Photograph courtesy of Camera. Victorian Eyewitness by Gus Macdonald  (ISBN No.0 7134 2095 2)
Talbots wife Constance. One of Talbots first Calotypes.

The Calotype process was invented by William Henry Fox Talbot in 1841.
A sheet of paper was treated with silver nitrate solution and then with potassium iodide.
When dry, this paper kept almost indefinitely.
Before exposure, the paper was sensitized by floating it on a solution of gallic acid, silver nitrate and acetic acid. It was then exposed for a short time in a camera to form a latent image which was not visible until it had been developed in the same solution as that used for sensitizing.
By this means, Talbot could obtain pictures in strong sunlight in one minute with a lens aperture of f/15.