Salt print

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 inventor Henry Fox Talbot. He made what he called "sensitive paper" for "photogenic drawing" by wetting a sheet of writing paper with a weak solution of ordinary table salt (sodium chloride), blotting and drying it, then brushing one side 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 sufficient, the exposure was ended and the result was stabilized by applying a strong solution 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 the most effective way to make the results truly 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 then 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 yield a negative showing much more than objects silhouetted against the sky.

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