Silvering is the chemical process of coating a non-conductive substrate such as glass with a reflective substance, to produce a mirror. While the metal is often

silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...

, 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 aberrations 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, ultrav ...

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 a ...

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 transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Med ...

had manufactured small glass mirrors backed by

lead Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...

, tin, or antimony. 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 trad ...

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 world ...

, 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 subcontinent of Eurasia and it is located enti ...

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, 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 scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe ...

Justus von Liebig 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 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 telescopes. 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'' 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 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 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 di ...

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

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 as we ...

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 Big Bear Solar Observatory (BBSO) is a university-based solar observatory in the United States. It is operated by New Jersey Institute of Technology (NJIT). BBSO has a clear aperture Goode Solar Telescope (GST), which has no obscuration in ...

, 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 ''How It's Made'' (''Comment c'est fait'' in French) is a Canadian documentary television series that premiered on January 6, 2001, on the Discovery Channel in Canada and the Science Channel in the United States. The program is produced in t ...

, 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 ''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 Liebi ...

, 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 , 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 (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

Tions.net
Diy mirror / mirroring / silvering Chemical processes Mirrors Silver