Calotype

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Calotype or talbotype is an early photographic process introduced in 1841 by William Henry Fox Talbot, using paper coated with silver iodide. The term calotype comes from the Greek καλός for 'beautiful', and τύπος for 'impression'.

How calotypes work

The sensitive element of a calotype is silver iodide. With exposure to light, silver iodide decomposes to silver leaving iodine as a free element. Excess silver iodide is washed away after oxidizing the pure silver with an application of gallo-nitrate (a solution of silver nitrate, acetic, and gallic acids). As silver oxide is black, the resulting image is visible. Potassium bromide then is used to stabilize the silver oxide.

In the case of salted paper, the sensitive element is silver chloride formed when the salt (sodium chloride) reacts with silver nitrate. Silver chloride decomposes when in contact with light forming silver and chlorine evaporates. Excess silver chloride is washed out of the paper and the silver oxidizes in contact with gallo-nitrate. The silver oxide is stabilized on the paper with hyposulphite of soda.

Silver chloride is sometimes favored over silver iodide because it is less sensitive to temperature. During long exposures in direct sunlight the temperature on the paper can be quite high.

The calotype created a negative image on the silver iodide from which positives could be printed (onto silver chloride paper). This made the calotype superior in one aspect to the daguerreotype which only made one positive image (whereby it was difficult to get multiple copies).

References

  • Aronold, H. J. P. William Henry Fox Talbot pioneer of photography and man of science (London: Hutchinson Benham, 1977).
  • Baxter, W. R. The Calotype familiarly explained, Photography: including the Daguerreotype, Calotype & Chrysotype (London: H. Renshaw, 1842, 2nd edition).
  • Buckland, G. Fox Talbot & the invention of photography (Boston: Gootine, London Scholar Press, 1980).
  • Eder, Josef Maria. History of Photography (New York: Dover Publications, 1978). Translated by Edward Epstean.
  • Lassam, and Seabourne. W H Fox Talbot: Scientist, photographer, classical scholar 1800 - 1877: a further assessment (Lacock, 1977).
  • Marshall, F. A. S. Photography: the importance of its applications in preserving pictorial records. Containing a practical description of the Talbotype process (London: Hering & Remington; Peterborough, T. Chadwell & J. Clarke, 1855).
  • Meier, Alf B. Basic Photography — a manual for the training of fashion photographers (Frankfurt/M.: Jentzen oHG, 1992).

External links

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