Before the 1930s, when the vacuum aluminizing technique was developed, the only method available for coating mirrors was silvering in which the silver was deposited chemically by a process developed by the German chemist Justus von Liebig. The initial reflectivity of silver (89 to 93%) is slightly higher than that of aluminum throughout most of the visible spectrum, but this advantage is short-lived because silver quickly tarnishes, whereas the reflectivity of an aluminum coating drops by only a few percent per year. Silver, too, is much more expensive. See also speculum.
Related category• TELESCOPE EQUIPMENT AND TECHNIQUES
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