Desensitizing of Photographic Emulsions

by L.P. Clerc, Photography, Theory and Practice (chapter 27, p. 222)

Use of Coloured Developers

Carey-Lea, as far back as 1877, showed that after immersing a photographic plate in ferrous oxalate developer it was possible, without risk of fog, to illuminatethe dark-room much more brightly than would have been possible during the handling of the sameplate when dry. This effect was attributed to the orange-red colouration of the developer (it has since been recognized that ferrous oxalate developer is a very efficient desensitizer), and little attention was paid to it, since the emulsions were so insensitive that in all cases a bright illumination could be used.

About 1889 various attempts were made to introduce into photographic practice the use of developers which had been coloured red by the addition of certain dyes, these dyes being subsequently destroyed in an acid fixing bath (coralline, croceine). The method, however, was not successful, since the dyes transmitted both blue and violet light. In 1900 Auguste and Louis Lumière and A. Seyewetz, with the same object in view, recommended the use of an orange-coloured compound, magnesium picrate, which allows the development to be observed at a distance of about 45 cm from a candle or at 12 cm from a 16 c.p. lamp, provided development is sufficiently rapid and the plate remains covered by a layer of developer about 1:27 cm thick. If these distances are doubled, it is possible to remove the negative from the bath and examine it very quickly by transmitted light. This method of working, although it afforded interesting demonstrations, was not used in practice. Continue reading →