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.