Part II.
TONING
As distinct from tinting, a toned image consists of a color image embedded within a layer of colorless gelatine, so that while the highlights are clear, the shadows are colored.
The coloring matter may consist of an inorganic colored compound or a dye, or of a mixture of both, and by wholly or partially replacing the black and white silver image by one or more of these substances, a colored image is produced.
It is, of course, important that the colored compound should be as transparent as possible so that the full tone will be apparent on projection. In this respect, while some samples of toned film appear colored in the hand, they produce a substantially black and white image on projection, so that when judging any particular tone, it is absolutely necessary to view the image on the screen.
Of the various inorganic colored compounds, the metallic ferrocyanides are the most suitable for toning purposes of which:
Iron (ferric) ferrocyanide is blue,
Copper ferrocyanide is red, and
Uranium ferrocyanide is reddish brown.
It is the object in toning to replace the metallic silver composing the image more or less by one of the above compounds or by a mixture of these. In this way intermediate tones are obtained.
Toning may be effected either by a single solution process or a two solution process.
Two Solution Process
This consists of first converting the silver image into silver ferrocyanide by a suitable bleaching bath, thoroughly washing, and acting upon the ferrocyanide image with a metallic salt, usually in presence of an acid, the metallic ferrocyanide being produced by double decomposition. The reaction, however, is never complete, so that the image is mixed with undecomposed silver ferrocyanide which tends to make the image opaque, but at the same time it also adds “body” to the latter. Providing allowance is made in the original positive for this intensification, good tones are obtained. (See Tone G.)