ORIGINALLY POSTED IN FEBRUARY 2015
It's been a while since I wrote that glass is a solid and not, as some people believe, a liquid. It's a conversation I have quite regularly these days when people ask what I do for a living and I explain that I melt glass in a kiln to create art. A common reply is something along the lines of "oh yes, because glass is a liquid isn't it". This is usually said in the form of a statement rather than a question. What has brought me to reaffirm my point in writing was an appearance last week on BBC Radio 2 of a marketing manager from the UK's most well known window glass supplier, Pilkington, who stated that glass was a supercooled liquid...
It amazes me that this notion is still being repeated but since it is I will copy, below, the dictionary definition of glass which clearly states that although some of the temperature-dependant transitions in physical state are indistinct, glass is an amorphous non-crystalline solid.
In physics, the standard definition of a glass (or vitreous solid) is a solid formed by rapid melt quenching. However, the term glass is often used to describe any amorphous solid that exhibits a glass transition temperature : Tg. If the cooling of the molten state is sufficiently rapid (relative to the crystalisation time) then crystallization is prevented and instead the disordered atomic configuration of the liquid is 'frozen' into the solid state at Tg. The tendency for a material to form a glass while quenched is called glass-forming ability. This ability can be predicted by the rigidity theory. Generally, the structure of a glass exists in a metastable state with respect to its crystalline form, although in certain circumstances, for example in atactic polymers where there is no crystalline analogue of the amorphous phase.
Some people consider glass to be a liquid due to its lack of a first-order phase transition where certain thermodynamic variables such as volume, entropy and enthalpy are discontinuous through the glass transition range. However, the glass transition may be described as analogous to a second-order phase transition where the intensive thermodynamic variables such as the thermal expansivity and heat capacity are discontinuous. Despite this, the equilibrium theory of phase transformations does not entirely hold for glass, and hence the glass transition cannot be classed as one of the classical equilibrium phase transformations.
Glass is an amorphous non-crystalline solid. It exhibits an atomic structure close to that observed in a supercooled liquid phase but displays all the mechanical properties of a solid.
The notion that glass flows to an appreciable extent over extended periods of time is not supported by empirical research or theoretical analysis (see viscosity of amorphous materials).
Laboratory measurements of room temperature glass flow show motion consistent with a material viscosity on the order of 1017–1018 Pa s. Although the atomic structure of glass shares some of the characteristics of supercooled liquids, glass behaves as a solid below its transition temperature. A supercooled liquid behaves as a liquid, but it is below the freezing point of the material, and in some cases will crystallize almost instantly if a crystal is added as a core. The change in heat capacity at a glass transition and a melting transition of comparable materials are typically of the same order of magnitude, indicating that the change in active degrees of freedom is comparable as well. Both in a glass and in a crystal it is mostly only the vibrational degrees of freedom that remain active, whereas rotational and translational motion is arrested. This helps to explain why both crystalline and non-crystalline solids exhibit rigidity on most experimental time scales.
So, any questions or shall we put this topic to bed?