The second ways these postcards were coloured was through toning. Toning is a method of processing photographic prints, where the silver particles that make up the image are converted into a silver compound, which changes the colour and stability of the finished print. The most familiar type of toning is sepia-toning, where the final image is made up of silver sulfide, which produces the sepia colour. Instead of the darks being black, they are dark brown.
Though sepia toning is sometimes seen in these postcards, a different type of toning was much more common: dye toning. This process was frequently used to colour films in the period, but its use in photography seems almost entirely confined to these postcards. When a print (or film reel) is dye-toned, the original silver of the image is converted to silver ferrocyanide. Silver ferrocyanide is itself colourless, but reacts with dyes. The dye particles attach themselves to the silver ferrocyanide particles, resulting in concentrations of colour in the same areas in which the exposed silver was concentrated–the image areas. The end result is a photograph where the visible image is made up of dye.
As in a sepia print, the blacks are no longer black. Rather, they are dark pink (or blue, or green, etc.). This is the key difference with a tinted photograph. In a dye-toned print, the colour is not applied over a black-and-white image; rather the image is made of the colour.
(left: tinted print, ca. 1910, from a friend’s collection; right: dye-toned print).
I have yet been able to find any information on the dye-toning of photographs, although the method of dye-toning can be read about in manuals for its use in film (see, for instance, this 1927 manual from Kodak, which includes examples of films that have been toned and tinted.
The manual notes that the effect of the dye increases with the length of time the object spends in the dye bath. This can certainly be seen in the postcards, with a range from images that still have much of the detail and contrast of an untreated print:
to images fading into the dye:
In fact, the difference of degree can often be seen within the same postcard:
The frequency of lines like these suggests that postcards were treated in large batches, where edges could easily have been left out of the dye bath.
The most frequent colours of dye-toned postcards were bright pink:
blue (which may sometimes also have been created by iron toning, a cousin of dye-toning):
and an interesting combination of purple and blue that was probably created by a treatment into two separate dye baths (a process described in the film toning manual):
detail, showing the combination of purple and blue:
Metallic postcards were also toned red:
Tinting was also often added to dye-toned postcards, with results that were sometimes lovely:
and sometimes blinding: