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Calotype Negative

Willliam Henry Fox Talbot’s next breakthrough came between the years of 1840 and 1841, when he further refined the steps of the negative-to-positive process. (During the course of his experiments he even considered the option of creating direct positive images which he called leucotypes.) The inventor, however, began to have better success with a new technique: the developed out negative, known as a calotype. The calotype was a latent image process. The visible image was produced only by further chemical development. Like the photogenic drawing negative, Talbot could generate multiple positive prints of a single image from the calotype.

To produce a calotype, Talbot created a light-sensitive surface by coating a sheet of paper, usually writing paper, with a solution of silver nitrate. He dried the paper to some degree and coated it with potassium iodide to produce silver iodide. Prior to exposing the negative in the camera, Talbot gave the paper a final coating of a solution of silver nitrate mixed with acetic and gallic acids. The sensitized paper was then exposed in the camera.

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“Cart on a road leading to a stone building, possibly a church in southeast England,” George Shepherd, calotype (paper process), Houghton Library, Harvard University
After exposure, Talbot discovered he could bring out the latent image, which had formed on the negative but was not yet visible, by washing the paper again in silver nitrate mixed with small quantities of acetic and gallic acids. Talbot’s application of latent image technology greatly increased the photographic sensitivity of the negative and thus reduced the necessary exposure time in the camera. This technique, known as a developed out process, brought out the visible image with the use of chemicals. 

The final step entailed fixing the calotype with hyposulfite of soda (hypo) or halides (potassium iodide and potassium bromide). Calotypes fixed with potassium iodide exhibited a yellow highlight tone. To produce a print with greater detail, Talbot often waxed the negative after processing. Wax penetrated the paper fibers making the negative more translucent. This process allowed more light to come through during printing and produced a print with less visible paper fibers. 

While historically some have used the word “calotype” in reference to both early negative and positive images, the term “calotype” here refers to the paper negatives created with the process as developed by Talbot and used by the inventor, his circle of friends, and other amateur photographers of the time.