Speculum metal

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.

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.

[5] It was certainly in use by the European Middle Ages, giving better reflectivity than the usual bronze mirrors, and tarnishing more slowly.

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.

His first reflecting telescope (a design which came to be known as a Newtonian reflector) had a 33-mm (1.3-inch) diameter speculum metal primary mirror of his own formulation.

Speculum also had the unfortunate property of tarnishing in open air with a sensitivity to humidity, requiring constant re-polishing to maintain its usefulness.

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 ]
Modern replica of Herschel's 6.2"-aperture Newtonian telescope which had a speculum metal main mirror
Telescopes with speculum metal mirrors were a large breakthrough in aperture, but their drawbacks fueled competition from refractors
The metal mirror of the Leviathan, the largest telescope mirror until the 100-inch Hooker telescope of 1917 (a metal-on-glass mirror)
Looking down the insides of an old reflecting telescope. It's not clear whether the reflector in this case is speculum metal, but it illustrates how a reflecting mirror rests at the inside of tube. Dating to the 18th century, this telescope would have originally used a metal mirror.