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 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, although there are other mixtures containing silver, lead, or zinc. This is about twice the proportion of tin to copper typically used in bronze alloys. Archaeologists and others prefer to call it "high-tin bronze",[2] although this broad term is also used for other alloys such as bell metal, 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 the mid-17th and mid-19th centuries, 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. 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,[3] although it could also be an invention of western civilizations.[4] Remarks in Pliny the Elder may refer to it.[5] It was certainly in use by the European Middle Ages, 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 in them.[6]
It was not suitable for "cold-working" techniques such as repoussé and chasing, being much too hard, but worked well if cast into small objects, and was also used for "Dark Age belt fittings, buckles, brooches" and similar small items, giving an attractive silver-white colouring.[7]
Use in telescopes
Speculum metal found an application in
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 183-cm (72-inch) mirror of his "Leviathan of Parsonstown" of 1845, impracticalities in using the metal made most astronomers prefer their smaller refracting telescope counterparts.[11] 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.[11] 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.[12]
In 1856–57 an improvement over speculum mirrors was invented when
See also
- Liquid-mirror telescope
- List of largest optical telescopes in the 19th century
- List of largest optical telescopes in the 18th century
References
- Science Museum. Archived from the originalon 2009-09-02. Retrieved 2008-11-23.
- ^ Meeks, 63-65
- ISBN 978-0-521-08571-7.
- ^ The Journal of the Anthropological Institute of Great Britain and Ireland. Vol. 64. Anthropological Institute of Great Britain and Ireland. 1934. p. 71.
- ^ Meeks, 63-64
- ISBN 0198661134; Meeks, 65
- ^ Meeks, 65
- ^ Robert Chambers; Thomas Thomson (1875). A Biographical Dictionary of Eminent Scotsmen. p. 175.
- ISBN 978-0-486-43265-6.
- ^ Norman W. Henley et al: Speculum Metal.
- ^ a b c Edison Pettit: The Reflector. Astronomical Society of the Pacific Leaflets. Vol. 7, No. 331, pp. 249–256. December 1956.
- ^ Voyage through the universe: The Visible Universe. Time-Life Books, 1990. (Web clip).
- 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