Speculum metal

The speculum metal mirror from William Herschel's 1.2-meter (49.5-inch) diameter "40-foot telescope", at the Science Museum in London.^[1]

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 is used primarily to make different kinds of mirrors including early reflecting telescope 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 when evaporated onto the surface.^[2]

Overview

Speculum metal mixtures usually contain two parts copper to one part tin along with a small amount of arsenic, although there are other mixtures containing silver, brass, lead, or zinc. 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^[3] although it could also be an invention of western civilizations as well. ^[4]. Such metals were used in sculpture and to make more effective mirrors than the more common yellow easily tarnishing bronze mirrors. Mirrors of speculum metal or any precious metal were rare and only owned by the wealthy ^[5]

Speculum and telescopes

Speculum metal found an application in early modern Europe as the only known good reflecting surface for mirrors in reflecting telescopes. Such telescopes needed first surface mirrors that could be ground and polished into complex shapes such as parabolic reflectors. 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 could not be built because Gregory could not find a craftsman capable of fabricating the complex speculum mirrors needed for the design.^[6] Isaac Newton 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.^[7] 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^[8] 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's 126-cm (49.5-inch) "40-foot telescope" of 1789 and Lord Rosse 1845 183-cm (72-inch) mirror of his "Leviathan of Parsonstown", impracticalities in using the metal made most astronomers prefer their smaller refracting telescope counterparts.^[9] 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 tarnishing 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 to the correct shape. This sometimes proved difficult, with the telescope mirrors sometimes having to be abandoned.^[10] This also meant two or more mirrors had to be fabricated for each telescope so that one could be used wile the other was being polished.

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 (silvering) 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 last large metal mirror telescope of the era, the Great Melbourne Telescope with a 122-cm (48-inch) aperture being completed in 1867. The era of 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 Hale telescope of 1908, and the 100 inch (2.5 m) Mount Wilson Hooker telescope in 1917.^[11]

See also

• Bronze
• Silvering
• Liquid mirror

References

External links

• National Pollutant Inventory — Copper and compounds fact sheet