calotype, early photographic technique invented by William Henry Fox Talbot of Great Britain in the 1830s. In this technique, a sheet of paper coated with silver chloride was exposed to light in a camera obscura; those areas hit by light became dark in tone, yielding a negative image. The revolutionary aspect of the process lay in Talbot’s discovery of a chemical (gallic acid) that could be used to “develop” the image on the paper—i.e., accelerate the silver chloride’s chemical reaction to the light it had been exposed to. The developing process permitted much shorter exposure times in the camera, down from one hour to one minute.
The developed image on the paper was fixed with sodium hyposulfite. The “negative,” as Talbot called it, could yield any number of positive images by simple contact printing upon another piece of sensitized paper. Talbot’s process was superior in this respect to the daguerreotype, which yielded a single positive image on metal that could not be duplicated. Talbot patented his process in 1841.