How mirrors are made

A bathroom mirror is plain window glass with a film of silver on the back, hidden under a coat of paint.

stations: 5
things upstream: 78
raw sources: 28
steps from the ground: 10

The short version

A household mirror is a plate of soda-lime glass coated on its back with a thin film of metal, usually silver, and then sealed under paint. The silver is deposited by a chemical reduction of silver nitrate, a method Justus von Liebig invented in 1835. Telescope mirrors instead get a vacuum-deposited aluminum coating.

Cut the glass plate

The base is a plate of transparent glass, usually soda-lime float glass. Lead glass is sometimes used for decorative effect, and plastic is used where weight or breakage matters.

Goes in
Window glass
Silver the back

In the wet process, a solution of silver nitrate is chemically reduced onto the glass, leaving a thin layer of metallic silver. This replaced the old tin-mercury amalgam and made mirrors affordable.

Goes in
Silver
Or vacuum-coat aluminum

Mirrors for optical instruments are usually made in a vacuum chamber, where aluminum is sputtered or evaporated onto the glass. Many of them carry the metal on the front face so light does not have to cross the glass twice.

Goes in
Aluminum
Paint over the metal

A layer of paint goes over the metal to shield it from abrasion and corrosion. Without it the thin film would scratch or tarnish within months.

Goes in
Paint
Bond a safety film

A flexible transparent plastic film is sometimes bonded to the front or back, so that if the glass breaks, the pieces stay in place instead of falling.

End of the lineMirror

Worth repeating

Obsidian mirrors found at Çatalhöyük in Anatolia date to around 6000 BCE.
In the late seventeenth century, the Countess de Fiesque reportedly traded an entire wheat farm for a Venetian mirror and considered it a bargain.
Aluminum reflects 85 to 90% of visible light, and it resists tarnishing better than silver does.