Why telescope mirror glass shipped as randomly broken chunks of a constant size in cardboard boxes?

Question

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?

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

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: https://www.youtube.com/watch?v=c-lBKuHqHk0 From light reflection, it looks like the glass surface is far from smooth. — Wayfaring Stranger, Mar 06 '21 at 16:06
@WayfaringStranger oh I didn't think to check for more videos, thanks* — uhoh, Mar 06 '21 at 21:14
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. — Wayfaring Stranger, Mar 06 '21 at 21:38
I remember some story from my parents about locals throwing rocks at it as it passed, but I hadn't heard about that! — uhoh, Mar 06 '21 at 21:53
@WayfaringStranger I updated #4 in the block of six images with arrows pointing at styrofoam bits adhering to the glass (after removal from the box). It seems the second person uses an airgun to blow on each piece in front of an air intake funnel, but they aren't doing it very thoroughly. Hopefully there are some subsequent cleaning steps; soap and filtered water then rinse perhaps. — uhoh, Mar 06 '21 at 22:58
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. — notovny, Mar 07 '21 at 01:56
@uhoh I expect that a tiny bit of polystyrene residue won't hurt the mirror. Polystyrene boils and depolymerises at 430°C, which is much lower than the 1165°C used to melt the E6 glass. The styrene monomer (which boils at 145°C) is a hydrocarbon, and burns to carbon dioxide & water if the temperature is > 1000°C. — PM 2Ring, Mar 09 '21 at 22:15
(cont) OTOH, residues from soap (or detergent) and filtered water may not be so benign. Filtered water contains metal ions, including iron, calcium, and sodium, which may impact the glass properties, in particular, its very low thermal expansion. Soap also contains sodium (or potassium), and so do many common detergents. I think E6 is a low sodium glass, but I couldn't easily find that info on the Ohara site, it may be hiding in a PDF. ;) Even small amounts of these contaminants could affect the homogeneity of the glass. — PM 2Ring, Mar 09 '21 at 22:18
@PM2Ring I think it will be hard to demonstrate that borosilicate glass will suffer from washing with soapy water containing traces of those then rinsing for several reasons; 1) it's full of sodium and aluminum oxides already, a few more metal atoms won't hurt, and 2) these chunks have low surface/volume ratio, so won't pick up much. Yes this is a telescope but the optical properties of the glass are irrelevant as it's a mirror, and the surface is covered by an aluminum coating. And they use plenty of wet processes each time they re-silver it, including soapy water! — uhoh, Mar 09 '21 at 23:52
@PM2Ring But they can always use de-ioniozed water if you like but later the mirror's surface will see plenty of wet chemical action. Search for "soap" in this answer for example or look for soap suds at about 01:14 in this video of the 200" Hale telescope mirror cleaning after the aluminum is chemically stripped with nasty chemicals: https://youtu.be/lkBgQaJVKjc?t=70 — uhoh, Mar 09 '21 at 23:58
Sure, but that cleaning happens after the mirror has been formed, so it only affects the surface of the glass, it doesn't introduce contaminants into the bulk of the glass. And yes, deionised water would be better than filtered water. But I still don't think any cleaning is actually necessary. — PM 2Ring, Mar 10 '21 at 00:12
@PM2Ring If you look at the fourth image in the group of six you can see the larger styrofoam particles stuck to the glass indicated with two little blue arrows. If you watch the video the technician is very lazy with the air gun, and so there could be quite sizable carbon containing particles of large size still adhering to the glass. I don't know what happens to styrofoam in molten glass. Since the air gun activity is so unenthusiastic, I have a very strong hunch the glass is washed with soapy or other surfactant-laden water to release styrofoam and cardboard particles from the packaging. — uhoh, Mar 10 '21 at 00:17
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. — PM 2Ring, Mar 10 '21 at 00:18
@PM2Ring yes, if there are contaminants concentrated at the edges of these big chunks, then during the melt they may only mix or diffuse through the viscous mess by millimeters, "baking in" an inhomogeneous component to the distribution of the different kinds of oxides. Okay I've got it now, thanks! — uhoh, Mar 10 '21 at 00:21
@WayfaringStranger I just ran across this, not exactly bullet holes, but still some interesting stories from telescope history: Who Would Kidnap a Space Telescope? — uhoh, Mar 13 '21 at 04:01

score 7 · Accepted Answer · answered Mar 09 '21 at 21:04

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.

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! — uhoh, Mar 09 '21 at 23:38

Why telescope mirror glass shipped as randomly broken chunks of a constant size in cardboard boxes?

1 Answers1

Linked