Also, I have heard all the classical “proofs” of it being debunked, except one. It came from an antique dealer saying that when you buy dirty glass antiques, don’t expect to be able to clean them completely as the dirt will have started to sink in the fluid glass. Is there any basis to it?
Do a search on Cecil’s columns. He addressed this. Glass is a fluid if you choose to define fluid in that way. That hasn’t been debunked. What has been debunked is the notion that glass flows at normal temperatures. It does not. Which answers your last question. Dirt can’t sink into glass because glass doesn’t flow.
I’ve heard that it’s because very old window panes tend to be thicker at the bottom than the top. The assumption was that over many decades the glass gradually oozed downward. In reality what happened is that it’s very hard to produce a pane of uniform thickness using traditional glassworking techniques. When glaziers would install the nonuniform panes they would put the thicker edge down to make the window more stable.
It may have originated from materials science, unfortunately. If you melt a pure crystalline solid such as a salt or an elemental metal, it has a precise, fixed melting point. On melting, its structure changes from an organised lattice in the solid to an amorphous mass of atoms in the liquid, and its properties change abruptly.
Glasses on the other hand have an amorphous structure at all times. They don’t have a fixed melting point. heat them up and they become bendy, then gooey, then runny… their viscosity decreases progressively with increasing temperature, but their structure doesn’t change.
So extrapolating this process in reverse, it’s tempting to regard solid glass as being the same thing as liquid glass, but at a temperature where its viscosity is very, very high.
This is a misconception though. However high the viscosity of a liquid, it will still flow under a load, or under its own weight. Just very slowly. And glasses do not flow at all at room temperatures. If they did, we could demonstrate it by experiment. Using interference patterns of reflected or refracted light, we could measure dimension changes in glass on the order of a wavelength. But we don’t need to, we have antique glass, and even better, volcanic glass tools, to inspect for dimension changes. (Broken glass is very useful in this respect, because we can see whether flow has affected the acute edges formed by fracture.)
As far as cleaning antique glass, it may be true that it cannot be effectively cleaned. There are chemical mechanisms that can degrade and stain glass, so if it’s been stored incorrectly, cleaning may not be possible. But as to dirt “sinking” into the glass, nah. Doesn’t happen.
I am aware of all the general conceptions about the glass as fluid idea. My question, though, is about the origin of this idea. Can we trace it back to someone, a time period? When did people start having this idea? It is widespread enough to be interesting as an example of a modern flat earth or cheese moon.
I don’t know whether this actually means any realistic chance of dirt or its components penetrating glass if left in contact for a long time, but it is possible for diffusion to occur in solids - it just doesn’t typically happen very fast at all.
I wouldn’t like anyone to take this and imagine that definitely means dirt can sink into glass though - semiconductor manufacture is an incredibly precise business and I have no doubt that the materials used are specifically chosen for their ability to diffuse into one another.
This was where I picked it up from as well – a friend who lived in a pre-Civil War era home with window panes as you describe. I was given the “glass as liquid” explanation.
One of my first posts on this board was regarding this “phenomenon”, backed up with Cecil’s pre-updated column on the matter. I actually got Cecil to post in my thread to semi-correct his original column
Dammit, though, glass should be a liquid! That was one of the coolest “facts” I remember learning as a child. Of course, some liquids are more viscous than others, and there is no reason (that I know of) that a substance couldn’t behave the way glass is erroneously thought to. So in the interest of redeeming my childhood, I think we should discover: is there such a substance? A common one?
That is very recent. Although if it was already being asked on “Ask …” columns, it probably was already a common idea. When did science start having a solid grasp (pun intended) on the idea of the internal crystalline structure of solids?
Crude oil. My dad worked on the oil industry. We have plenty of those acrylic blocks with crude samples trapped inside. Some of the heavier ones have been upside down since my childhood (20+ years ago) and they show no signs of wanting to drip down.