properties of glass

[shelli]

Glass is a liquid, not a solid. Take a window pane that has been in position for a couple of hundred years and look at it closely. You will always find that it is thinner at the top than at the bottom. Presumably, if you take it out and turn it around, it will flow back the other way over the next few generations

Is this true? - it came from a fairly reliable source (although he's a bit of a hippy!)

 

[Gervase]

It's a bit of an urbn myth, I'm afraid. Glass is an amorphous solid which doesn't flow at room temperature (otherwise our mediaeval stained glass would have fallen out by now).
There's a kernel of truth to the tale, as explained here, but no more.

 

[biffvernon]

One possible source for this urban myth is that crown glass, made by spinning a disc three or four feet across, is not of exactly even thickness. The disk gets thinner towards the edges. Panes cut from a disk of crown glass may be slightly thicker one side than the other and the glazier may have been accustomed to placing the glass heavy edge downwards. A few hundred years later someone noticed that panes were often thicker at the bottom and drew the wrong conclusion.

 

[shelli]

Biff - you are clever!

that sounds like perfectly sensible explanation to me.

 

[alice]

A very interesting article, Gervase.

I like to work with glass and have done some stained glass work and quite a bit of glass slumping and have used it as a sculpture medium. It's fascinating stuff.

In the article, it mentioned Bevin's Glass in West Virginia, US that still makes window panes in the old method for historical rehab. Is there a company in the UK that does the same thing? I've seen clear wavy glass in stained glass shops, but the waves are pretty regular and a bit TOO wavy and I don't know if it would be good for windows. It's pretty brittle. But I thought of replacing my transom lights with it over my front door. Maybe one day I might even replace my PVC windows!

alice

 

[Nemesis]

Tatra glass - does 'seed' or cylinder glass. Crown glass is very hard to come by now and is probably not right for your building.

Other firms also sell cylinder glass but I think Tatra is the least expensive.

 

[MdB]

From Wikipedia

http://en.wikipedia.org/wiki/Glass#Behavior_of_antique_glass

The observation that old windows are often thicker at the bottom than at the top is often offered as supporting evidence for the view that glass flows over a matter of centuries. It is then assumed that the glass was once uniform, but has flowed to its new shape, which is a property of liquid. The likely source of this belief is that when panes of glass were commonly made by glassblowers, the technique used was to spin molten glass so as to create a round, mostly flat and even plate (the Crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk would be thicker because of centrifugal forces. When actually installed in a window frame, the glass would be placed thicker side down for the sake of stability and visual sparkle. Occasionally such glass has been found thinner side down, as would be caused by carelessness at the time of installation.[citation needed]

Mass production of glass window panes in the early twentieth century caused a similar effect. In glass factories, molten glass was poured onto a large cooling table and allowed to spread. The resulting glass is thicker at the location of the pour, located at the center of the large sheet.[citation needed] These sheets were cut into smaller window panes with nonuniform thickness. Modern glass intended for windows is produced as float glass and is very uniform in thickness.

Several other points indicate that the 'cathedral glass' theory is misconceived:

* Writing in the American Journal of Physics,[8] physicist Edgar D. Zanotto states "...the predicted relaxation time for GeO2 at room temperature is 1032 years. Hence, the relaxation period (characteristic flow time) of cathedral glasses would be even longer" (Am. J. Phys, 66(5):392–5, May 1998). In layman's terms, he wrote that glass at room temperature is very strongly on the solid side of the spectrum from solids to liquids.
* If medieval glass has flowed perceptibly, then ancient Roman and Egyptian objects should have flowed proportionately more—but this is not observed. Similarly, prehistoric obsidian blades should have lost their edge; this is not observed either.
* If glass flows at a rate that allows changes to be seen with the naked eye after centuries, then changes in optical telescope mirrors should be observable (by interferometry) in a matter of days—but this also is not observed. Similarly, it should not be possible to see Newton's rings between decade-old fragments of window glass—but this can in fact be quite easily done.[citation needed]
* Glass in refracting telescopes, with objective lenses of large diameter, are observed to sag under their own weight (causing a loss of focus), but this is due to elastic deformation and not because of the glass flowing over time; this (along with chromatic aberration and other effects) limits the size of refracting telescopes, with the largest refractor in the world being the Yerkes Observatory telescope with a diameter of 102 centimetres (40 in).
* The "cathedral glass" phenomenon that is often cited as a demonstration of flow generally refers to old leaded glass windows in churches. The windows often appear to have sagged at their bottoms. On closer examination, it is found that the individual pieces of glass have remained flat, and that the bending has occurred at the soft lead "cames" that join the pieces. This bending may be largely due to successive thermal expansions and contractions of the glass over time, combined with the constant weight of the glass above. The lead cames are essentially plastic; that is, they tend not to recover their original shape after being distorted. Thus, successive temperature fluctuations are able to create progressive deformations, and the illusion of flow.

 

[Penners]

Glass is certainly a liquid at 1723 °C. But provided you keep your windows below that temperature, you should be OK.

 

[biffvernon]

From Wikipedia

http://en.wikipedia.org/wiki/Glass#Behavior_of_antique_glass

...the edges of the disk would be thicker because of centrifugal forces.[citation needed]

Hmmm...I don't think that is true. The good folk at Norgrave tried making some crowns and I'm pretty sure the ones I saw were thickest in the middle where the pontil was attached (the bull's eye) and the disk got thinner towards the edge. I would imagine that if the edge were thicker than the centre, the centrifugal force imposed when spinning would tear the hot sheet apart, with interesting results.

Despite the valiant attempts at Norgrove there is no production of Crown glass anywhere and probably noone who has the knowledge and skill to make a large sheet of the quality regularly achieved in the 18th century.

Cylinder glass is still made at Norgrave, and in Poland (available through Tatra Glass of Loughborough) and in Germany. The Polish glass, retails for about £80 per square metre.

 

[Penners]

thickest in the middle where the pontil was attached (the bull's eye) and the disk got thinner towards the edge

I'm sure that's correct. I've seen pieces of bullseye glass a number of times (genuine - not that ridiculous phoney stuff) and it certainly tapers in thickness away from the bullseye.

 

[Johneds]

The velocity at the edge of the disk is very much higher than at the centre - thus the edge glass thins much more than at the centre.

 

[Nemesis]

As with the base of antique drinking glasses.

 

[Penners]

A subject that is dear to your heart, no doubt.

 

[Nemesis]

I do have a collection of antique drinking vessels yes...

 

[Penners]

Personally, I find the age of the contents much more interesting than the age of the vessel.

 

[Nemesis]

I find booze tastes better out of a nice glass.

Even if it's bottle shaped.