Silvering

Silvering is the chemical process of coating a non-conductive substrate such as glass with a reflective substance, to produce a mirror.

Most common household mirrors are "back-silvered" or "second-surface", meaning that the light reaches the reflective layer after passing through the glass.

Therefore, precision optical mirrors normally are "front-silvered" or "first-surface", meaning that the reflective layer is on the surface towards the incoming light.

A hard, protective, transparent overcoat may be applied to prevent oxidation of the reflective layer and scratching of the metal.

[4] In the early 10th century, the Persian scientist al-Razi described ways of silvering and gilding in a book on alchemy,[citation needed] but this was not done for the purpose of making mirrors.

[5] When glass mirrors first gained widespread usage in Europe during the 16th century, most were silvered with an amalgam of tin and mercury,[6] In 1835 German chemist Justus von Liebig developed a process for depositing silver on the rear surface of a piece of glass; this technique gained wide acceptance after Liebig improved it in 1856.

In 1856-1857 Karl August von Steinheil and Léon Foucault introduced the process of depositing an ultra-thin layer of silver on the front surface of a piece of glass, making the first optical-quality first surface glass mirrors, replacing the use of speculum metal mirrors in reflecting telescopes.

[14] Silver would be ideal for telescope mirrors and other demanding optical applications, since it has the best initial front-surface reflectivity in the visible spectrum.

Conversely, a thin gold coating is used to create optical filters which block infrared (by mirroring it back towards the source) while passing visible light.