Editing Glass (section)

== Microscopic structure ==
[[File:Silica.svg|thumb|left|The amorphous structure of [[Silicon dioxide|glassy silica (SiO<sub>2</sub>)]] in two dimensions. No long-range order is present, although there is local ordering to the [[tetrahedral]] arrangement of oxygen (O) atoms around the silicon (Si) atoms.|alt=A graphic showing the lack of periodic arrangement in the microscopic structure of glass]]
[[File:Crystalline polycrystalline amorphous2.svg|thumb|upright=1.25|Microscopically, a [[single crystal]] has atoms in a near-perfect [[Periodic function|periodic]] arrangement; a [[polycrystal]] is composed of many microscopic crystals; and an [[amorphous]] solid such as glass has no periodic arrangement even microscopically.|alt=A graphic visually showing the difference between the microscopic arrangement of single crystals, polycrystals, and amorphous solids, as explained in the caption]]
{{Main|Structure of liquids and glasses}}
The standard definition of a ''glass'' (or vitreous solid) is a non-crystalline solid formed by rapid melt [[quenching]].<ref>[[ASTM]] definition of glass from 1945</ref><ref name="Zallen83">{{cite book |last=Zallen |first=R. |title=The Physics of Amorphous Solids |publisher=John Wiley |place=New York |year=1983 |pages=1–32|isbn=978-0-471-01968-8}}</ref><ref name="Cusack87">{{Cite book |last=Cusack |first=N.E. |title=The physics of structurally disordered matter: an introduction |publisher=Adam Hilger in association with the University of Sussex press |year=1987 |page=13 |isbn=978-0-85274-829-9}}</ref><ref name="Horst Scholze 1991">{{Cite book |last=Scholze |first=Horst |title=Glass – Nature, Structure, and Properties |publisher=Springer |year=1991 |pages=3–5 |isbn=978-0-387-97396-8}}</ref> However, the term "glass" is often defined in a broader sense, to describe any non-crystalline ([[amorphous solid|amorphous]]) solid that exhibits a [[glass transition]] when heated towards the liquid state.<ref name="Horst Scholze 1991" /><ref name="Elliot84">{{Cite book |last=Elliot |first=S.R. |title=Physics of Amorphous Materials |publisher=Longman group ltd |year=1984 |pages=1–52 |isbn=0-582-44636-8}}</ref>

Glass is an [[amorphous solid]]. Although the atomic-scale structure of glass shares characteristics of the structure of a [[supercooled liquid]], glass exhibits all the mechanical properties of a solid.<ref>{{cite web|last=Neumann |first=Florin |url=http://dwb.unl.edu/Teacher/NSF/C01/C01Links/www.ualberta.ca/~bderksen/florin.html |title=Glass: Liquid or Solid – Science vs. an Urban Legend |access-date=8 April 2007 |url-status=dead |archive-url=https://web.archive.org/web/20070409022023/http://dwb.unl.edu/Teacher/NSF/C01/C01Links/www.ualberta.ca/~bderksen/florin.html |archive-date=9 April 2007}}</ref><ref name="Gibbs">{{cite web |last=Gibbs |first=Philip |url=http://math.ucr.edu/home/baez/physics/General/Glass/glass.html |title=Is glass liquid or solid? |access-date=21 March 2007 |url-status=live |archive-url=https://web.archive.org/web/20070329154027/http://math.ucr.edu/home/baez/physics/General/Glass/glass.html |archive-date=29 March 2007}}</ref><ref>"Philip Gibbs" ''Glass Worldwide'', (May/June 2007), pp. 14–18</ref> As in other [[amorphous solid]]s, the atomic structure of a glass lacks the long-range periodicity observed in [[Crystal structure|crystalline solids]]. Due to [[chemical bonding]] constraints, glasses do possess a high degree of short-range order with respect to local atomic [[polyhedra]].<ref>{{cite journal |last=Salmon |first=P.S. |title=Order within disorder |doi=10.1038/nmat737 |journal=Nature Materials |pmid=12618817 |volume=1 |issue=2 |year=2002 |pages=87–8|bibcode=2002NatMa...1...87S |s2cid=39062607 |issn = 1476-1122 }}</ref> The notion that glass flows to an appreciable extent over extended periods well below the glass transition temperature is not supported by empirical research or theoretical analysis (see [[viscosity#In solids|viscosity in solids]]). Though atomic motion at glass surfaces can be observed,<ref>{{cite journal |last1=Ashtekar |first1=Sumit |last2=Scott |first2=Gregory |last3=Lyding |first3=Joseph |last4=Gruebele |first4=Martin |year=2010 |title=Direct Visualization of Two-State Dynamics on Metallic Glass Surfaces Well Below Tg |journal=J. Phys. Chem. Lett. |volume=1 |issue=13 |pages=1941–1945 |doi=10.1021/jz100633d |arxiv=1006.1684 |s2cid=93171134 }}</ref> and viscosity on the order of 10<sup>17</sup>–10<sup>18</sup>&nbsp;Pa·s can be measured in glass, such a high value reinforces the fact that glass would not change shape appreciably over even large periods of time.<ref name=Elliot84 /><ref>{{cite journal |last1=Vannoni |first1=M. |last2=Sordini |first2=A. |last3=Molesini |first3=G. |year=2011 |title=Relaxation time and viscosity of fused silica glass at room temperature |journal=Eur. Phys. J. E |volume=34 |issue=9 |pages=9–14 |doi=10.1140/epje/i2011-11092-9|pmid=21947892 |s2cid=2246471 }}</ref>

=== 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" />