Speculum metal

Metal mirror is a very hard, well polishable, whitish bronze alloy of copper, tin and other metals additives that improve the hardness and polishing.

Alloys

Some alloys are used for mirror:

• Simple tin bronze from copper and tin in the ratio of 4:1 to 2:1, in part, with the addition of 1-2% arsenic.
• 71-72 % copper, 18-19 % tin, 4-4.5 % antimony and lead ( ancient Roman mirror ).
• 80.8 % copper, 9.1% lead, 8.4% antimony ( a Chinese metal mirror ).
• Platinum and steel in a 1:1 ratio ( results in a particularly white metal ).
• 350 parts of copper, 165 parts of tin, 20 parts of zinc, 10 parts of arsenic, 60 parts of platinum.

Use

Because of the good polishing ( speculum ) could be produced from the alloy metal mirror; even for the production of primary mirrors of telescopes was used. For example, Lord Rosse a 1845 this alloy for the production of the 1.8 -meter and 3.5 -ton mirror of the "Leviathan".

The disadvantage was the alloy that they started off relatively quickly in the air and so subsided that the reflectivity; a constant polishing the mirror was required. Metal mirrors were replaced by silver-plated glass mirror with their development by Carl August von Steinheil and Leon Foucault, based on a method of Justus Liebig, 1859.