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Calotype process

"The calotype negative process was sometimes called the Talbotype, after its inventor. It was not Talbot's first photographic process (introduced in 1839), but it is the one for which he became most known. Henry Talbot devised the calotype in the autumn of 1840, perfected it by the time of its public introduction in mid-1841, and made it the subject of a patent (the patent did not extend to Scotland).

The base of a calotype negative, rather than the glass or film to which we have become accustomed, was high quality writing paper. The sheet of paper was carefully selected to have a smooth and uniform texture and, wherever possible, to avoid the watermark. The first stage, conducted in candlelight, was to prepare what Talbot called his iodized paper. The paper was washed over with a solution of silver nitrate and dried by gentle heat. When nearly dry, it was soaked in a solution of potassium iodide for two or three minutes, rinsed and again dried. As long as this iodized paper was stored carefully, it could be kept for some time, so it was generally prepared in batches ahead of time.

Immediately before taking a photograph, a fresh solution of gallo-nitrate of silver was mixed up. This was made from equal quantities of a solution of silver nitrate and one of gallic acid; the solution was unstable and had to be used right away. Under weak candlelight, a sheet of iodized paper was coated with this solution, left to sit for about thirty seconds and then dipped in water. It was then partially dried in the dark, often using blotting paper. The calotype paper could be employed completely dry, but was more sensitive when moist, and in any case had to be exposed in the camera within a few hours of preparation (Talbot found that he could sometimes put it away for future use but its keeping qualities were never predictable)."

full text: http://special.lib.gla.ac.uk/hillandadamson/calo.html

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