Types of reflecting surfaces:

Alloy of copper (68%) and tin (32%) initially used to manufacture mirrors of reflecting optical instruments. Before Mudge developed the alloy in the proportions listed above, Newton (1642-1727), inventor of the optical combination that bears his name, used bell alloy made smoother by the addition of arsenic, thanks to indications provided by alchemists. Herschel (1738-1822) used a 75% copper and 25% tin blend. The alloy was reasonably efficient but deteriorated very rapidly, requiring the mirror to be repolished and therefore, in practice, reprofiled. The very low reflectiveness of all-metal mirrors (~60%) prevented the dissemination of the reflecting telescope for nearly two centuries—until the introduction of silvering, which allowed the use of glass mirrors.

Procedure for imparting a reflecting surface to optical-instrument mirrors. Consists in depositing a thin layer of extra-pure silver obtained by chemical reduction of silver nitrate. After the introduction of aluminum coating, silvering has now been virtually abandoned for professional purposes. This is because of the very brief life-span of the silver layer (about six months), which quickly deteriorates and thus drastically reduces the mirror's reflectivity, in particular at shorter wavelengths. The procedure was introduced in 1856 by Karl August Steinheil (1801-70) and, independently, the following year, by Jean Bernard Léon Foucault (1819-68). Author of the famous pendulum experiment, Foucault employed the glass silvering technique perfected a few years earlier by the German chemist Liebig (1803-73). By allowing the production of high-reflectivity mirrors, silvering spelled the abandonment of mirrors made entirely of molten metal, and thus promoted the development of the modern reflector.

Process to impart a reflecting surface to optical-instrument mirrors. Consists in depositing a thin layer of extra-pure aluminum via evaporation under vacuum. Aluminum coating, developed by John Strong in 1931, has replaced the use of silvering thanks to its greater resistance and longevity. The duration is extremely variable depending on atmospheric pollution conditions, but averages 5 years. Aluminum-coated surfaces can also be covered with a thin layer of quartz or silicon monoxide, which extends its useful life to 10-20 years.