The Pitch Drop Experiment

[Michael GilliganParticipant @michaelgilligan61133]

With thanks to Alan Charleston, who inadvertently led me to this ‘museum’

I would like to share: **LINK**

https://physicsmuseum.uq.edu.au/famous-pitch-drop-experiment

Such dedication !!

MichaelG.

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Special warning to Bazyle … The live video stream linked from that page could waste a lot of your precious time, so beware !

[Michael GilliganParticipant @michaelgilligan61133]

[Adrian DownesParticipant @adriandownes83818]

'live video stream linked from that page could waste a lot of your precious time, so beware !'

As would the experiment itself 🤣🤣🤣🤣

[old martParticipant @oldmart]

Interesting experiment, I don't think I will be trying it myself. I have heard similar said about glass, that it is not a true solid.

[Howard LewisParticipant @howardlewis46836]

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

[Grindstone CowboyParticipant @grindstonecowboy]

Now… the dangers of reading things on the internet about to be demonstrated…

I was told about the window glass thing by my school physics teacher, and never had reason to doubt him UNTIL I read "somewhere on the internet" that the super-cooled liquid story was a lot of nonsense and had since been disproved, it was just the way window glass was made back then. They quoted old church windows in Germany, but maybe they should have looked at Howard's house too (Sorry, don't have a link)

So the upshot of all this is I just don't know who to believe anymore, although I prefer to stick with what I've 'known' longer.

Rob

[old martParticipant @oldmart]

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.

[Howard LewisParticipant @howardlewis46836]

My information came from my A level Physics and Chemistry masters, (Long before computers, or Berners Lee inventing the internet ), which inspired me to examine the windows.

And they were visibly thicker at the bottom!

The effect is less marked in ancient glass, because the pieces of glass in leaded lights iare so much smaller. and lighter

I'd rather believe a Physics master than something on t'internet, unless the provenance can be proved.

Howard

[Grindstone CowboyParticipant @grindstonecowboy]

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

[Nick Clarke 3Participant @nickclarke3]

Whether glass is a supercooled liquid is untrue as one definition of such is a liquid where the substance remains liquid below the temperature where it transforms from a liquid with no rigid crystal structure to a solid with one. This transformation can take place in a supercooled liquid below the freezing point if a ‘seed’ of the crystalline form of the substance is introduced. As glass appears to be solid it does not fit the definition. A better term might be an amorphous solid – a solid which while hard and brittle does not have a defined structure.

I doubt if the ‘thicker at the bottom’ idea of glass is due to flow for many reasons. There is no flow over window frames, some examples such as old lenses, obsidian (natural glass) objects from the stone age, Roman or Egyptian glass objects do not exhibit flow. I agree with those people who say it is down to manufacturing defects. Why is it at the bottom? Well if I was lifting a sheet of glass it would be easier if the heavy end was at the bottom perhaps – who knows!

The main reason for saying no flow though is that the ultimate limit for flow in glass is the glass transition temperature Tg – above which glass is extremely viscous and below which it is brittle. Tg for Soda Lime glass is quoted on Wikipedia as 520-600C so any flow at normal temperatures is unlikely, and according to some theorists, impossible.

Regarding the validity of my comments they are based upon my training as an engineer and as an A level physics teacher – but as someone who has spent a lot of the last 30 years also teaching ICT I suggest that automatically rejecting information from the Internet is as wrong as accepting it without question.

Edited By Nick Clarke 3 on 06/01/2021 19:08:14

[Neil WyattModerator @neilwyatt]

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

[Mike PooleParticipant @mikepoole82104]

Well I don’t know how I am going to get over this, I have believed glass flowed for over 50 years and now my world is in tatters.

Mike

[Jeff DaymanParticipant @jeffdayman43397]

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

[Howard LewisParticipant @howardlewis46836]

Mike,

Shattered perhaps? Just crack on regardless

Howard

[Nigel Graham 2Participant @nigelgraham2]

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.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Nigel Graham 2 on 07/01/2021 20:39:01:

[…]

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 ? )

[…]

.

I’m not 100% certain, but I believe that encastré strictly applies to beams that are fixed [embedded] at both ends.

… That might be logically extended to the infinity of diametral lines in a circular blank, but in both cases it is presumed that those fixings are ‘mechanically earthed’

MichaelG.

[Sam StonesParticipant @samstones42903]

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

[Nigel Graham 2Participant @nigelgraham2]

Sam –

Your second sentence sent my mind off in a different direction again….

Pitch as in bitumen…? Fine.

Pitch as in sweet singing in the choir…? Fine.

Pitch as on threads and gears..? Fine.

Pitch as in a vertical shaft in a cave…? Errrr. That puts a rather different slant on the title " Pitch Drop Experiment " !

[Sam StonesParticipant @samstones42903]

The pitch was musical after all, Nigel.

Sam

[Jeff DaymanParticipant @jeffdayman43397]

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.

[Jeff DaymanParticipant @jeffdayman43397]

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 CharlestonParticipant @alancharleston78882]

Hi,

Glass does indeed flow over time. I used to work in a lab and there was a room set aside for glassblowing. About 20 years before I got there, the staff used to make glass coils for a gas chromatograph from straight lengths of thin wall soda glass tubing (OD about 4mm) which were about 6 feet long. These were stored in a wooden box which was about 5 feet long, so one end was in the box while the other end was supported by the top of the box. After 20 years or so, each of the tubes was permanently curved, with the middle of them being at least 6 inches away from a straight line between the two ends. This was presumably due the plastic deformation (flow) of the glass over that time.

If you want to see a really long term experiment, have a look at this:

https://en.wikipedia.org/wiki/Oxford_Electric_Bell

Truly amazing.

Regards,

Alan

[PeroParticipant @pero]

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

[Nick Clarke 3Participant @nickclarke3]

Posted by Alan Charleston on 08/01/2021 04:31:39:

Hi,

Glass does indeed flow over time. I used to work in a lab and there was a room set aside for glassblowing. About 20 years before I got there, the staff used to make glass coils for a gas chromatograph from straight lengths of thin wall soda glass tubing (OD about 4mm) which were about 6 feet long. These were stored in a wooden box which was about 5 feet long, so one end was in the box while the other end was supported by the top of the box. After 20 years or so, each of the tubes was permanently curved, with the middle of them being at least 6 inches away from a straight line between the two ends. This was presumably due the plastic deformation (flow) of the glass over that time.

My contention that glass does not flow and that it does not fit the definition of a supercooled liquid is supported by the science (where have I heard that before?) as I understand it so reading your post made me think.

I suspect that the glass was not flowing as if that were the case it would also be thicker at the bottom and develop a 'club foot' so some other mechanism might be at work. I consider this to be confirmed firstly by thinking about wood and similar materials which if stored as you suggest would likely also the have developed a set as you describe with no thought of them flowing.

Certainly bowing of a thin object could be caused by elastic and extraelastic deformation, but this also occurs in solids where the molecules, unlike in a glass, are bound in a lattice and the lattice itself is distorted. For flow to occur the molecules would probably need to move relative to each other.

[Alan CharlestonParticipant @alancharleston78882]

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

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