Calotype

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The Calotype is an early negative-positive photographic process invented by William Henry Fox Talbot over a period from January 1834 to publishing and patenting the method in January 1838. The Calotype was the first viable negative-positive process. Fox Talbox called his images Photogenic Drawings, and had been experimenting for some time with the earlier ideas of Thomas Wedgewood[1]. The process was also known as the Talbotype after its inventor.

Preparation

Calotype negatives were made on paper. Best quality writing paper was brushed with a solution of silver iodide and potassium iodide, and then dried. The paper could then be stored until needed. Before exposure, the paper was run through a bath containing a solution of silver nitrate, gallic acid and acetic acid. Exposures could then be made with the paper still wet, or after drying. Typical exposures could be around five minutes in sunlight.

Developing

The latent image on the paper could be developed in a mixture quite similar to the sensitisation bath - a solution of silver nitrate and gallic acid. After development, the paper was rinsed and fixed - in early versions using a salt solution, but later in sodium thiosulphate ("hypo").

The resulting negatives would be contact-printed on to paper treated with silver chloride. The negative was often waxed, to make it transparent, to speed up printing. Later, the negatives were waxed before sensitisation, which both increased sensitivity and reduced the effect of the paper grain.

The Calotype process was superceded by the invention of the wet-collodion process.

Note

  1. examples of Fox Talbot photogenic drawings at the University of Oxford Museum of the History of Science

Bibliography