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Glass is a liquid
Or is it? Read on...

Strange as it may seem, glass is a liquid. How can this be? It is surely a brittle transparent solid? But no, as surely as a dolphin is not a fish, glass is not solid but liquid. To understand this, it's important to see that liquids aren't all the same as each other. (But hang on a bit, this can't be right, can it? Note: Please read the whole page before passing comment on this!) Water, the obvious typical liquid which others are compared against, isn't the same as liquid nitrogen or liquid metal mercury. More notably in the strange business about glass, syrup and treacle are notably different to water. It's much more difficult to stir treacle. This property of being difficult to stir is known scientifically as Viscosity. Some liquids have a very high viscosity and flow so slowly that by the time a can of liquid falls over there is time to pick it up before it spills! The ultimate example of a high viscosity liquid is glass. The viscosity of glass is so high that even though it's a liquid it looks like a solid. But if you could see glass from the comfy seat of a time machine, you'd see glass flow like syrup.

If this seems too strange to be true, take a look at reflections in window panes and glass on framed paintings that are very old. Glass that's a hundred years old is rippled. Two hundred year old glass has a distinctively undulating surface. You can use this knowledge to tell how old some antiques are, and how old the glass in the windows are on some buildings.

Taken to its extreme, if you had a large cup made of stone which could be left undisturbed, if you put pieces of smashed glass into the cup and left it there, if you could come back in a million years, you'd find the glass had flowed and taken a form like ice frozen from liquid water.

However, as with many of these things, it's not as simple as that. There are different beliefs about this, ranging from the totally liquid to the totally solid. Part of the doubt comes because of the notion of "cathedral glass" being originally stated by tourist guides to be liquid because it's thicker at the bottom than the top, and then later this idea being scotched because some of the panes are the other way up, suggesting the phenomenon to be more to do with imperfections in the glassmaking!

Antique glass windowpanes have undulating surfaces, and this has also been variously attributed to the "liquid" theory or to the "imperfections" theory.

One thing that's certain is that it makes a difference what type of glass is considered. Glassware a few thousand years old is sometimes apparently less morphed by time than windowpanes 100 years old. This and other anomalous evidence have resulted in some claims that the "glass is a liquid" theory is entirely false and that glass is not a liquid at room temperature.

So, rather than just believing one theory or another because of the ever-changing fashion of belief, it would be good if the room temperature liquid glass theory was to be put the the test. A scientific experiment could be done, and it needn't take hundreds of years. A thin pane of window glass could be placed on a table with a third of it extending over the edge, and a laser bounced off the surface to see if, over a period of a few years, the glass bent by a small amount. As it's possible to measure distances very accurately using a laser, it should be possible to prove one way or the other whether glass is a liquid or not.

Other experiments into measuring the viscosity of glass include such ideas as: 1. Getting two thin glass rods fixed horizontally at one end, hang a weight on one of them, and see if over a period of months there is a difference of bending. 2. Mounting a glass rod between two pillars and hanging a weight on a cheese-wire over it to see if after a few years any impression has been made. 3. Taking a hologram of a windowpane and then another hologram of the same thing after a year to see if it's "flowed" (accurate to fractions of a wavelength of light).

It would also be interesting to find out if fibre-optic lamps start to droop more over the years!

If it's found that glass is not liquid at room temperature at all, then the next question is: What is it's true melting point? At what special temperature is it no longer truly solid? Some guesses in the "solid glass" persuasion put this at about 300 degrees C.

Or, in the other belief/theory, in which glass is a very thick liquid at room temperature, then would glass be a liquid at any temperature? How about at liquid nitrogen temperatures?