Calotype

Paper negative process discovered by William Henry Fox Talbot in September 1840 and patented by him in 1841. Good-quality writing paper was treated with a silver nitrate and potassium iodide solution. After drying, the paper could be stored indefinitely in the dark or used immediately. Prior to exposure in the camera the paper was usually sensitized using a freshly prepared solution of silver nitrate, gallic acid and acetic acid. After exposure a similar gallo-nitrate of silver solution was used to develop the image, which was fixed using a solution of potassium bromide or more commonly ‘hypo’ (hyposulphite of Soda or sodium thiosulphate). The final negative was usually waxed to make it translucent. The positive prints made from these negatives were printed on to salted paper as described by Talbot for his ‘photogenic drawing’ process. The calotype process was the dominant negative-positive process practised between 1841 and 1851. It was popular with amateur landscape and architectural photographers, particularly in Britain and France. The inexact chemical knowledge of the time affected the stability of the calotype so that it had a tendency to fade, while the irregular paper grain produced a less finely detailed representation of visible reality.
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