Silvering is the

chemical A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...

process of coating a non-conductive substrate such as

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

with a reflective substance, to produce a

mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...

. While the metal is often

silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

, the term is used for the application of any reflective metal.

Process

Most common household mirrors are "back-silvered" or "second-surface", meaning that the light reaches the reflective layer after passing through the glass. A protective layer of

paint Paint is a material or mixture that, when applied to a solid material and allowed to dry, adds a film-like layer. As art, this is used to create an image or images known as a painting. Paint can be made in many colors and types. Most paints are ...

is usually applied to protect the back side of the reflective surface . This arrangement protects the fragile reflective layer from corrosion, scratches, and other damage. However, the glass layer may absorb some of the light and cause distortions and

optical aberration In optics, aberration is a property of optical systems, such as Lens (optics), lenses and mirrors, that causes the ''image'' created by the optical system to not be a faithful reproduction of the ''object'' being observed. Aberrations cause the i ...

s due to

refraction In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commo ...

at the front surface, and multiple additional reflections on it, giving rise to "ghost images" (although some optical mirrors such as Mangins take advantage of it). Therefore, precision optical mirrors normally are "front-silvered" or " first-surface", meaning that the reflective layer is on the surface towards the incoming light. The substrate normally provides only physical support, and need not be transparent. A hard, protective, transparent overcoat may be applied to prevent

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

of the reflective layer and scratching of the metal. Front-coated mirrors achieve reflectivities of 90–95% when new.

History

Ptolemaic

Egypt Egypt ( , ), officially the Arab Republic of Egypt, is a country spanning the Northeast Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via the Sinai Peninsula. It is bordered by the Mediterranean Sea to northe ...

had manufactured small glass mirrors backed by

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

, tin, or

antimony Antimony is a chemical element; it has chemical symbol, symbol Sb () and atomic number 51. A lustrous grey metal or metalloid, it is found in nature mainly as the sulfide mineral stibnite (). Antimony compounds have been known since ancient t ...

. In the early 10th century, the Persian scientist al-Razi described ways of silvering and

gilding Gilding is a decorative technique for applying a very thin coating of gold over solid surfaces such as metal (most common), wood, porcelain, or stone. A gilded object is also described as "gilt". Where metal is gilded, the metal below was tradi ...

in a book on

alchemy Alchemy (from the Arabic word , ) is an ancient branch of natural philosophy, a philosophical and protoscientific tradition that was historically practised in China, India, the Muslim world, and Europe. In its Western form, alchemy is first ...

, but this was not done for the purpose of making mirrors. Tin-coated mirrors were first made in

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

in the 15th century. The thin tinfoil used to silver mirrors was known as "tain". When glass mirrors first gained widespread usage in Europe during the

16th century The 16th century began with the Julian calendar, Julian year 1501 (represented by the Roman numerals MDI) and ended with either the Julian or the Gregorian calendar, Gregorian year 1600 (MDC), depending on the reckoning used (the Gregorian calend ...

, most were silvered with an amalgam of tin and mercury, In 1835 German

chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a graduated scientist trained in the study of chemistry, or an officially enrolled student in the field. Chemists study the composition of ...

Justus von Liebig Justus ''Freiherr'' von Liebig (12 May 1803 – 18 April 1873) was a Germans, German scientist who made major contributions to the theory, practice, and pedagogy of chemistry, as well as to agricultural and biology, biological chemistry; he is ...

developed a process for depositing silver on the rear surface of a piece of glass; this technique gained wide acceptance after Liebig improved it in 1856. The process was further refined and made easier by the chemist Tony Petitjean (1856). This reaction is a variation of the

Tollens' reagent Tollens' reagent (chemical formula Ag(NH3)2OH) is a chemical reagent used to distinguish between aldehydes and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of silver n ...

for aldehydes. A diamminesilver(I) solution is mixed with a sugar and sprayed onto the glass surface. The sugar is oxidized by silver(I), which is itself reduced to silver(0), i.e. elemental

, and deposited onto the glass. In 1856-1857 Karl August von Steinheil and Léon Foucault introduced the process of depositing an ultra-thin layer of silver on the front surface of a piece of glass, making the first optical-quality first surface glass mirrors, replacing the use of speculum metal mirrors in

reflecting telescope A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...

s. These techniques soon became standard for technical equipment. An aluminum vacuum-deposition process invented in 1930 by Caltech physicist and astronomer John Strong, led to most reflecting telescopes shifting to aluminum. Nevertheless, some modern telescopes use silver, such as the

Kepler Space Telescope The Kepler space telescope is a defunct space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...

. The ''Kepler'' mirror's silver was deposited using ion assisted evaporation.

Modern silvering processes

General processes

Silvering aims to produce a non-crystalline coating of amorphous metal (metallic glass), with no visible artifacts from grain boundaries. The most common methods in current use are

electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the redox, reduction of cations of that metal by means of a direct current, direct electric cur ...

, chemical "wet process" deposition, and vacuum deposition. Electroplating of a substrate of glass or other non-

conductive In physics and electrical engineering, a conductor is an object or type of material that allows the flow of Electric charge, charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. The flow ...

material requires the deposition of a thin layer of conductive but transparent material, such as carbon. This layer tends to reduce the adhesion between the metal and the substrate. Chemical deposition can result in better adhesion, directly or by pre-treatment of the surface. Vacuum deposition can produce very uniform coating with very precisely controlled thickness.

Metals

Silver

The reflective layer on a second surface mirror such as a household mirror is often actual silver. A modern "wet" process for silver coating treats the glass with

tin(II) chloride Tin(II) chloride, also known as stannous chloride, is a white crystalline solid with the formula . It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl2 is widely used as a reducing agent (in ac ...

to improve the bonding between silver and glass. An activator is applied after the silver has been deposited to harden the tin and silver coatings. A layer of copper may be added for long-term durability. Silver would be ideal for telescope mirrors and other demanding optical applications, since it has the best initial front-surface reflectivity in the visible spectrum. However, it quickly oxidizes and absorbs atmospheric

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

to create a dark, low-reflectivity tarnish.

Aluminum

The "silvering" on precision optical instruments such as telescopes is usually aluminum. Although aluminum also oxidizes quickly, the thin aluminum oxide (sapphire) layer is transparent, and so the high-reflectivity underlying aluminum stays visible. In modern aluminum silvering, a sheet of glass is placed in a

vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

chamber with electrically heated nichrome coils that can evaporate aluminum. In a vacuum, the hot aluminum atoms travel in straight lines. When they hit the surface of the mirror, they cool and stick. Some mirror makers evaporate a layer of

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

or beryllia on the mirror; others expose it to pure

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

or air in an oven so that it will form a tough, clear layer of aluminum oxide.

Tin

The first tin-coated glass mirrors were produced by applying a tin-mercury amalgam to the glass and heating the piece to evaporate the mercury.

Gold

The "silvering" on infrared instruments is usually gold. It has the best reflectivity in the infrared spectrum, and has high resistance to oxidation and corrosion. Conversely, a thin gold coating is used to create optical filters which block infrared (by mirroring it back towards the source) while passing visible light.

References

{{reflist, 25em, refs= {{cite web , title=Ball Aerospace completes primary mirror and detector array assembly milestones for Kepler Mission , date=25 September 2007 , publisher=Ball Aerospace and Technologies Corp. , website=spaceref.com , url= http://www.spaceref.com/news/viewpr.html?pid=23610 , access-date=6 April 2013 {{cite web , title=Daily events and images of the installation of the BBSO New Solar Telescope , publisher= Big Bear Solar Observatory , type=blog , url=http://www.bbso.njit.edu/telblog/section4.html , website=www.bbso.njit.edu , access-date=6 January 2020 {{cite web , first=Kathleen Payne , last=de Chavez , date=Spring 2010 , title=Historic mercury amalgam mirrors: History, safety, and preservation , website=Williamstown Art , url=http://www.williamstownart.org/techbulletins/images/WACC%20Historic%20Mercury%20Mirrors.pdf , access-date=2014-03-11 {{cite magazine , first=Jim , last=Destefani , date=March 2008 , title=Mirror, mirror: Keeping the Hale Telescope optically sharp , magazine= Products Finishing Magazine , id=PF mag article 030805 , url=http://www.pfonline.com/articles/030805.html , archive-url=https://web.archive.org/web/20091011082949/http://www.pfonline.com/articles/030805.html , archive-date=2009-10-11 {{cite web , first=Helen , last=Fioratti , title=The Origins of Mirrors and their uses in the Ancient World , website=L'Antiquaire & the Connoisseur , url=http://www.lantiquaire.us/origins-of-mirrors.html , access-date=2009-08-14 , url-status= dead , archive-url=https://web.archive.org/web/20110203052336/http://www.lantiquaire.us/origins-of-mirrors.html , archive-date=2011-02-03 {{cite journal , last1=Fulton , first1=L. Michael , last2=Dummer , first2=Richard S. , year=2011 , title=Advanced Large Area Deposition Technology for Astronomical and Space Applications , journal=Vacuum & Coating Technology , volume=2011 , issue=December , pages=43–47 , url=http://e-ditionsbyfry.com/Olive/ODE/VTC/Default.aspx?href=VTC/2011/12/01 , access-date=6 April 2013 , url-status=dead , archive-url=https://web.archive.org/web/20130512233653/http://e-ditionsbyfry.com/Olive/ODE/VTC/Default.aspx?href=VTC%2F2011%2F12%2F01 , archive-date=12 May 2013 {{cite AV media , number=305 , series= How It's Made , title=Episode 305 , place= Anjou, Quebec, Canada , quote=Episode 305 filmed a
Verrerie-Walker
{{cite journal , first = Justus , last = Liebig , author-link = Justus Liebig , year = 1835 , title = Ueber die Producte der Oxydation des Alkohols , language=de , trans-title = Regarding the products of oxidation of alcohols , journal = Annalen der Chemie , volume = 14 , issue = 2 , pages = 133–167 , doi = 10.1002/jlac.18350140202 , bibcode = 1835AnP...112..275L , url = https://zenodo.org/record/1426914 {{cite journal , first = Justus , last = Liebig , author-link = Justus Liebig , year = 1856 , title = Ueber Versilberung und Vergoldung von Glas , language=de , trans-title = Regarding the silvering and gilding of glass , journal = Annalen der Chemie und Pharmacie , volume = 98 , issue = 1 , pages = 132–139 , doi = 10.1002/jlac.18560980112 , url = https://zenodo.org/record/1427076 {{cite patent , country = GB , number = 1681 , status = patent , title = Silvering, Gilding, and Platinizing Glass , gdate = 1856-01-12 , fdate = 1855-07-24 , inventor-last = Petitjean , inventor-first= Tony , url = https://books.google.com/books?id=nQcADAPqbO0C&q=anthony+petitjohn+silvering&pg=RA29-PA7 {{cite book , first1=H.K. , last1=Pulker , date=1999-03-29 , title=Coatings on Glass , publisher=

Elsevier Science Elsevier ( ) is a Dutch academic publishing company specializing in scientific, technical, and medical content. Its products include journals such as ''The Lancet'', '' Cell'', the ScienceDirect collection of electronic journals, '' Trends'', t ...

, url=https://books.google.com/books?id=ho2pYhOiEMcC , isbn=9780080525556 {{cite web , title=Era of huge reflectors, page 2 , series=Amazing-space , website= Space Telescope Science Institute (stsci.edu) , place=Baltimore, MD , url=http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/eras/huge-reflectors/page2.php {{cite book , last=Watson , first=Don Arthur , date=January 1986 , title=Construction Materials and Processes , publisher=McGraw-Hill , department=Gregg Division , isbn=9780070684768 , language=en , url=https://books.google.com/books?id=NKVRAAAAMAAJ , via=Google Books

External links

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