Silver salts darken in light
c. 1717 (published 1727)
Johann Heinrich Schulze showed that silver salts darken because of light, not heat, and printed words onto them with paper stencils. Silver became photography's metal.
How it began
Around 1717 the German professor Johann Heinrich Schulze was trying to make a glowing substance. He mixed chalk with nitric acid that happened to contain silver. The mixture turned dark purple, but only on the side of the flask facing the window. When he heated a flask in the dark, nothing happened, so the cause was light, not heat.
How it works
Silver salts such as silver nitrate and silver chloride are unstable in light. Light frees metallic silver, which looks dark because it is finely divided. Schulze stuck paper letters to the flask and found the words written in dark silver when he peeled them off. He had made the first image by light, but it lasted only until the rest darkened too.
Why it mattered
This was the chemical half of photography. In 1777 Carl Wilhelm Scheele showed that violet light acts on silver chloride more strongly than red, and that the darkened part is metallic silver. Every silver-based process that followed, from the daguerreotype to colour film, builds on this.
Limits and what replaced it
The images could not be kept: light went on darkening the unexposed silver. Making them permanent waited for a fixer that removes the unexposed salts, and for development, which made photographs fast enough to use.
Today
Black-and-white film and paper still record light with silver halide crystals.
Sources
- Wikipedia: Johann Heinrich Schulze ↗
- Eder, ch. IX–X (pp. 57–82). Eder dates the discovery by its 1727 publication.