Editing Glass (section)

=== Formation from a supercooled liquid ===
{{Main|Glass transition|Glass formation}}
{{Unsolved |physics |What is the nature of the [[Glass transition|transition]] between a fluid or regular solid and a glassy phase?
"The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition." —[[Philip Warren Anderson|P.W. Anderson]]<ref>{{cite journal |last=Anderson |first=P.W. |journal=Science |volume=267 |year=1995 |doi=10.1126/science.267.5204.1615-e |pmid=17808155 |issue=5204 |pages=1615–16 |title=Through the Glass Lightly|s2cid=28052338 }}</ref> }}
For melt quenching, if the cooling is sufficiently rapid (relative to the characteristic [[crystallization]] time) then crystallization is prevented and instead, the disordered atomic configuration of the [[supercooled]] liquid is frozen into the solid state at T<sub>g</sub>. 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 (physics)|rigidity theory]].<ref name="phillips1979">{{cite journal |last=Phillips |first=J.C. |title=Topology of covalent non-crystalline solids I: Short-range order in chalcogenide alloys |journal=Journal of Non-Crystalline Solids |year=1979 |volume=34 |issue=2 |page=153 |doi=10.1016/0022-3093(79)90033-4 |bibcode=1979JNCS...34..153P }}</ref> Generally, a glass exists in a structurally [[metastability in molecules|metastable]] state with respect to its [[Crystallinity|crystalline]] form, although in certain circumstances, for example in [[atactic]] polymers, there is no crystalline analogue of the amorphous phase.<ref name="Folmer">{{cite journal |last1=Folmer |first1=J.C.W. |last2=Franzen |first2=Stefan |title=Study of polymer glasses by modulated differential scanning calorimetry in the undergraduate physical chemistry laboratory |journal=Journal of Chemical Education |year=2003 |volume=80 |issue=7 |page=813 |doi=10.1021/ed080p813 |bibcode=2003JChEd..80..813F}}</ref>

Glass is sometimes considered to be a liquid due to its lack of a first-order [[phase transition]]<ref name=Gibbs /><ref>{{cite web|last=Loy |first=Jim |url=http://www.jimloy.com/physics/glass.htm |title=Glass Is A Liquid? |access-date=21 March 2007 |url-status=dead |archive-url=https://web.archive.org/web/20070314004114/http://www.jimloy.com/physics/glass.htm |archive-date=14 March 2007}}</ref>
where certain [[thermodynamics|thermodynamic]] [[thermodynamic variable|variables]] such as [[volume]], [[entropy]] and [[enthalpy]] are discontinuous through the glass transition range. The [[glass transition]] may be described as analogous to a second-order phase transition where the intensive thermodynamic variables such as the [[thermal expansion|thermal expansivity]] and [[heat capacity]] are discontinuous.<ref name=Zallen83 /> However, the equilibrium theory of phase transformations does not hold for glass, and hence the glass transition cannot be classed as one of the classical equilibrium phase transformations in solids.<ref name="Horst Scholze 1991" /><ref name="Elliot84" />