A widespread urban legend is that stained glass windows in medieval cathedrals are thicker at the bottom because the glass flows slowly downward like a very viscous liquid. Appealing as it is, this is unfortunately a myth: Until the 19th century, panes of glass were often made by the Crown glass process. A lump of molten glass would be blown, flattened and finally spun into a disc before being cut into panes, resulting in sheets were thicker towards the edge of the disc. Other techniques of forming glass panes have been used but it is only the relatively recent float glass processes which have produced good quality flat sheets of glass. As to why most windows are thicker at the bottom and not the top: for structural stability, it would make more sense to install those thick portions in the bottom of the pane, wouldn’t it?
Another interesting observation is that astronomers have been making and using telescopes – with large glass lenses – for well over a century. These lenses need a shape that is accurate to about one tenth of a wavelength of visible light (≈ 10-10 m). But not one astronomer has ever complained that their old lens is unusable because the glass had deformed over time. So that settles the myth. But wait… there’s actually much more to this problem than windows!
Philip W. Anderson, a Nobel Prize-winning physicist at Princeton, wrote in 1995: “The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition… This could be the next breakthrough in the coming decade.” However, even until today, scientists still disagree, with some vehemence, about the nature of glass. To begin to answer the question “Glass: Liquid or solid?”, we will have to begin by looking at its material and thermodynamic properties:
Crystalline vs. amorphous solids
Glass is an amorphous material, that is to say non-crystalline – its microscopic structure is such that there is no long-range order in a glass. A glass can be seen as a three-dimensional network similar to that of a crystal, but in which only the short-range order is preserved. Compare, for example, the structure of the silica (SiO2) in its crystalline form (as found in cristobalite) and amorphous silica glass (Figure 1).
Because of its amorphous structure, glass under X-ray diffraction produces a diffusion halo, unlike crystals which give the narrow and intense peaks.
Many solids have a crystalline structure on microscopic scales with the particles arranged in a regular lattice. As the solid is heated the particles vibrate about their position in the lattice until, at the melting point, the crystal breaks down and the substance start to flow. There is a sharp distinction between the solid and the liquid state called a first order phase transition, i.e. a discontinuous change in the properties of the material such as density.
A liquid has viscosity, which simply put is a measure of its resistance to flow (water @ rtp: 0.01 poises, thick oil: ≈ 1.0 poise). Cooling a liquid increases its viscosity, which has a tendency to prevent crystallisation. Crystallisation usually occurs when a liquid is cooled to below its melting point, but it can remain as a supercooled liquid if there are no nucleation sites to initiate crystallisation. If the viscosity rises enough as it is cooled further, it may never crystallise, forming instead an amorphous solid. The molecules then have sufficient cohesion to maintain some rigidity, but are in fact in the same disordered arrangement as was in the liquid!
Some people claim that glass is actually a supercooled liquid because there is no first order phase transition as it cools. However, there is in fact a second order phase transition between the supercooled liquid state and the glass state, so a distinction can still be drawn. The transition is not as dramatic as the phase change that takes you from liquid to crystalline solids. There is no discontinuous change of density and no latent heat of fusion. The transition can instead be detected as a marked change in the thermal expansivity and heat capacity of the material.
Thus, to sum up, the structures of “liquid” and “solid” glass are virtually the same, but thermodynamically they are not the same. To examine the thermodynamic properties in greater detail, we can look at this second order phase transition, otherwise known as the glass transition:
The glass transition
Glass does not have a precise structural setting point or melting point; rather, it has what’s known as a glass transition temperature, 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.
The temperature at which the glass transition takes place varies according to how slowly the liquid is cooled (Figure 2). If it is cooled slowly it has longer to relax, the transition occurs at a lower temperature and the glass formed is more dense. However, if it is cooled very slowly it will crystallise, so there is a minimum limit to the glass transition temperature.
Figure 2. Density as a function of temperature in the phases of glassy materials
A liquid to crystal transition is a thermodynamic one: i.e. the crystal is energetically more favourable than the liquid when below the melting point. The glass transition is purely kinetic: i.e. the disordered glassy state does not have enough kinetic energy to overcome the potential energy barriers required for movement of the molecules past one another. The molecules of the glass thus take on a fixed but disordered arrangement. Glasses and supercooled liquids are both metastable phases rather than true thermodynamic phases like crystalline solids. In principle, a glass could undergo a spontaneous transition to a crystalline solid at any time. Sometimes old glass devitrifies in this way if it has impurities.
Conclusion and further reading
It would be convenient if we could conclude that the glass transition clearly defines the boundary separating supercooled liquids from amorphous solids, but this is very difficult to justify. For example, polymerised materials such as rubber show a clear glass transition at low temperatures but are normally considered to be solid in both the glass and rubber states.
As of now, there is still no definitive answer to the question “Glass: Solid or liquid?” In terms of thermodynamics and material properties it is possible to justify various different views that it is a highly viscous liquid, an amorphous solid, or simply another state of matter that is in between. The difference is semantic, for all such phases or states of matter are idealisations of real material properties.
Note: For further reading, you may want to go to http://dracutplumbing.com/glass and scroll down to the section title “Glass transition” for a more detailed discussion comparing how specific volume and enthalpy vary with temperature for the liquid-crystal and liquid-glass transitions.
Other good articles and sources used: