Is a glass solid or a liquid?

Lot of times we hear that the glass panes in the old buildings like churches, are thicker at the bottom. People believe that glass is a liquid and with time, it flows to the bottom of the glass panes making it thicker. But the question is, is glass really a liquid or is it a solid? The belief about the glass pane is actually not true. During the Medieval age, the glass panes were made by the process of Crown Glass, in which, they used to take a small amount of molten glass which was then rolled, blown, expanded, flattened and spun and then finally were cut like panes. These panes were mostly thicker towards their edge and while installing them, the side which was heavier, was put at the bottom. Although there are other techniques as well to make glasses, but the float glass technique produces glasses of good quality which are flat. Now to know if a glass is liquid or a solid, we need to understand the material as well as the thermodynamic properties of glass.

Thermodynamic properties

Few people say that glass is a result of a super cooled liquid due to the lack of first order phase transition while it cools. As a matter of fact, a difference can be seen as there is a second order transition which happens between the super cooled liquid state and the glass state. This transition is not as prominent as it happens when a liquid changes to a crystalline solid. Also, there is no discontinuous change of density and also no latent heat of fusion. This transition can be seen as the change in the thermal expansion as well as heat capacity of the material.

The transition into glass takes place at a temperature which can change with the speed of the cooling process of the material. In case the material cools slowly, it gets more time to relax, the transition takes place at a lower temperature which helps in making denser glasses. In case it cools very slowly, crystallization will occur. Hence, there is a minimum limit to the glass transition temperature.

The transition to glass from a liquid state, is purely a kinetic change. Hence, the glasses with disordered molecules, do not possess so much of kinetic energy so that it can overtake the barriers of the potential energy which is needed so that the molecules are able to make a free movement past each other. The molecules of the glass arrange themselves in a fixed but in a disordered way. Glasses and super cooled liquids, both are considered to be in a metastable phase instead of a true thermodynamic phase as that of a crystal. In principle, a glass could undergo spontaneous transition to a crystalline solid at any time. Sometimes if impurities are present, the old glasses become opaque.

In fact, glasses change from being a super cooled liquid to an amorphous solid at the glass transition, but justifying this, becomes very difficult. Polymerized materials like rubber have a visible glass transition at low temperatures; however, they are seen as solid both in glass as well as in a rubber condition. So, at times it is also said that glass is neither considered a liquid nor a solid. It is, in fact, of a different structure which is neither a solid nor a liquid. But this is not agreed by everybody.

Material properties

Whenever somebody talks about solid or liquid, they actually talk about their macroscopic material properties and not their molecular arrangement. In fact, when it comes to glasses, it was known as a material, much before we could understand its actual molecular physics.

Solids show a property of elasticity when small stress is applied, where they initially deform but again come back to their normal position as soon as the stress is removed. Under strong stress some solids break while some behave like a plastic. Copper is one such substance which shows this property. The resistance to flow under the deformation of plastic is known as viscoplasticity, which is almost like viscosity except that there is a little stress called elastic limit, below which the property of plasticity is not found. Materials which show the property of plasticity do not flow; however, they may creep, i.e. they show deformity but only when constant stress is applied.

Hence, an arbitrary measure of viscosity or viscoplasticity is not a good way to differentiate solids and liquids. It can also be said that in case there is less shearing stress needed to produce a permanent deformation then it is a solid. This is another way of saying that solids are rigid. Thus, a definition of liquid will be, such material which has the ability to flow. In case it is put in a container, it flows to fill the lower part of the container first until its surface itself becomes flat.

There are even few materials which have a limited flow known as viscoelasticity. The material deforms elastically under stress. In situation where the stress is put for a longer time, the deformation becomes a permanent one even if the stress was small. Materials with viscoelasticity may flow slowly for some time but then it stops. Hence, in case of such behaviours, it becomes pointless to even try to make a differentiation between liquids as solids.

Glasses – Their types

In order to be sure that glass in the old windows did not flow, different properties of different glasses needs to be recognised. Pure silica can be used to make glass; however, fused silica has a high transition point of around 1200^oC due to which it becomes difficult to mould it into bottles or panes. Almost 2000 years ago, people had learned how to lower the softening temperature by adding lime and soda before heating, which resulted in a glass containing sodium and calcium oxides. The glass made by adding soda-lime, used for making windows as well as bottles, contain even other oxides, nowadays. It is not easy to measure the glass transition temperature for different glasses as it changes according to how slowly the glass is cooled. In case of glasses which are made up by adding soda-lime, a fast cooling can produce a glass transition at about 550^oC. It is thought that the minimum glass transition temperature is almost 270^oC. In case the glass is cooled very slowly, it can still be found as a super cooled liquid just above that temperature. Glass such as Pyrex, which is used for test-tubes and ovenware, is normally based on boro-silicates or alumina-silicates which withstand heating better and typically have a higher glass transition temperature. The glasses, for e.g. leaded variety, have a low transition temperature.

It is also said that the best example that show that glass does not flow is provided by the lenses in the telescopes, which, even after 150 years, show an excellent optical qualities. These lenses get spoiled if the glass gets deformed even slightly. In fact, normally the glass that is used in making windows and bottles are not the optical glasses. Boro-silicate or soda-lime can be the base of such glasses, which might also be mixed with different metallic oxides so as to enhance its thermal as well as optical properties. So, the best example that states that glasses do not flow are the old telescope lenses and mirrors, but there is very little evidence supporting that glass do not flow in old window panes.

Early glasses were used in making bottles and windows, and were normally made by adding soda and lime to silicates. At times, Potash was also added to it. There were other impurities also which made the glass softer than the modern soda-lime glass. Other compounds were added to give it colour or improve its properties. Many people claimed that such glasses deform because the glass has flowed slowly over the centuries. This is a normal myth among people; however, if checked closely it is seen that the property of flowing like that around as well as out of the frame, does not exist. In fact, the methods with which the glass panes were made at that time, show a more consistent deformation along with lot of imperfections. In some cases, gaps also appear between glass panes and their frames, but this is due to deformation of the lead framework rather than the glass. Other examples of rippling in the windows of old homes can be accounted for because the glass was imperfectly flattened by rolling before the float glass process came into use. It is difficult to verify with absolute certainty that no examples of glass flow exist, because there are almost always no records of the original state. Very rarely it is found that lead is present in the stained glass panes which makes them heavy by lowering their viscosity.

The difference between the solids and liquids which are highly viscous, do not have any exact definition. All such phases or states of matter are idealizations of real material properties. Nevertheless, from a more common point of view, glass should be considered as a solid since it is rigid according to our everyday experience. In any case, it is not proved that the glass panes of the old windows deform as a result of the flow of the glass through years and also it cannot be said whether a glass is a solid or a liquid.