Is Glass Liquid or Solid?

A controversy has erupted in many areas of science over the definition of glass. The old saying is that glass is actually a liquid because of windows from old buildings (built 150 years or more ago) have glass in it that is thicker at the bottom than at the top. The theory is that the "liquid" moves ever so slightly down from the pull of gravity, until when we see it today, the glass is thin where the gravity is pulling from. But is it really that simple? Is that even true?

There are a few things that have to be looked at before we can make any assumptions.

First, there's the properties of liquids and solids on the molecular level. Solids contain their molecules in such a way that looks like a lattice, with a pattern than looks the same no matter where you look at it and which is why solids are firm. Liquids contain their molecules in such a way to have no structure; the molecules are free to roam around, which is why liquids cannot be picked up by themselves. It is this molecular structure that usually shows the difference between a liquid and solid. Glass, however, is unique in this area. Glass has a crystalline lattice for its molecules just like a solid does, but the difference is that instead of it being a perfectly structured lattice like ice would be, it is misshapen. So in one argument, it is rigid like a solid, but it is random like a liquid.

Molecular Lattice of a Solid

Molecular Structure of Glass

Second to consider is viscosity. Viscosity is the measurement of a liquid's resistance to flow. For example, vegetable oil has a higher viscosity than water. Temperature has a high impact on viscosity also, because typically the warmer a liquid is, the lower its viscosity. The phrase "slower than molasses in winter" is used to show how slow something is because the viscosity of molasses in a cold environment is very high. The interesting thing is that sometimes viscosity has the tendency to prevent solidifying a liquid. In water's case, when water goes before its freezing point (or alternatively its melting point for ice), it crystallizes and becomes a solid. However, in some cases, viscosity can prevent a liquid from crystallizing. In those cases, the liquid is considered to be "supercooled" and remain liquid past the freezing/melting point. In these supercooled liquids, if the viscosity rises high enough, the liquid gets thicker and thicker, but never crystallizes, and instead is considered to be an amorphous solid. In these amorphous solids, the liquids retain some rigidity.

Third to consider is the different methods of making glass. In the glass that most people assume to be a liquid because of the change of thickness from top to bottom, you would find it to be very old - from the medieval times through the nineteenth century. Because it is old, most people assume that this glass process of being pulled down by gravity over hundreds of years is the way that it happens. If indeed that was true, then it would be a process that couldn't be proven in a lab. In actuality, the method of glass-making has changed since then. Back then, the method was the crown glass method, which involved a lump of molten glass that is rolled, blown, expanded, flattended, then spun into a disk before it was cut into the desired shapes. The problem with this was two-fold: it limited the size a pane of glass could be, and it didn't distribute the glass evenly so many pieces of glass had a thicker side of glass than another side. Even though there have been other glass-making procedures, only recently has the float glass method been used, which ensures an even thickness of glass.

So, when you combine all that together, it still is an interesting debate. The method of making glass in history provided uneven glass which accounts for the differing thicknesses of the glass, so that should have dismissed the idea of the solid vs. liquid debate. But it doesn't because of the molecular structures and viscosity elements to the argument makes the debate even more interesting.

And once again, the question remains so it is asked again: is glass a liquid or solid? Many people (prominent scientists even) still debate that to this day. In the end, one could call glass a solid for the simple fact that it is rigid and feels solid. But also, one could call glass a supercooled liquid because of its extremely high viscosity. But then, it could also be called a solid because it has a crystalline structure. But it's an amorphic structure that's characteristic of supercooled liquids. So, one could call it either a solid or a liquid, but it's not a liquid because of the "thickness differentiation" theory. Some people have even called glass a fourth state of matter that's neither solid nor liquid but having properties of both.

Either way, I think it's just semantics. And until the scientists come up with an exact definition, we can debate all day long what it is and isn't.

Bet you didn't know that!