How Mirrors Are Actually Made
Before 1835, mirrors were made with mercury and tin. The Venetian workers who made them got shaky hands, lost teeth, and died young.
For most of mirror history, a mirror was a "tin amalgam" — a sheet of tin foil laid on a flat stone, flooded with liquid mercury, and a pane of glass pressed carefully onto the pool. The mercury wet the tin, the excess was tilted off, and the remaining alloy hardened into a reflective layer behind the glass. Venice dominated the trade from Murano for three centuries. The mirror makers got shaky hands, lost teeth, and died young. Mercury poisoning was the occupational hazard; Venetian records list hatters and mirror-silverers together.
In 1835 a German chemist named Justus von Liebig worked out a reaction between silver nitrate, ammonia, and an aldehyde — glucose, Rochelle salt, later formaldehyde — that would precipitate a mirror-bright layer of metallic silver onto clean glass at room temperature. No mercury, better optics, and a thinner coating. The "silvered" mirror is still essentially his process, scaled to sheet-glass production.
Modern float-glass mirrors add a protective copper layer behind the silver, then a two-coat paint back to keep air and humidity off. That's why a bathroom mirror blackens from the edges over the years: the paint seal is failing and the silver is oxidizing.
Telescope mirrors are different. They're front-silvered so light doesn't pass through glass at all, and they use aluminum deposited in a vacuum chamber, with a protective SiO2 overcoat. The Hubble's 2.4-meter primary and the James Webb's gold-coated beryllium segments are engineering reversals of the household mirror, but the geometry is the same: atoms of metal, laid down one layer thick, turning a transparent substrate into a reflector.
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