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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 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.〔http://www.iop.org/EJ/abstract/0950-7671/24/9/308〕
==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〔(by Joseph Needham, Gwei-djen Lu, Science and civilisation in China, Volume 5, page 238 )〕 although it could also be an invention of western civilizations.〔(The Journal of the Anthropological Institute of Great Britain and Ireland - Volume 64 - Page 71 )〕 Such metals were used in sculpture and to make more effective mirrors than the more common yellow easily tarnishing bronze mirrors. In that era mirrors of speculum metal, or any precious metal, were rare and only owned by the wealthy 〔(Albert Allis, The Scientific American cyclopedia of formulas, page 89 )〕
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