because the broken surfaces of the chunks melt together smoothly without discontinuities.

This somewhat more modern article (2019) goes deeper into detail:

A significant advantage for our process is that Ohara delivers the glass as blocks broken out of a set of one-ton melts. All surfaces are pristine fracture surfaces which melt together seamlessly in the honeycomb mold, leaving no trace of the original blocks.

And this work from the Arizona University explains further:

...E6 borosilicate glass... is melted in clay pots,..., with vigorous strirring that yields high homogeneity and low striae. It is supplied to us in the form of roughly cubical blocks with smooth, uncontaminated fracture surfaces and weighing typically 4 kg each.

So using pristine chunks of E6 borosilicate seems to greatly reduce the possible contamination (in comparison to using powder or not pristine chunks), seems to melt rather very smoothly and is probably an easy size to transport. The reason of why they use this material is explained in depth in all 3 articles mentioned.

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  • $\begingroup$ Yes this makes sense; high volume/surface reduces contamination pickup and those smooth surfaces are easier to wash, compared to bags of pourable ground up glass particles. Thanks for your thoughtful and well-sourced answer! $\endgroup$ – uhoh Mar 9 at 23:38

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