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Technicolor Company in the production of films for the Associated Artists, we can give precise particulars of the chemical side of the transformation of black images into monochromes; these particulars are gathered from numerous experiments made by the writer, experiments supported by highly demonstrative specimens. I will limit myself here to giving an idea of the processes followed, while referring readers who are anxious for further details to a monograph I have recently published (i).
Double-faced films, developed in the usual way, in a bath of hydroquinone and metol, in such a manner as to obtain bold but unexaggerated pictures, is immersed in a transformation bath, with the object of substituting sulphocyanide of copper for reduced silver, the former being an energetic mordant for basic tar dyes. Sulphate of copper, neutral citrate of sodium and sulphocyanide of ammonium are used for this transformation bath according to a receipt which is an emendation of that given by Christensen.
The film must be wound round a drum with a non metallic surface; it is stirred about for a time in the bath until the images are completely bleached. When one series of images has been bleached, the film is reversed so as to be able to bleach those on the opposite side.
A short rinsing is sufficient after this process; the white image produced by the treatment is formed by sulphocyanide of copper and replaces the mordant needed to fix the colours.
While fixing them, the film must be wound round a drum with a rubber surface in such a way as not to allow the coloured liquid to pass behind. The drum is revolved in the colouring solution and the colour is thus fixed to the image. Green malachite and safranine red are the best dyes for the purposes of bi-chromatic photography. Experience has shown me that by the use of sufficiently diluted dyeing solutions (i : 10,000), the colours may be fixed to the image without any colouring of the
(1) Cf. Prof. R. Namias : La tecnica e la pratica dei raggi e della fissazione dei colori sulle diapositive e sui films cinematografici, pubd. by the Progresso Fotografico, Milan, 1928.