Gemini North Telescope 8-meter mirror being aluminized.
Aluminizing is the process of coating a mirror with aluminum. The aluminum is vaporized in a vacuum, causing a film of metal, about 100 nanometers thick, to be deposited on the glass. Layers of other materials, such as silicon dioxide, may subsequently be laid down to give protection. Aluminum offers consistently high reflectance throughout the visible, near-infrared, and near-ultraviolet regions of the spectrum.
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.