Speculum metal

Speculum metal

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Speculum metal is a mixture of around two-thirds copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 and one-third tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

 making a white brittle alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...

 that can be polished to make a highly reflective surface. It is used primarily to make different kinds of mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

s including early reflecting telescope
Reflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...

 optical mirrors. Speculum metal can also be used as the metallic coating on glass mirrors (as opposed to silver or aluminium) giving a reflectivity of 68% at 6000 angstroms
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

 when evaporated onto the surface.

Overview


Speculum metal mixtures usually contain two parts copper to one part tin along with a small amount of arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

, although there are other mixtures containing silver, brass, lead, or zinc. The knowledge of making very hard white high luster metal out of bronze
Bronze
Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...

 type high tin alloys may date back more than 2000 years in China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

 although it could also be an invention of western civilizations as well. Such metals were used in sculpture and to make more effective mirrors than the more common yellow easily tarnishing bronze
Bronze
Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...

 mirrors. Mirrors of speculum metal or any precious metal were rare and only owned by the wealthy

Speculum and telescopes


Speculum metal found an application in early modern Europe
Early modern Europe
Early modern Europe is the term used by historians to refer to a period in the history of Europe which spanned the centuries between the end of the Middle Ages and the beginning of the Industrial Revolution, roughly the late 15th century to the late 18th century...

 as the only known good reflecting surface for mirrors in reflecting telescope
Reflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...

s. Such telescopes needed first surface mirrors that could be ground and polished into complex shapes such as parabolic reflector
Parabolic reflector
A parabolic reflector is a reflective device used to collect or project energy such as light, sound, or radio waves. Its shape is that of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis...

s. For nearly 200 years speculum metal was the only mirror substance that could perform this task. One of the earliest designs, James Gregory
James Gregory (astronomer and mathematician)
James Gregory FRS was a Scottish mathematician and astronomer. He described an early practical design for the reflecting telescope – the Gregorian telescope – and made advances in trigonometry, discovering infinite series representations for several trigonometric functions.- Biography :The...

’s Gregorian telescope
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...

 could not be built because Gregory could not find a craftsman capable of fabricating the complex speculum mirrors needed for the design. Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

 was the first to successfully build a reflecting telescope, his 1668 Newtonian reflector with a 33-mm (1.3-inch) diameter speculum metal mirror of his own formulation. 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. Compositions varied, with more copper making it more yellow and more tin more blue in reflection with ratios of up to 45% tin for resistance to tarnishing.

Although speculum metal mirror reflecting telescopes could be built very large, such as William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

's 126-cm (49.5-inch) "40-foot telescope
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 120 cm diameter primary mirror with a 12 m long focal length . It was the largest telescope in...

" of 1789 and Lord Rosse
William Parsons, 3rd Earl of Rosse
William Parsons, 3rd Earl of Rosse, Knight of the Order of St Patrick was an Irish astronomer who had several telescopes built. His 72-inch telescope "Leviathan", built 1845, was the world's largest telescope until the early 20th century.-Life:He was born in Yorkshire, England, in the city of...

 1845 183-cm (72-inch) mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

 of his "Leviathan of Parsonstown
Leviathan of Parsonstown
Leviathan of Parsonstown is the unofficial name of the Rosse six foot telescope. This is a historic reflecting telescope of 72 in aperture, which was the largest telescope in the world from 1845 until the construction of the 100 in Hooker Telescope in 1917...

", impracticalities in using the metal made most astronomers prefer their smaller refracting telescope
Refracting telescope
A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image . The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses...

 counterparts. Speculum metal was very hard to cast and shape. The substance only reflected 66 percent of the light that hit it. Speculum also had the unfortunate property of tarnish
Tarnish
Tarnish is a thin layer of corrosion that forms over copper, brass, silver, aluminum, and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen in the air. For example, silver needs hydrogen sulfide to tarnish; it...

ing in open air with a sensitivity to humidity, requiring constant re-polishing to maintain its usefulness. This meant the telescopes mirrors had to be constantly removed, polished, and re-figured
Figuring
Figuring is the process of final polishing of an optical surface to remove imperfections or modify the surface curvature to achieve the shape required for a given application.-Types of figuring:...

 to the correct shape. This sometimes proved difficult, with the telescope mirrors sometimes having to be abandoned. This also meant two or more mirrors had to be fabricated for each telescope so that one could be used while the other was being polished.

In 1856-57 an improvement over speculum mirrors was invented when Karl August von Steinheil and Léon Foucault
Léon Foucault
Jean Bernard Léon Foucault was a French physicist best known for the invention of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation...

 introduced the process of depositing an ultra thin layer of silver (silvering
Silvering
Silvering is the chemical process of coating glass with a reflective substance. When glass mirrors first gained widespread usage in Europe during the 16th century, most were made of an amalgam of tin and mercury, but by the 19th century mirrors were commonly made through a process by which silver...

) on first surface of glass telescope mirrors. Silvered glass mirrors were a vast improvement since silver reflects 90 percent of the light that hits it and is much slower to tarnish than speculum. Also silver coatings can be removed from glass without changing the delicate shape of the glass substrate. This marked the end of the speculum-mirror reflecting telescope, with the last large one, the Great Melbourne Telescope
Great Melbourne Telescope
The Great Melbourne Telescope was built by Thomas Grubb in Dublin, Ireland in 1868, and installed at the Melbourne Observatory in Melbourne, Australia in 1869.The telescope had a speculum primary mirror, and was mounted on an equatorial mounting...

 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 the 36 inch (91 cm) Crossley Reflector (1895), 60 inch (150 cm) Mount Wilson Observatory
Mount Wilson Observatory
The Mount Wilson Observatory is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a 5,715 foot peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles...

Hale telescope of 1908, and the 100 inch (2.5 m) Mount Wilson Hooker telescope in 1917.

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