Is Glass a Liquid?

Glass is probably one of the most commonly used materials, known to us. Right from simple cookery utensils to eye spectacles, lighting systems, construction works (doors, windows), camera components, and sophisticated machinery, it is used in every aspect of life.

Even though we have been using it in so many ways, not many of us actually know whether it is a solid or a liquid.

The primary component of glass is a large proportion of silica, along with smaller amounts of soda (sodium bicarbonate or potash), and lime. In nature, these particles, when subjected to lightning and volcanic eruption, lead to its formation in the form of beads, specks, and other shapes. Basically, all types are available in optically transparent forms and are characterized by very high viscosity and susceptibility to breakage, except for few types (e.g., borosilicate).

Each of the component elements plays a crucial role in maintaining the amorphous solid state of glass. Like for instance, soda reduces melting point, and lime stabilizes the chemical composition of the resulting compound.

During commercial production, additives are added in specific concentrations to impart certain characteristics, such as color, luster, reflectiveness, and hardness. Also, the production steps, which include heating to a very high temperature (about 1700° C ), cooling time and treating time, are monitored strictly to get the desired characteristics.

The origin of this common physics question is attributed to two main sources - the first is that glasses used in very old churches are thicker at the base than the top portion and the second is with reference to the statement, 'glass is a frozen supercooled liquid', mentioned in a book, written by Gustav Tammann (1861-1938), a German physicist.

In an explanation to the first source, scientists are of the opinion that glasses used in those times were manufactured by the Crown glass process, which resulted in thick panes which were not uniform.

The heavier side or the thicker side was oriented at the bottom. Thus, the bottom portion is relatively thicker due to production error, not because of its liquid state.

One of the main features of glasses is that they are amorphous solid material, meaning the constituent atoms are not arranged in an orderly manner. In crystalline solids, the atoms and molecules are assembled in a proper manner, which at the time of melting, move and flow freely.

Thus, there is a distinctive demarcation between the two phases, solids and liquids in crystalline substances. And each phase has a distinct density and viscosity, which become higher as the liquid cools down.

In case of glass, the molecules even after cooling down below its melting point, do not crystallize, because of the lack of nucleation sites. With the rapid increase in viscosity, the molecules solidify slowly and form a thick amorphous solid.

No doubt, the molecules that make up it, are arranged in an disordered fashion, but there is sufficient cohesion between them to exhibit a solid, yet amorphous state. This disordered arrangement of molecules gives glass, its characteristic reflective property.

Thus, we have seen there is no definite answer to whether glass is a liquid or solid. Strictly speaking, we can assume it to be in a phase that is neither liquid nor solid. Or else, it is an intermediary state of matter between an ideal liquid and an ideal solid, wherein the molecules are disorderly arranged, but with sufficient amount of rigidity to hold the molecules together in a specific shape.