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Speculum metal is a mixture of around two-thirds
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
and one-third tin, making a white brittle
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductili ...
that can be polished to make a highly reflective surface. It was used historically to make different kinds of
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 ...
s from personal grooming aids to optical devices until it was replaced by more modern materials such as metal-coated glass mirrors. Speculum metal mixtures usually contain two parts copper to one part tin along with a small amount of
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, b ...
, although there are other mixtures containing silver, lead, or zinc. This is about twice the proportion of tin to copper typically used in
bronze Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (including aluminium, manganese, nickel, or zinc) and sometimes non-metals, such as phosphorus, or metalloids suc ...
alloys. Archaeologists and others prefer to call it "high-tin bronze", although this broad term is also used for other
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductili ...
s such as
bell metal Bell metal or bell bronze is an alloy used for making bells and related instruments, such as cymbals. It is a form of bronze with a higher tin content, usually in approximately a 4:1 ratio of copper to tin (typically, 78% copper, 22% tin by mas ...
, which is typically around 20% tin. Large speculum metal mirrors are hard to manufacture, and the alloy is prone to tarnish, requiring frequent re-polishing. However, it was the only practical choice for large mirrors in high-precision optical equipment between mid-17th and mid-19th century, before the invention of glass silvering. Speculum metal was noted for its use in the metal mirrors of reflecting telescopes, and famous examples of its use were Newton's telescope, the Leviathan of Parsonstown, and William Herschel's telescope used to discover the planet
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of ...
. A major difficulty with its use in telescopes is that the mirrors could not reflect as much light as modern mirrors and would tarnish rapidly.


Early history

The knowledge of making very hard white high luster metal out of bronze-type high-tin alloys may date back more than 2000 years in China although it could also be an invention of western civilizations. References in
Pliny the Elder Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman author, naturalist and natural philosopher, and naval and army commander of the early Roman Empire, and a friend of the emperor Vespasian. He wrote the encyclopedic ' ...
may refer to it. It was certainly in use by the European
Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire ...
, giving better reflectivity than the usual bronze mirrors, and tarnishing more slowly. However, tin was expensive, and the composition of the alloy had to be controlled precisely. Confusingly, mirrors made of speculum metal were known at the time, and often later, as "steel mirrors", although they had no
steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistan ...
in them. It was not suitable for "cold-working" techniques such as repoussé and chasing, being much too hard, but worked well if
cast Cast may refer to: Music * Cast (band), an English alternative rock band * Cast (Mexican band), a progressive Mexican rock band * The Cast, a Scottish musical duo: Mairi Campbell and Dave Francis * ''Cast'', a 2012 album by Trespassers William ...
into small objects, and was also used for "Dark Age belt fittings, buckles, brooches" and similar small items, giving an attractive silver-white colouring.


Use in telescopes

Speculum metal found an application in
early modern Europe Early modern Europe, also referred to as the post-medieval period, is the period of European history between the end of the Middle Ages and the beginning of the Industrial Revolution, roughly the late 15th century to the late 18th century. Histor ...
as the only known good reflecting surface for mirrors in reflecting telescopes. In contrast to household mirrors, where the reflecting metal layer is coated on the back of a glass pane and covered with a protective varnish, precision optical equipment like telescopes needs first surface mirrors that can be ground and polished into complex shapes such as
parabolic reflector A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface genera ...
s. For nearly 200 years speculum metal was the only mirror substance that could perform this task. One of the earliest designs, James Gregory’s
Gregorian telescope The Gregorian telescope is a type of reflecting telescope designed by Scottish mathematician and astronomer James Gregory in the 17th century, and first built in 1673 by Robert Hooke. James Gregory was a contemporary of Isaac Newton. Both oft ...
could not be built because Gregory could not find a craftsman capable of fabricating the complex speculum mirrors needed for the design.
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, Theology, theologian, and author (described in his time as a "natural philosophy, natural philosopher"), widely ...
was the first to successfully build a reflecting telescope in 1668. His first reflecting telescope (a design which came to be known as a Newtonian reflector) had a 33-mm (1.3-inch) diameter speculum metal primary mirror of his own formulation. Newton was likewise confronted with the problem of fabricating the complex parabolic shape needed to create the image, but simply settled on a spherical shape. The composition of speculum metal was further refined and went on to be used in the 1700s and 1800s in many designs of reflecting telescopes. The ideal composition was around 68.21% copper to 31.7% tin; more copper made the metal more yellow, more tin made the metal more blue in color. Ratios with up to 45% tin were used for resistance to tarnishing. Although speculum metal mirror reflecting telescopes could be built very large, such as William Herschel's 126-cm (49.5-inch) "
40-foot telescope William Herschel's 40-foot telescope, also known as the Great Forty-Foot telescope, was a reflecting telescope constructed between 1785 and 1789 at Observatory House in Slough, England. It used a diameter primary mirror with a focal length (h ...
" of 1789 and
Lord Rosse William Parsons, 3rd Earl of Rosse (17 June 1800 – 31 October 1867), was an Irish astronomer, naturalist, and engineer. He was president of the Royal Society (UK), the most important association of naturalists in the world in the nineteenth ...
1845 183-cm (72-inch) mirror of his " Leviathan of Parsonstown", impracticalities in using the metal made most astronomers prefer their smaller
refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses an ...
counterparts.Edison Pettit
The Reflector
''Astronomical Society of the Pacific Leaflets''. Vol. 7, No. 331, pp. 249–256. December 1956.
Speculum metal was very hard to cast and shape. It only reflected 66% of the light that hit it. Speculum also had the unfortunate property of tarnishing in open air with a sensitivity to humidity, requiring constant re-polishing to maintain its usefulness. This meant the telescope mirrors had to be constantly removed, polished, and re-figured to the correct shape. This sometimes proved difficult, with some mirrors having to be abandoned. It also required that two or more mirrors had to be fabricated for each telescope so that one could be used while the other was being polished. Rapidly cooling night-time air would cause stresses in large speculum metal mirrors, distorting their shape and causing them to produce poor images. Lord Rosse had a system of adjustable levers on his 72-inch metal mirror so he could adjust the shape when it was unreliable at producing an acceptable image.Voyage through the universe: ''The Visible Universe''. Time-Life Books, 1990.
Web clip
.
In 1856–57 an improvement over speculum mirrors was invented when Karl August von Steinheil and Léon Foucault introduced the process of depositing an ultra-thin layer of silver on the front surface (first surface) of a ground block of glass. Silvered glass mirrors were a vast improvement, since silver reflects 90% of the light that hits it and is much slower to tarnish than speculum. Silver coatings can also be removed from the glass, so a tarnished mirror could be resilvered without changing the delicate precision-polished shape of the glass substrate. Glass is also more thermally stable than speculum metal, allowing it to hold its shape better through temperature changes. This marked the end of the speculum-mirror reflecting telescope, with the last large one, the Great Melbourne Telescope with its 122 cm (48-inch) mirror, being completed in 1867. The era of the large glass-mirror reflector had begun, with telescopes such as
Andrew Ainslie Common Andrew Ainslie Common FRS (1841–1903) was an English amateur astronomer best known for his pioneering work in astrophotography. Biography Common was born in Newcastle Upon Tyne on 7 August 1841. His father, Thomas Common, a surgeon known for ...
's 1879 36-inch (91 cm) and 1887 60-inch (152 cm) reflectors built at
Ealing Ealing () is a district in West London, England, west of Charing Cross in the London Borough of Ealing. Ealing is the administrative centre of the borough and is identified as a major metropolitan centre in the London Plan. Ealing was his ...
, and the first of the "modern" large glass-mirror research reflectors, 60-inch (150 cm) Mount Wilson Observatory Hale telescope of 1908, the 100-inch (2.5 m) Mount Wilson
Hooker telescope The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles. The observato ...
in 1917 and the 200-inch (5 m) Mount Palomar Hale telescope in 1948.


See also

* Liquid-mirror telescope * List of largest optical telescopes in the 19th century * List of largest optical telescopes in the 18th century


References

*Meeks, Nigel, "Patination phenomena on Roman and Chinese bronze mirrors and other artefacts", in ''Metal Plating and Patination: Cultural, Technical and Historical Developments'', ed. Susan La-Niece, 2013, Elsevier, ISBN 9781483292069
google books


External links

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Bronze Copper alloys Optical materials