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The idea that glass after being formed is in a state of flux concerns the belief, held by many, that because glass is a "super cooled liquid" it actually has a degree of "flow" at temperatures you and I find comfortable. This urban legend would have us believe that the windows in colonial homes and in very old stained glass windows are thicker at the bottom than at the top because glass is a "supercooled liquid" and is slowly flowing. Ahh that pesky gravity. The acceptance of that story is not based on fact but on a mythological story that has been circulating for many many years. (I read it on the internet so it must be true). There was a time, in the dim dark past that, in my ignorance I believed in the myth of glass flow. Without question I accepted the myth of glass flow. My blind acceptance of what I thought passed for fact should not be a trait of the curious mind. What could I have been thinking when I accepted the "fact" that the Rose Window would soon spill out of the confines of the lead cames that have held it in place all these years. Okay, lets get to the heart of the myth of glass flow, but before reading on do some research and look up the word "poise" and then when you understand what poise is then look up the poise of lead and glass. If you're still a believer in glass flow after that then Read On!

I was made aware of the fallacy of the glass flows myth many years ago by the late great glass chemist, Nick Labino. Nick offered this simple analogy, "...if the windows found in early Colonial American homes were thicker at the bottom than the top because of "flow" then the glass found in Egyptian Tombs should be a puddle." Wow, that was a shocker. And there's this one from Wikipedia, "If glass flows at a rate that allows changes to be seen with the naked eye after centuries, then the effect should be noticeable in antique telescopes.
Any slight deformation in the antique telescopic lenses would lead to a dramatic decrease in optical performance, a phenomenon that is not observed". I just love real world examples! For those of you that are still skeptical I cite research that tells us that although 1/2 of the glass in old stained glass windows is thicker at the bottom, take three guesses where the other half are thicker and the first two guesses don't count. You got it, sides and top. Hmmm, what do you suppose that indicates? In other words, if glass flowed then ALL the glass in antique windows would be thicker at the bottom and research tells us that's not true. If you still don't believe it, read below what the brainiacs have to say. When that know - it - all antique dealer whips out the urban legend of glass flow, send him or her to this page. After they read all the information below and they still refuse to believe that glass doesn't flow have them email me with a reason other than "thicker at the bottom."

Finally, Dr. Neumann relates the following. Read this carefully: "In other words, while some antique windowpanes are thicker at the bottom, there are no statistical studies to show that all or most antique windowpanes are thicker at the bottom than at the top. The variations in thickness of antique windowpanes has nothing to do with whether glass is a solid or a liquid; its cause lies in the glass manufacturing process employed at the time, which made the production of glass panes of constant thickness quite difficult."
What Dr. Neuman and Labino is saying and is that if glass flowed, all the glass that comprised antique windows should be thicker at the bottom, but we know that is just not true.
Analysis Shatters Cathedral Glass Myth
By C. Wu
A new (not so new anymore) study debunks the persistent belief that stained glass windows in medieval cathedrals are thicker at the bottom because the glass flows slowly downward like a very viscous liquid.

Edgar Dutra Zanotto [1] of the Federal University of Sao Carlos in Brazil calculated the time needed for viscous flow to change the thickness of different types of glass by a noticeable amount. Cathedral glass would require a period "well beyond the age of the universe," he says.

Suffice it to say that the glass could not have thickened since the 12th century. Zanotto reports his finding in the American Journal of Physics -- May 1998 -- Volume 66, Issue 5, pp. 392-395.
The study demonstrates dramatically what many scientists had reasoned earlier. "You would have to bring normal glass to 350 deg. Celsius (662 deg. F.) in order to begin to see changes," says William C. LaCourse, assistant director of the NSF Industry-University Center for Glass Research at Alfred (N.Y.) University.

Viscosity depends on the chemical composition of the glass. Even germanium oxide glass, which flows more easily than other types, would take 10^32 years (100,000,000,000,000,000,000,000,000,000,000) to sag. Zanotto calculates. Medieval stained glass contains impurities that could lower the viscosity and speed the flow, but even a significant reduction wouldn't alter the conclusion, he remarks, since the age of the universe is only 10^10 (100,000,000,00).

The difference in thickness sometimes observed in antique windows probably results from glass manufacturing methods, says LaCourse. Until the 19th century, the only way to make window glass was to blow molten glass into a large globe then flatten it into a disk. Whirling the disk introduced ripples and thickened the edges.[2] For structural stability, it would make sense to install those thick portions in the bottom of the pane, he says.

Later glass was drawn into sheets by pulling it from the melt on a rod, a method that made windows more uniform (Foucault process). Today, most window glass is made by floating liquid glass on molten tin. This process, developed in the 50's by Sir Alastair Pilkington, and named for him makes the surface extremely flat. (Pilkington process)

The origins of the stained glass myth are unclear, but the confusion probably arose from a misunderstanding of the amorphous atomic structure of glass, in which atoms do not assume a fixed crystal structure. "The structure of the liquid and the structure of the [solid] glass are very similar," says LaCourse, "but thermodynamically they are not the same."

Glass does not have a precise structural setting point or conversely a melting point; rather, it has what's known as a glass transition temperature (a temperature when it begins to undergo a viscosity change), typically a few hundred degrees Celsius. Cooled below this temperature, glass retains its amorphous structure yet takes on the physical properties of a solid rather than a supercooled liquid.

"At first, I thought that the [sagging window idea] was a Brazilian myth," Zanotto wrote, but he soon learned that people all over the world share the belief. Repeated in reference books, in science classes, and recently over the Internet, the idea has been repeatedly pulled out to explain ripply windows in old houses. "For the lay person, it makes a lot of sense," says LaCourse.

In 1989, Robert C. Plumb of Worcester (Mass.) Polytechnic Institute suggested in the Journal of Chemical Education that definitive proof might require an instruction book written in the Middle Ages advising glaziers to install glass panes with the thick end at the bottom. Now if only such a handbook could be found.
From Science News, Vol. 153, No. 22, May 30, 1998, p. 341. Copyright © 1998 by Science Service.
[1] Edgar Dutra Zanotto is a Brazilian materials engineer from the Universidade Federal de São Carlos (Federal University of Sao Carlos) (UFSCar) in Brazil. He currently teaches glass related subjects in that University for both graduation and post-graduation as he is the head of LaMaV, the Vitreous Materials Laboratory. In May 1998, Zanotto wrote an article in the American Journal of Physics relating to the false notion that observations of thick glass in old windows translated to the fact that glass is a liquid. Zanotto sought to calculate the flow of glass and found that at 414 Celsius (777 °F) the glass would move a visible amount in 800 years, yet at room temperature he found that it would take glass 10,000 trillion times the age of the earth.

[2] Editors note: Mr. Wu has it backwards, when you spin a disc of glass into what is known as a rondel the disc is thinner at the edge, not thicker, and thickens as you move toward the center.
This illustration depicts a Glass Crown Disc and the resulting layout of windows cut from it. As you can see from the side view the disc gets thicker toward the center. The most coveted panes were the ones closest to the outer edge as that is where the glass is thinnest. Crown disc's can have diameters of 4 ft. (1.2 meters). A rondel is much smaller and usually has a diameter of about 1.5 ft. (45 cm) more or less.
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Origin of an Urban Legend?
©1996 Florin Neumann

How did the "glass is a supercooled liquid" urban legend originate? It is possible it began with an erroneous reading of an influential book by Gustav Tammann (1861-1938), a German physicist who was among the first to study glass as a thermodynamic system (Tammann, 1933). I was unable to locate a copy of Tammann's book to verify this, so the following is speculation. One or two papers I consulted attributed to Tammann the statement "Glass is a supercooled [or undercooled] liquid." But, from other papers, it appears that what Tammann actually wrote was "Glass is a frozen supercooled liquid" [my emphasis]. My speculation is that an author misquoted Tammann, and this misquotation was repeated by later authors who, since copies of Tammann's book are rather rare, did not refer directly to Tammann.

Until about 20 years ago supercooling a glass melt was the only way to obtain glass, and the behavior of melts as they passed through the glass transition (i.e., solidified) was very different from crystallization. But solid-state physics was almost entirely based on the study of crystalline solids, which made the behavior of glass melts appear paradoxical. To emphasize this a professor would state "Glass is a liquid which has lost the ability to flow", and some undergraduate, with his mind more on the Friday night date than on the physics of glass, would remember only "glass is a liquid"... Perhaps now we can finally put this legend to its well-deserved rest.

What does the Corning Museum of Glass have to say?


Glasses are amorphous solids. There is a fundamental structural divide between amorphous solids (including glasses) and crystalline solids. Structurally, glasses are similar to liquids, but that doesn't mean they are liquid. It is possible that the "glass is a liquid" urban legend originated with a misreading of a German treatise on glass thermodynamics.

Now that you've gone through this page and still remain skeptical I would recommend you click on the links below and read up on the physics of the question. Reading what these gentleman have to say requires that you know a bit about physics or maybe more than a bit.

Florin Neumann Link
Phillip Gibbs Link (This is a must read)
Robert C. Plumb Link
Dr. Brill's paper is must read if you want conclusive evidence of why glass does not flow.
Download the file by clicking on the link below.
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This article from the NY Times deals with the enigmatic aspects of glass. A brief quote from the article tells us. “We don’t understand why glass should be a solid and how it forms.” The article was brought to my attention by Scott Novota on the CraftWeb glass forum.
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If, after reading all of the above information you're still a skeptic and have proof that glass flows, I have provided a space for your experiment that proves glass does flow.
Send me your experiment and I will submit it to my board of certified rocket scientists for peer review. If they feel your experiment passes a rigid set of scientific standards I will post it to this site. Below are two experiments that have met our rigid standards. I post them for your consideration.
Dear Sir, Madam,

Concerning the article "The "Glass Flows" Myth" I experimented (observed) that flat glass panels about 1 meter long horizontally adjusted on the top of an aquarium end up after about a few months with about half a centimeter sag on the middle part, after that the curvature could be observed whatever the position and could be reversed with a few months up side down! Temperature was sometimes going up to about 50 or 60 degree Centigrades because of the close proximity of the artificial UV lights. So from then flowing glass was not so strange to me, and I think cathedrals have special conditions :
1. Dark glass exposed to the sun can probably heat up a lot, may be 90 degrees C. (*a)
2. Being surrounded with dark lead would increase the heat.
3. The pressure on the bottom pieces could be very important because of lead flexibility and because of the great height of some stained glasses. Otherwise where does the 10 power 32 number came from? Otherwise, it would be very interesting to see some viscosity curves depending on temperature and pressure, I guess the glass viscosity for ambiance temperature could be interpolated
(*a) in fact if very dark south exposed glass panels get more asymmetric that the clear north exposed we'll get an interesting clue.

Best Greetings,
Name Withheld
I have experimentally proven that glass flows. About 30 years ago I placed a 20 ml pipette in the back of my toilet for safe keeping, as it is the best protected place in my house for such a thin and long piece of glassware. After about 15 years I noted that the glass rod had sagged under it's own weight, by around 1/4 of an inch in the center.

Name Withheld

Editors note: What I want to know is what else he stores in the back of his toilet and what was that pipette used for anyway!
This page is by far the most popular page on my site. The number below shows how many people have clicked through to this page. In the years that this page has been up I have received mail from a handful of skeptics, unconvinced if you will, that glass did not flow.
Editors note: Many people equate sag with flow. They are not the same thing. If the thickness of the glass changed then you would have something but even though glass can “sag” it is not changing thickness.

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