Editing Glass (section)

== Physical properties ==
=== Optical ===
{{Main|Optical glass}}
Glass is in widespread use in optical systems due to its ability to refract, reflect, and transmit light following [[geometrical optics]]. The most common and oldest applications of glass in optics are as [[Lens (optics)|lenses]], [[window]]s, [[mirror]]s, and [[Prism (optics)|prism]]s.<ref name="Bach12">{{cite book |title=The Properties of Optical Glass |first1=Hans |last1=Bach |first2=Norbert |last2=Neuroth |publisher=Springer |year=2012|url=https://books.google.com/books?id=y3nnCAAAQBAJ&pg=PA1 |pages=1–11 |isbn=978-3-642-57769-7}}</ref> The key optical properties [[refractive index]], [[Dispersion (optics)|dispersion]], and [[Transparency and translucency|transmission]], of glass are strongly dependent on chemical composition and, to a lesser degree, its thermal history.<ref name=Bach12 /> Optical glass typically has a refractive index of 1.4 to 2.4, and an [[Abbe number]] (which characterises dispersion) of 15 to 100.<ref name=Bach12 /> The refractive index may be modified by high-density (refractive index increases) or low-density (refractive index decreases) additives.<ref>{{Cite book |url=https://books.google.com/books?id=-0DOBQAAQBAJ&pg=PA70 |title=Physical Properties of Materials, Second Edition |last=White |first=Mary Anne |authorlink1=Mary Anne White |year=2011 |pages=70 |publisher=CRC Press|isbn=978-1-4398-9532-0}}</ref>

Glass transparency results from the absence of [[grain boundary|grain boundaries]] which [[diffuse reflection|diffusely scatter light]] in polycrystalline materials.<ref name="Carter-Norton">{{Cite book |url=https://books.google.com/books?id=aE_VQ8I24OoC&pg=PA583 |title=Ceramic Materials: Science and Engineering |last1=Carter |first1=C. Barry |first2=M. Grant |last2= Norton |year=2007| publisher=Springer Science & Business Media| pages=583|isbn=978-0-387-46271-4 }}</ref> Semi-opacity due to crystallization may be induced in many glasses by maintaining them for a long period at a temperature just insufficient to cause fusion. In this way, the crystalline, devitrified material, known as Réaumur's glass [[porcelain]] is produced.<ref name="EB1911-incorp">{{EB1911|inline=1 |wstitle=Glass |volume=12 |page=86}}</ref><ref name="Mysen05">{{cite book|last1=Mysen|first1=Bjorn O.|last2=Richet|first2=Pascal|title=Silicate Glasses and Melts: Properties and Structure|publisher=Elsevier|year=2005|pages=10}}</ref> Although generally transparent to visible light, glasses may be [[Opacity (optics)|opaque]] to other [[Electromagnetic spectrum|wavelengths of light]]. While silicate glasses are generally opaque to [[infrared]] wavelengths with a transmission cut-off at 4 μm, heavy-metal [[Fluoride glass|fluoride]] and [[Chalcogenide glass|chalcogenide]] glasses are transparent to infrared wavelengths of 7 to 18 μm.<ref name=brittanica-industrial /> The addition of metallic oxides results in different coloured glasses as the metallic ions will absorb wavelengths of light corresponding to specific colours.<ref name=brittanica-industrial />

=== Other ===
{{See also|List of physical properties of glass|Corrosion#Corrosion of glass|Strength of glass}}
[[File:Artesanía en vidrio (Unsplash).jpg|thumb|Glass can be fairly easily melted and manipulated with a heat source]]
In the manufacturing process, glasses can be poured, formed, extruded and moulded into forms ranging from flat sheets to highly intricate shapes.<ref>{{Cite book |url=https://books.google.com/books?id=BZUtFQNuNgMC&pg=PA60 |title=Handbook of Physical Vapor Deposition (PVD) Processing |last=Mattox |first=D.M. |year=2014 |page=60 |publisher=Cambridge University Press |isbn=978-0-08-094658-0}}</ref> The finished product is brittle but can be [[laminated glass|laminated]] or [[Tempered glass|tempered]] to enhance durability.<ref>{{Cite book|url=https://books.google.com/books?id=D7Z8ywb3QggC&pg=PA361|title=Glasses and the Vitreous State|last=Zarzycki|first=Jerzy|year=1991|publisher=Cambridge University Press|isbn=978-0-521-35582-7|pages=361}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=CL8Z38FaPsAC&pg=PA365|title=Collision Repair and Refinishing: A Foundation Course for Technicians|last1=Thomas|first1=Alfred|last2=Jund|first2=Michael|year=2013|pages=365|publisher=Cengage Learning |isbn=978-1-133-60187-6}}</ref> Glass is typically inert, resistant to chemical attack, and can mostly withstand the action of water, making it an ideal material for the manufacture of containers for foodstuffs and most chemicals.<ref name="Harvey09" /><ref name="Gardner-1949">{{Cite book|url=https://books.google.com/books?id=yYQ3BMs9Ql0C&pg=PA13|title=Research and Development in Applied Optics and Optical Glass at the National Bureau of Standards: A Review and Bibliography|last1=Gardner|first1=Irvine Clifton|last2=Hahner|first2=Clarence H.|date=1949|publisher=U.S. Government Printing Office|page=13|isbn=9780598682413}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=rJTBCQAAQBAJ&pg=PA550|title=Food Safety in the 21st Century: Public Health Perspective|last1=Dudeja|first1=Puja|last2=Gupta|first2=Rajul K.| page=550 |last3=Minhas|first3=Amarjeet Singh|year=2016|publisher=Academic Press|isbn=978-0-12-801846-0}}</ref> Nevertheless, although usually highly resistant to chemical attack, glass will corrode or dissolve under some conditions.<ref name="Gardner-1949" /><ref name="Bengisu 2013">{{Cite book|url=https://books.google.com/books?id=PXD8CAAAQBAJ&pg=PA360|title=Engineering Ceramics|last=Bengisu|first=M.|year=2013|publisher=Springer Science & Business Media|isbn=978-3-662-04350-9|page=360}}</ref> The materials that make up a particular glass composition affect how quickly the glass corrodes. Glasses containing a high proportion of [[alkali metal|alkali]] or [[Alkaline earth element|alkaline earth]] elements are more susceptible to corrosion than other glass compositions.<ref>{{Cite book|url=https://books.google.com/books?id=IVe7CgAAQBAJ&pg=PA141|title=Materials Degradation and Its Control by Surface Engineering|last1=Batchelor|first1=Andrew W.|last2=Loh|first2=Nee Lam|last3=Chandrasekaran|first3=Margam|year=2011|publisher=World Scientific|page=141|isbn=978-1-908978-14-1}}</ref><ref name="Chawla93">{{Cite book|url=https://books.google.com/books?id=_NXYRgHnnqkC&pg=PA328|pages=327–328|title=Materials Selection for Corrosion Control|last=Chawla|first=Sohan L.|date=1993|publisher=ASM International|isbn=978-1-61503-728-5}}</ref>

The density of glass varies with chemical composition with values ranging from {{convert|2.2|g/cm3|kg/m3}} for [[Fused quartz|fused silica]] to {{convert|7.2|g/cm3|kg/m3}} for dense flint glass.<ref>{{cite Q|Q87511351}}<!--"Density of Glass" in The Physics Factbook--></ref> Glass is stronger than most metals, with a theoretical [[tensile strength]] for pure, flawless glass estimated at {{convert|14|to|35|GPa|psi}} due to its ability to undergo reversible compression without fracture. However, the presence of scratches, bubbles, and other microscopic flaws lead to a typical range of {{convert|14|to|175|MPa|psi}} in most commercial glasses.<ref name="brittanica-industrial">{{Cite web|url=https://www.britannica.com/topic/glass-properties-composition-and-industrial-production-234890|title=Industrial glass – Properties of glass|website=Encyclopedia Britannica}}</ref> Several processes such as [[Toughened glass|toughening]] can increase the strength of glass.<ref>{{cite web|url=https://www.pilkington.com/en-gb/uk/architects/glass-information/functions-of-glass/mechanicalfunctionsofglass/glass-strength|title=Glass Strength|website=www.pilkington.com|access-date=2017-11-24|url-status=live|archive-url=https://web.archive.org/web/20170726123604/http://www.pilkington.com/en-gb/uk/architects/glass-information/functions-of-glass/mechanicalfunctionsofglass/glass-strength|archive-date=26 July 2017}}</ref> Carefully drawn flawless [[glass fibre]]s can be produced with a strength of up to {{convert|11.5|GPa|psi}}.<ref name=brittanica-industrial />
{{Further|topic=the tiny glass flakes formed during glass vial manufacturing |Spicule (glass manufacture)|label1=Spicule}}

=== Reputed flow ===
The observation that old windows are sometimes found to be thicker at the bottom than at the top is often offered as supporting evidence for the view that glass flows over a timescale of centuries, the assumption being that the glass has exhibited the liquid property of flowing from one shape to another.<ref>{{cite news|url=https://www.nytimes.com/2008/07/29/science/29glass.html?ex=1375070400&en=048ade4011756b24&ei=5124&partner=permalink&exprod=permalink|title=The Nature of Glass Remains Anything but Clear|work=The New York Times|access-date=29 July 2008|date=29 July 2008|author=Kenneth Chang|url-status=live|archive-url=https://web.archive.org/web/20090424094929/http://www.nytimes.com/2008/07/29/science/29glass.html?ex=1375070400&en=048ade4011756b24&ei=5124&partner=permalink&exprod=permalink|archive-date=24 April 2009}}</ref> This assumption is incorrect, as once solidified, glass stops flowing. The sags and ripples observed in old glass were already there the day it was made; manufacturing processes used in the past produced sheets with imperfect surfaces and non-uniform thickness (the near-perfect [[float glass]] used today only became widespread in the 1960s).<ref name=Gibbs />

A 2017 study computed the rate of flow of the medieval glass used in [[Westminster Abbey]] from the year 1268. The study found that the room temperature viscosity of this glass was roughly 10<sup>24</sup>{{nbsp}}[[Pascal (unit)|Pa]]·[[Second|s]] which is about 10<sup>16</sup> times less viscous than a previous estimate made in 1998, which focused on soda-lime silicate glass. Even with this lower viscosity, the study authors calculated that the maximum flow rate of medieval glass is 1 [[Nanometer|nm]] per billion years, making it impossible to observe in a human timescale.<ref>{{cite journal | last1=Gulbiten | first1=Ozgur | last2=Mauro | first2=John C. | last3=Guo | first3=Xiaoju | last4=Boratav | first4=Olus N. | title=Viscous flow of medieval cathedral glass | journal=Journal of the American Ceramic Society| volume=101 | issue=1 | date=3 August 2017 | issn=0002-7820 | doi=10.1111/jace.15092 | pages=5–11}}</ref><ref>{{Cite web |title=Glass viscosity calculations definitively debunk the myth of observable flow in medieval windows |last=Gocha |first=April |work=The American Ceramic Society |date=3 August 2017 |url= https://ceramics.org/ceramic-tech-today/glass-viscosity-calculations-definitively-debunk-the-myth-of-observable-flow-in-medieval-windows}}</ref>