The gelatin dry plate
1871 (Maddox's proposal); 1878 (Bennett's fast emulsion; factory plates)
Silver bromide held in gelatin made plates that were factory-made, kept for months and fast enough to expose in a fraction of a second. The snapshot became possible.
How it began
In 1871 Richard Leach Maddox, an English physician and microscopist, described an emulsion of silver bromide in gelatin to replace collodion. It was slow at first. In 1878 Charles Harper Bennett found that keeping the emulsion warm for days, “ripening” it, made it far more sensitive. Factories in Britain, the United States and Germany began making plates, among them a young George Eastman in Rochester from 1880.
How it works
Gelatin, a protein from animal tissue, holds tiny silver halide crystals evenly in a dry layer: an emulsion. Ripening makes the crystals larger, and traces of sulphur compounds in the gelatin create sensitivity specks, so a short exposure produces a latent image that development can bring out. Gelatin swells in water to let developer in, then dries hard again.
Why it mattered
Plates no longer had to be made on the spot, and exposures fell to fractions of a second. Photographers needed shutters, cameras could be held in the hand, and motion could be frozen. Gelatin emulsion on flexible roll film led directly to the Kodak and amateur photography.
Limits and what replaced it
Glass was heavy and breakable. Celluloid roll film replaced plates for amateurs from 1889, and sheet film for most professionals in the 20th century. The gelatin emulsion itself has never been replaced in film.
Today
Gelatin silver halide emulsion is still the light-sensitive layer of every roll of film.
Sources
- Wikipedia: Dry plate ↗
- Eder, ch. LIX–LXI (pp. 421–439)