Films coloured by toning.
Toning was widely used in the silent period, perhaps less frequently than tinting because it sometimes called for several baths for the (already developed) parts to be coloured. However some single solution toning processes were common especially Iron-tone Blue and the various red-brown tones.
Toning is the process of exchanging part or the entire original silver image for another coloured material. It had already been widely used for making coloured still paper prints but not all the paper techniques were suitable as the replacement material has to be at least partially translucent to project a coloured image. Totally opaque dyes would simply be seen as black on projection. However, the lantern slide industry had already tried out most techniques and undoubtedly the new motion picture industry learnt from this.
Toning colours the dark parts of the image leaving the clear parts unaltered since only the silver image, or part of it, is removed and replaced with another metal salt, a coloured dye or a coloured silver salt. It's therefore usually quite easy to distinguish a toned image from a tinted one provided the tone has not faded. Sepia Toning, in which the silver image is replaced by reddish brown silver sulphide, was originally quite a fiery ginger colour by reflected light but this fades over the years to a dark brown that is sometimes mistaken for untoned silver.
The most common form of toning (even now sometimes used in still photography) consisted of transforming the silver (which is neutral and opaque) into inorganic coloured salts of other metals. This is sometimes called metallic toning.
Using this procedure only a restricted number of colours could be achieved: red or red-orange (an image of copper or uranium ferrocyanide), and blue (an image of ferric ferrocyanide, also called Prussian blue]. Less commonly, vanadium [greenish-brown] and chromium [brown] salts were also used. The Eastman Kodak manual on tinting and toning recommends vanadium for a greenish-brown, but this colour is rarely seen [or rarely recognised].
Generally, metallic toned films are rather unstable and tend to decay more quickly than the tinted films probably because some metals accelerate [i.e. are catalysts for] the cellulose nitrate base decomposition process. Sometimes the decay of the toning means that the dark parts are affected most with a corresponding disappearance of the image or the production of a striking "solarisation" effect. All toned images fade in time and some are very susceptible to fading accelerated by the intense ultra-violet emission of projection equipment. It is therefore essential to look at the extreme edges of any frame to see if any less faded or altered toning or dye can be seen and never to assume that a tone is unaltered. Prussian blue seems to fade patchily, sometimes vanishing entirely, sometimes darkening to muddy neutral colours.
Another procedure, called loosely mordanting or mordant toning, permitted the transformation of some or all of the silver salts or silver into an insoluble salt that was almost colourless which were capable of absorbing certain basic dyes selectively. In this way, it was possible to tone an image with a much wider range of colours. A considerable range of colours was possible, some appeared very similar to tinting in that the perforated edges, and highlights were stained as well, but to a lesser extent. This makes it very difficult to be certain about some effects.
A great colour range was possible using colour development, a process later used to develop primary colours in true colour film. .
Several textbooks mention the use of bichromated gelatine and the wash-off process for selectively imbibing dyes but it seems that these processes were not used for colouring films although both techniques were used for three colour subtractive films in the 1930's, the last being widely used in the Technicolor imbibition process.
In all toning processes, the final image is a dye or coloured salt and some remaining silver image. Several manuals produced in the 1920's describe a technique for the removal of the silver image leaving just the coloured image behind. The procedure consisted of fixing the film in a conventional fix solution [of sodium thiosulphate]. There seems no good reason why this should have worked, as silver is only sparingly soluble in fix solutions. In practice, the bleaching effect is quite dramatic and may rely on some residual ferricyanide salts unwashed from the emulsion forming a powerful silver bleach or solvent known to photographers as Farmer's Reducer. Prussian Blue, copper and uranium tones, and mordant dye tones were probably often intensified by this technique.
It is possible to carry out all these procedures today using modern black and white print stocks but it is by no means certain that the colours and contrasts generated are the same as those that were produced with the film stocks, developers and sources of chemicals originally.
Occasionally a toning technique was used for colouring an entire film. "Good Earth" in 1937 was toned an overall brown using mordant dye tone used in a "modern" developing machine at MGM laboratories in Los Angeles. The film was 12,000 ft long, of 14 reels, and 500 prints were made and toned, a total of 6,000,000 ft probably taking about 2,000 hours just to tone!