[this querry was conveyed by Traherne's note - see Doc. No: 03834].
8th March 1839
The exact proportion between the salt and water, and between the nitrate of silver and water which he considers best.<1>
And the time between repeating the washings which produces the greatest sensibility.<2>
Calvert R. Jones
1. WHFT's photogenic drawing utilised silver choride as the light-senstive agent. However, this was insoluble in water, making it impossible to coat onto the paper, and necessitating a two-step process. First the plain writing paper was brushed or soaked in a solution of table salt, and then silver nitrate was brushed over. A chemical reaction occurred, precipitating light-sensitive silver chloride directly into the fibres of the paper, where it was physically trapped. The ratio was an exquisitely tricky one, a question explored in Dr. Mike Ware's Mechanisms of image deterioration in early photographs: the sensitivity to light of W.H.F. Talbot's halide-fixed images, 1834-1844 (London: National Museum of Science and Industry, 1994).
2. WHFT found that by alterating several applications of salt and silver that the sensitivity of the paper was increased, a crucial step towards its use in the camera.