Yes, Glass is a supercooled liquid.

If you look at OLD panes of glass, it is possible to see that they are thicker at the bottom than the top.

The house in which I grew up in the 50s, still had the original glass from when it was built in 1923, and the change in thickness, from top to bottom, was visible. At about the same time, some of the panes became brittle and would crack for no obvious reason (Unless the wooden frames were distorting )

Howard

The trouble with the glass is that you would have to start with optical flats on edge to test the theory properly. I used to live in a tudor cottage and the diamond panes in the windows were hand formed in small individual batches and were too uneven to be able to see through properly.

Had a quick look and it's a very common question out there on't web.

This article at the Corning Museum of Glass site (and you'd think they would know) states it is not true, although there are quite a lot of guesses, opinions and estimates in there.

Scientific American and the Fiber Optics Association have some things to say about it too.

Still confused,

Rob

I heard that the majority of panes are thicker at the bottom, but not all.

The suggested reason is that it's logical to fit a pane of glass with the heavy end at the bottom.

Neil

There's probably a support group help line phone number Mike, I know how upsetting it can be.

Has anyone actually carried out proper research though? Not sure how, short of measuring many hundreds of old windows and doing a lot of statistics.

New glass is very slightly elastic, and loses that, becoming more brittle with age – friends in the trade but with no commercial connection with me, have told me it's no use trying to re-cut old windows as they just break uncontrollably.

That elasticity was used in making the first big astronomical telescope mirrors or lenses. The method is described in one of the three Holtzappfel books reprinted by TEE Publishing. The blank was a disc made as flat and parallel as possible. This was clamped all round its perimeter, down to rings of precision jacks set very accurately higher than the edge. The top surface, now convex, was ground flat again. On very carefully releasing the clamps, I think I recall reading they were released very slowly, the glass would ease back to being flat underneath and concave on top.

I do not know if that method is still used. (Isn't the fancy word for perimetric clamping, encastre – with an acute accent on the last e ? )

If a relatively lightweight glass window can flow over the decades, then has such distortion been reported in the older, big optical telescopes still in use? Or are they always parked vertically so the object-lens or mirror spends the days in its most stable position?

Thinking of the relative weight across a window-pane, I would be very surprised if enough glass has crept for even an experience glazier to tell the difference by balance, even with something the size of a shop window. It would in case be hard to be definite about old windows because rolled glass was never perfect anyway. My parents' Edwardian home had wiggly areas in some of its sash windows.

The old leaded windows and bull's-eye panes were not made that way to delight County-magazine buyers centuries hence, but because large sheets could not be made in those days.

A fascinating subject, Michael.

For a few brief moments when I saw your subject title, my mind switched from musical pitch to thread pitch before ….. well here’s my contribution.

In light of pitch’s (historic) use as the thermoplastic (resin) component in dough moulding compounds, pitch is actually extremely brittle at room temperature. [I can’t find its Tg, can you Nick?]

Pitch was stored in large pieces outside at the plastics factory where I began work (1950). Some pieces were as large as footballs. Along with fillers of various kinds, it was steam-heated in ‘Z’ blenders. The fillers provided a support matrix not unlike glass reinforced resin.

In the northern reaches of England on those rare occasions when the sun shone through, the stored lumps of pitch could be heard snapping and tinkling as it/they responded to the changes of surface temperature.

Sam

Edited By Sam Stones on 07/01/2021 21:09:13

The pitch was musical after all, Nigel.

Sam

Strictly FYI about the old glass thicker at the bottom of window panes, and conclusions about it, were discussed in 2007 by Scientific Amurkan at the link below.

https://www.scientificamerican.com/article/fact-fiction-glass-liquid/#:~:text=Glass%2C%20however%2C%20is%20actually%20neither,for%20changes%20to%20be%20visible.

Here's an English sentence for you all: "would the musical pitch of a drop of bituminous pitch drop when the bituminous pitch was dropped on a football pitch at -40 deg C and cracked?"

I'll get my coat………

Edited By Jeff Dayman on 08/01/2021 00:30:50

Alan

I don't know the mechanism affecting the coiled glass but I do know that a similar effect can be seen in suspended lengths of most metals, plastic, wood etc. The thinner the section and the longer the unsupported length the greater the resulting bend.

The force acting of course is gravity but how the molecular structure responds ( differently I suspect in each of the above materials ) is beyond my mostly retired pay grade.

Pero

Hi Nick,

I don't think your assertion that glass is not a supercooled liquid is correct. If you look at the ternary phase diagram of the system Na2O – CaO – SiO2 (soda glass) there are a whole host of different compounds which can crystallise from it, most notably devitrite – Na₂Ca₃Si₆O₁₆. If molten glass is slowly cooled from above the liquidus temperature and held at or just below it, the mass will crystallise. The fact that it hasn't in your windows, is that the rate of cooling below the liquidus is such that the viscosity quickly rises to a level which prevents the atoms in the glass from migrating to take their places in a crystal lattice. Normally, a supercooled liquid is formed when the liquid is completely free of impurities which can act as seeds for crystal nucleation, and the system remains liquid below the liquidus temperature shown in its phase diagram. I would suggest that glass is in the same state – i.e. amorphous (non – crystalline) below the liquidus temperature.

As far as other materials (steel or wood for example) also becoming curved when supported at their ends, I would make the following argument.

I believe that liquid flow is the result of the elements within the substance rearranging themselves at an atomic or molecular level. This would be the case with an amorphous material such as glass. It would not however be the case in steel, where the rearrangement would involve crystalline components sliding past each other or wood which would involve the fibres sliding past each other.

Of course my argument is predicated on my definition of liquid flow which could be a load of bollocks.

Regards,

Alan