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 reflecting telescope optical mirrors until it was replaced by more modern materials.

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 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. 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. 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

Speculum metal found an application in early modern Europe 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 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.

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