On the stability of Robert Adamson's salted paper prints

Abstract

Since the inception of the silver salted paper print, its problems have always lain in the fixing — rather than the recording — of the image. The light-sensitive material is ‘photogenic drawing paper’, so-named by William Henry Fox Talbot, who discovered in June 1834 that the sensitivity to light was higher when the ratio for combining silver nitrate and sodium chloride (common salt) was deficient in the halide.1 The corollary to this observation, mutatis mutandis, was that excess halide should greatly suppress the sensitivity. This proved so, providing Talbot with the first methods of fixing silver photographs by means of strong solutions of sodium chloride or potassium iodide.2 Neither fixing treatment, however, secured a completely stable result: in daylight, the palest tones of chloride-fixed silver images tended to acquire a purplishgrey or lavender-coloured fog of photolytic silver,3 which could eventually obliterate details of the image; conversely, in iodide-fixed photographs the mid-tone details tended to fade to the uniform primrose-yellow colour of silver iodide.4 These disadvantages were overcome in January 1839 by Sir John Herschel's invention of ‘hypo’ fixation,5 using a solution of sodium thiosulphate (then called ‘hyposulphite of soda’), which is capable of completely dissolving the unreacted, water-insoluble silver chloride. In presenting this solution, Herschel was drawing on his pre-photographic chemical discoveries of 1819, published in the Edinburgh Philosophical Journal.6