In contrast to tinting, toning is not the simple immersion of a film into a dye bath but involves a chemical reaction converting the silver image. In this reaction the neutral silver image in the emulsion of the positive film is replaced by one consisting of colored metal compounds. These were usually iron ferrocyanide (Prussian Blue) for blue, copper ferrocyanide for red/brown, silver sulfide for sepia or rarely uranium ferrocyanide for reddish brown. Toning had been used in still photography before. But since film was projected on the screen it required translucent toning compounds.
For tinting, the positive print is immersed into a variety of dye baths, scene by scene. To this end, the print has to be cut into the corresponding fragments and reassembled after the dyeing process. The dye homogeneously attaches over the entire image’s gelatin including the perforation area. Usually synthetic dyes were dissolved in a weak acid solution to form a chemical bond with the gelatin.
“In 1898 William Friese-Greene, a professional portrait photographer by trade, demonstrated in London ‘the first process of true natural-color cinematography.’ His program consisted of ‘a series of animated natural-color pictures,’ and although this demonstration aroused considerable interest at the time, Friese-Greene was unable to exploit this system on a profitable basis. Undaunted, he eventually developed a total of four different color methods.”