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The salt print was the dominant paper-based photographic process for producing positive prints during the period from 1839 through approximately 1860.
The salted paper technique was created in the mid-1830s by English scientist and inventorHenry Fox Talbot. He made what he called "sensitive paper" for "photogenic drawing" by brushing a sheet of writing paper with a weak solution of ordinary table salt (sodium chloride), drying it, then brushing it with a strong solution of silver nitrate. This produced a tenacious coating of silver chloride in an especially light-sensitive chemical condition. The paper darkened where it was exposed to light. When the darkening was judged to be sufficiently strong, the exposure was ended and the result was stabilized by applying a concentratedsolution of salt, which altered the chemical balance and made the paper only slightly sensitive to additional exposure. In 1839, washing with a solution of sodium thiosulfate ("hypo") was found to be a more completely effective way to make the results light-fast.
The salt print process is often confused with Talbot's slightly later calotype or "talbotype" process, in part because it was normally used when making prints from calotype paper negatives. Calotype paper employed silver iodide instead of silver chloride, but the most important difference is that it was a developing out process, not a printing out process like the salt print, meaning that a much shorter exposure was used to produce an invisible latent image which was later chemically developed to visibility. This made calotype paper far more practical for use in a camera. Salted paper typically required at least an hour of exposure in the camera to produce a negative showing more than objects silhouetted against the sky.
- "Calotypes". Mhs.ox.ac.uk. Retrieved 2013-06-24.