Calotype
Calotype or talbotype is an early photographic process introduced in 1841 by William Henry Fox Talbot,[1] using paper[2] coated with silver iodide. The term calotype comes from the Greek καλός for "beautiful", and τύπος for "impression".
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[edit] 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.[3] 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 process created a translucent original negative image from which multiple positives could be made by simple contact printing. This gave it an important advantage over the daguerreotype process, which created an opaque original positive that could only be duplicated by copying it with a camera.
[edit] Popularity
Despite their flexibility and the ease with which they could be made, calotypes did not displace the daguerreotype.[4] In part, this was the result of Talbot having patented his processes, unlike Daguerre who had been granted a stipend by the French state in exchange for making his process publicly available.[5]. In addition, the calotype produced a less clear image than the daguerreotype. The use of paper as a negative meant that the texture and fibers of the paper were visible in prints made from it, leading to an image that was slightly grainy or fuzzy compared to daguerreotypes, which were usually sharp and clear[6][7] Nevertheless, calotypes -- and the salted paper prints that were made from them -- remained popular in the United Kingdom and on the European continent outside of France until the collodion process enabled photographers to make glass negatives later in the nineteenth century.
[edit] References
- ^ http://www.loc.gov/pictures/collection/dag/timeline.html
- ^ https://www.ncjrs.gov/pdffiles1/nij/225328.pdf
- ^ http://www.loc.gov/teachers/classroommaterials/lessons/world/studentprep.html
- ^ Carlebach, Michael L. (1992). The Orgins of Photojournalism in America. Smithsonian Institution Press. ISBN 1-56098-159-8.
- ^ Carlebach, Michael L. (1992). The Orgins of Photojournalism in America. Smithsonian Institution Press. ISBN 1-56098-159-8.
- ^ Carlebach, Michael L. (1992). The Orgins of Photojournalism in America. Smithsonian Institution Press. ISBN 1-56098-159-8.
- ^ http://www.loc.gov/rr/print/coll/589_calotype.html
- 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.
- Greene, A. Primitive Photography: A Guide to Making Cameras, Lenses, and Calotypes (Focal Press, 2002)
- 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).
- US Patent 5171, Henry Fox Talbert: "Improvement in Photographic Pictures" filing date Jun 26, 1847
