Topic: Aging glass

[Paul ADK]

I once read somewhere that at least part of the explanation for wavy window glass was that glass is a liquid and, given sufficient time, it actually runs or sags from the effect of gravity.  Does anyone know how long it takes for that effect to visibly appear?  What brought this to mind now are a pair of Chinese or Japanese (I don't know which) glass shadow boxes.  I would not have thought the shell art to be that old, no more than fifty years or so at the outside, and quite possibly much younger.  When I took the frames apart to clean the glass however, both panes of glass were noticeably thicker at the bottom then the top and looked puddled like the window glass in a 100 year old house.   

Although the pictures do not show the effect, I have attached a couple of photos.  Any information will be greatly appreciated.

 

[Ivo]

There may be several reasons for thickness deviation in plate glass. The one you refer to is seen in old church windows, where glass has become thicker towards the bottom over the years. It depends on the glass composition, but you would not expect this effect to become noticable within a few hundred years.

The other is in the glass manufacturing process. Currently all plate glass is made by casting liquid glass onto a bed of molten tin, which ensures a very even thickness within very narrow tolerances. (Float glass, since the 1950s)

Before that, window glass was made by drawing - the plate was pulled upward from the tank. This also had a reasonable quality - but had size limitations.

Up to the beginning of the 20th century, cylinder glass was made. This glas was blown as a very large cylinder which was cut open lengthwise and allowed to slump flat onto a marble table before being cut to individual panes. This glass is somewhat uneven in thickness - and probably the cause oof the effect you saw.

An even older and less current procedure consisted in blowing a bubble, opening it and spinning it round into a large plate - which was then cut to panes. This would  have a more pronounced thickness variance.

[Pinkspoons]

Isn't moving glass a myth? At least on a visible / macroscopic level?

I always thought it was because the panes in old church windows were made unevenly, usually using the Crown glass process (the centripetal method Ivo mentions last), and that the thicker end of the glass was always, sensibly, put at the bottom.

[Ivo]

I didn't think it was a myth, even in view of
http://www.glasslinks.com/newsinfo/physics.htm
this article. I remember reading an article with an opposite view on the subject - and I know for a fact glass is not stable and will self destruct in a few thousand years (or sometimes in a fraction of a second).

[aa]

So far as I know there has never been any empirical evidence to support this theory. However, you could write to the Society of Glass Technology http://www.societyofglasstechnology.org.uk/cgi-bin/open.cgi?page=index and ask for a definitive view.

I think the theory is based on the premise that glass is supposed to be a liquid. That is not so. A glass is a supercooled liquid which is a different state of matter from a liquid.

As has been pointed out, if you were a craftsman fixing a pane of glass into a window and you had the option of placing it with the thick side on the base, that would be the logical choice.

The magical properties of glass, and the idea of glass flowing ever so slowly back into the ground, has been exploited for marketing purposes, from time to time, by glassmakers who have wanted to add mysterious dimension to their work.

Recent research into glass recipes has produced soluble glasses, that are designed to dissolve. The idea of some early glass changing state, because of inherent instabilities, is not outside the bounds of possibility.....

[Frank]

There must be a fully quoted discussion on this myth elsewhere on the board... several years ago.

[flying free]

http://math.ucr.edu/home/baez/physics/General/Glass/glass.html

adding article for interest as referenced on a more recent thread.
m

[aa]

A glass is a supercooled liquid which is a different state of matter from a liquid.

"There is no clear answer to the question "Is glass solid or liquid?".  In terms of molecular dynamics and thermodynamics it is possible to justify various different views that it is a highly viscous liquid, an amorphous solid, or simply that glass is another state of matter that is neither liquid nor solid.  The difference is semantic.  In terms of its material properties we can do little better.  There is no clear definition of the distinction between solids and highly viscous liquids.  All such phases or states of matter are idealisations of real material properties.  Nevertheless, from a more common sense point of view, glass should be considered a solid since it is rigid according to everyday experience.  The use of the term "supercooled liquid" to describe glass still persists, but is considered by many to be an unfortunate misnomer that should be avoided"

So now perhaps we should re-define glass as a super-cool solid!!!

[oldglassman]

 Hi ,
        Crikey I had thought this old wife's tale had been laid to rest a long time ago , yes glass is unstable and full of stress which is why a knock can result in complete destruction sometimes ,but to say that all glass will self destruct eventually I believe is completely incorrect , only if the chemical composition of the molten glass batch and the subsequent annealing was wrong then as was discovered when compositions of metal ie the addition of lead oxide in the 17thc to try achieve clear crystal glass were being played around with glass was discovered to "fly" ie explode on the shelves without any outside shock,eventually with more experimentation and advances in annealing was this more or less overcome in the 1680s, also crizzeling was a great problem for the same reason ,this can take may years to appear and can be progressive which eventually will lead to the object disintegrating completely,again this was a problem with chemical imbalance and annealing,but if all the elements of manufacture were right there is no reason why glass will not remain in its current state forever, look at how much Roman period glass there is ,made all over Europe and still in perfect shape , I often wonder if those who thought that window glass slumped over the years because the glass is still fluid had given any thought to wine glasses/goblets that are several hundreds of years old with far greater weight than a small window pane and have not slumped , maybe one day my shelves of huge heavy baluster goblets will be reduced to blobs

cheers ,
 Peter.

[flying free]

Peter, I added this article to the thread yesterday
http://math.ucr.edu/home/baez/physics/General/Glass/glass.html

it concludes (my underlining)

'Conclusion

'There is no clear answer to the question "Is glass solid or liquid?". In terms of molecular dynamics and thermodynamics it is possible to justify various different views that it is a highly viscous liquid, an amorphous solid, or simply that glass is another state of matter that is neither liquid nor solid.  The difference is semantic.  In terms of its material properties we can do little better.  There is no clear definition of the distinction between solids and highly viscous liquids.  All such phases or states of matter are idealisations of real material properties.  Nevertheless, from a more common sense point of view, glass should be considered a solid since it is rigid according to everyday experience.  The use of the term "supercooled liquid" to describe glass still persists, but is considered by many to be an unfortunate misnomer that should be avoided.  In any case, claims that glass panes in old windows have deformed due to glass flow have never been substantiated.  Examples of Roman glassware and calculations based on measurements of glass visco-properties indicate that these claims cannot be true.  The observed features are more easily explained as a result of the imperfect methods used to make glass window panes before the float glass process was invented.'

Do you think there is something wrong with the article?  Should it be removed from the thread again?

m