Glass Flow
The "Glass Flows" Myth
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. Those who believe this urban legend point to the fact that the windows in colonial homes and in old stained glass windows are thicker at the bottom than at the top. 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 idea 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 this 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." A Great visual don't you think. 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? 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."
Analysis Shatters Cathedral Glass Myth
By C. WuA new 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 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. 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. Today, most window glass is made by floating liquid glass on molten tin. This process, developed about 30 years ago, makes the surface extremely flat.
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.
Origin of an Urban Legend?
©1996 Florin NeumannHow 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.
From "How Everything Works" (link below)
Q: I have read that very old panes of glass become thicker at the bottoms than the tops. Doesn't that show that glass flows? — MJ
A: While it is sometimes noted that old cathedral glass is now thicker at the bottom than at the top, such cases appear to be the result of how the glass was made, not of flow. Medieval glass was made by blowing a giant glass bubble on the end of a blowpipe or "punty" and this bubble was cut open at the end and spun into a huge disk. When the disk cooled, it was cut off the punty and diced into windowpanes. These panes naturally varied in thickness because of the stretching that occurred while spinning the bubble into a disk. Evidently, the panes were usually put in thick end down.
Modern studies of glass show that below the glass transition temperature, which is well above room temperature, molecular rearrangement effectively vanishes altogether. The glass stops behaving like a viscous liquid and becomes a solid. Its heat capacity and other characteristics are consistent with its being a solid as well.
Conclusion
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.
http://dwb4.unl.edu/Chem/CHEM869A/CHEM869ALinks/www.ualberta.ca/~bderksen/florin.html
http://math.ucr.edu/home/baez/physics/General/Glass/glass.html
http://dwb.unl.edu/Teacher/NSF/C01/C01Links/www.ualberta.ca/~bderksen/windowpane.html
http://www.howeverythingworks.org/pages.php?topic=windows_and_glass&page=1
Read what Dr. Robert Brill of Corning Research has to say. Download the pdf file below.
