The Pitch Drop Experiment

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 !

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.

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

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

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.

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

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

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

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

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

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

Mike,

Shattered perhaps? Just crack on regardless

Howard

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.

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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.

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

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 " !

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.

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