I know Cecil’s column on glass was a classic from 1982, but I have a question that dates back to then. In the early 80’s I lived in Europe becasue of my father’s job, and my parents took us everywhere. I remember going to an old castle, and there was a display of a very old window pane. The display noted that the glass was much wider at the bottom than the top, and suggested that the glass, being liquid, had flowed down the pane. The column said that it would take millions of years for this to happen, so what’s going on?
Welcome, FreeTheBirds.
It’s customary to link to the column so that others can follow along. It’s as simple as cutting and pasting the URL. (You can pretty it up if you want)
http://www.straightdope.com/columns/read/110/how-come-you-can-see-through-glass
Tour guides, and possibly whomever wrote that bit of mis-information, are horrible sources of information. Cecil, as usual, is correct.
Nothing happened - they started that way.
It’s generally believed that the crappy glassworking tech of the time did not permit perfectly even sheets of glass. There was always a thicker edge, and for structural reasons, panes were installed with this thicker edge down.
Aw hell, as preventive maintenance, every ten years or so, I’ve been turning all of my windows upside down in my house, so I wouldn’t have to go get new ones.
Not a question of “generally believed”. The fundamental method in use until about 1800 was to spin a blob of molten glass until it flattened into a disk, a method that always produces a thickness gradient. Later methods were to cut and flatten blown-glass cylinders, to run glass between rollers, and to dip a rod lengthwise in a pool of molten glass and draw it up like a flat “soap bubble”. The modern method, floating molten glass on molten tin, didn’t come into use until the 1950s.
Actually, one method once glass-blowing was common, was to have a mold shaped like a cube. The blower took a lump of molten glass, and blew it into a bubble, then stuck it into the mold and blew until he had a cube. When it cooled enogh to handle, they cut the sides into panes. hence those tiny little panes, leaded together like stained glass, that you see on very old houses, especially in Europe.
The bottom square was less clear, and had a ringed pattern from forming the bottom of the bubble. This was considered much less desirable then, and used in cheaper applications. Today “bottom glass” is often found (and imitated) as decorative panes.
Or so I was told once…
Simply put, glass doesn’t flow. At least not more than many metals. Lead is a billion times less viscous than glass, but it isn’t called a liquid nor does it flow in a manner anywhere like viscous liquides.
The viscosity of glass is so high that it doesn’t represent any physically significant reality. It simply isn’t appropriate to assign a viscosity number to glass when it has solidified. The chemical bonds in glass while irregular and not crystalline, do have extremely strong bonds that prevent flow in any practical sense.
As for why light passes through glass, it isn’t because it is a liquid. There are many crystalline transparent substances that bear no physical relationship to liquids yet they pass light perfectly fine too.
I remember a scientific documentary several years back that looked into the question of what glass is, and of course the question is really what is glass at a certain temperature. One of the interesting things that they showed in the program were stain glass windows of extremely old churches where holes had started to form in the center of the panes and the glass itself looked like melted wax. They did go onto mention that the windows were placed to be in direct sunlight as often as possible and that they probably held a consistently high (non room) temperature.
Debate erupts over the liquid nature of glass and Uncle Cecil’s answer
The debate breaks out again shortly thereafter.
Minor, brief discussion of the same thing one month later.
Enjoy the reading. 
Just to follow up (I’m not going to post the three OTHER links I have found, including the ones to threads no longer in the database), in one thread here, before Unca Cece posted his explanation weaseling out of being “wrong” on glass, he admitted that glass is a solid. We also had a Mailbag article (for newer members/guests, that’s the precursor to the Staff Reports) in which SDSAB member David B. chimed in on the issue on the solid side.
Over 10 years now, and we still see this debate flare up whenever the old column resurfaces. 
I scanned the most recent thread but and I don’t care about how glass is classified. I just would like to know if, in fact, glass flows or not. Was there ever a consensus?
If it’s not in a furnace is doesn’t flow.
Yeah. Regardless of whether glass is a solid or a liquid, the question was why you could see through it, and I couldn’t believe Cecil’s answer. You can see through glass because it’s a liquid? Really? Can you see through Merlot? Magma? How about mercury, can you see through mercury? And if you want to keep something hidden from prying eyes, should you hide it behind a nice solid sheet of clear plastic? Or behind walls of quiescently-frozen ice? Is bromine a solid? Is lead crystal a liquid?
I’m not sure I understand this. Holes obviously didn’t “start” to form in the panes. Did they explain what they assumed was happening?
Hmm, fascinating stuff, but the logical questions do often follow. For instance, why can we see through cellophane? and why we can’t see through mercury, are answered by the same simple fact. It is not solid or liquid that determnes see-through-ability, it is TRANSPARENCY.
Until I read the column and follow ups, I was certain glass flowed at a much higher rate. Empirical evidence such as the original window panes in my Grandmother’s 1850’s home suggests this due to very clear and thin top parts, while the lower ends of the panes are indeed measurably thicker, wavy, and slightly distort the objects on the other side. It never occured to me that they had always been like that, due to old-timey manufacturing techniques and installation.
I’m now a wee bit wiser; my thanks to JFK, md2000, and Nametag. Oh, you too, Cecil.
ajdebosco is correct and Cecil is WRONG! There is no question at all about this judgment.
As ajdebosco said, "It is not solid or liquid that determnes see-through-ability, it is TRANSPARENCY. "
There are solids that you can easily see through (several different types of crystals are transparent at visible wavelengths) and there are many liquids that are completely opaque or reflective at visible wave lengths.
Yes, it’s astounding that, at this late date, there is still such disagreement over the nature of glass. I could probably get some folks to agree with me, if I said glass is really imaginary.
I applaud ajdebosco’s "It is not solid or liquid that determnes see-through-ability, it is TRANSPARENCY. " Even transparency is not so easily nailed down. Politicians claim to be for transparency, and their opponents call them fakers. Bill O’Reilly scoffs, “Transparency? Don’t be ridiculous. Transsexuals cannot be parents. Next, they’ll want to marry ducks.” :dubious:
I could always bring-up the Monty Hall probability question! :eek: Of course, the biggest problem with that one is that the correct response depends very much upon a set of assumptions or pre-conditions. Whereas for glass, it is purely a problem of physics.
“Marry, sir, they have committed false report; moreover, they have spoken untruths; secondarily, they are slanders; sixth and lastly, they have belied a lady; thirdly, they have verified unjust things; and to conclude, they are lying knaves.”