Silvering is the

chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., w ...

process of coating a non-conductive substrate such as

glass Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...

with a

reflective Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The ' ...

substance, to produce a

mirror A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the im ...

. While the metal is often

silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...

, 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 any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture. Paint can be made in many ...

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 lenses, that causes light to be spread out over some region of space rather than focused to a point. Aberrations cause the image formed by a lens to be blurred or distorted, with t ...

s due to

refraction In physics, refraction is the redirection of a wave as it passes from one 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 commonly observed phenomen ...

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 Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...

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

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

History

Ptolemaic

Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a List of transcontinental countries, transcontinental country spanning the North Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via a land bridg ...

had manufactured small glass mirrors backed by

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...

, tin, or

antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient ti ...

. 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 Arabic: ''al-kīmiyā''; from Ancient Greek: χυμεία, ''khumeía'') is an ancient branch of natural philosophy, a philosophical and protoscientific tradition that was historically practiced in China, India, the Muslim wo ...

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

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

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 begins with the Julian year 1501 ( MDI) and ends with either the Julian or the Gregorian year 1600 ( MDC) (depending on the reckoning used; the Gregorian calendar introduced a lapse of 10 days in October 1582). The 16th centur ...

, most were silvered with an

amalgam Amalgam most commonly refers to: * Amalgam (chemistry), mercury alloy * Amalgam (dentistry), material of silver tooth fillings ** Bonded amalgam, used in dentistry Amalgam may also refer to: * Amalgam Comics, a publisher * Amalgam Digital, an in ...

of tin and mercury, In 1835

German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...

chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe th ...

Justus von Liebig Justus Freiherr von Liebig (12 May 1803 – 20 April 1873) was a German scientist who made major contributions to agricultural and biological chemistry, and is considered one of the principal founders of organic chemistry. As a professor at th ...

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 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 Jean Bernard Léon Foucault (, ; ; 18 September 1819 – 11 February 1868) was a French physicist best known for his demonstration of the Foucault pendulum, a device demonstrating the effect of Earth's rotation. He also made an early measurement ...

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 Speculum metal is a mixture of around two-thirds copper and one-third tin, making a white brittle alloy that can be polished to make a highly reflective surface. It was used historically to make different kinds of mirrors from personal grooming ...

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 disused 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 o ...

. 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 An amorphous metal (also known as metallic glass, glassy metal, or shiny metal) is a solid metallic material, usually an alloy, with disordered atomic-scale structure. Most metals are crystalline in their solid state, which means they have a high ...

(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 reduction of cations of that metal by means of a direct electric current. The part to be ...

, chemical "wet process" deposition, and

vacuum deposition Vacuum deposition is a group of processes used to deposit layers of material atom-by-atom or molecule-by-molecule on a solid surface. These processes operate at pressures well below atmospheric pressure (i.e., vacuum). The deposited layers can ...

. 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 charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gene ...

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

and absorbs atmospheric

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

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 is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...

chamber with electrically heated

nichrome Nichrome (also known as NiCr, nickel-chromium or chromium-nickel) is a family of alloys of nickel, chromium, and often iron (and possibly other elements) commonly used as resistance wire, heating elements in devices like toasters, electrical kett ...

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 atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...

or beryllia on the mirror; others expose it to pure

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

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 ''Products Finishing'' is a monthly American trade magazine and web site focused on reporting on the use of organic and inorganic finishings and the technologies used to deliver them. The Publisher is Todd Luciano, the Editor is Scott Francis. Ed ...

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 , lang=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 , lang=de , trans-title = Regarding the silvering and gilding of glass , journal =

Annalen der Chemie und Pharmacie ''Justus Liebigs Annalen der Chemie'' (often cited as just ''Liebigs Annalen'') was one of the oldest and historically most important journals in the field of organic chemistry worldwide. It was established in 1832 and edited by Justus von Liebig ...

, 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 , url=https://www.google.co.uk/books/edition/Coatings_on_Glass/ho2pYhOiEMcC , isbn=9780080525556 {{cite web , title=Era of huge reflectors, page 2 , series=Amazing-space , website=

Space Telescope Science Institute The Space Telescope Science Institute (STScI) is the science operations center for the Hubble Space Telescope (HST), science operations and mission operations center for the James Webb Space Telescope (JWST), and science operations center for the ...

(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 , lang=en , url=https://books.google.com/books?id=NKVRAAAAMAAJ , via=Google Books

External links

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