Mordant Dye Toning.
Mordant toning is a procedure based on the principle that certain organic dyes do not stain gelatine normally but will stain gelatine already impregnated with insoluble mordant compounds. The principle is similar to that of dyeing fabrics such as wool or other similar staples. There appear to have been several different routes that would not have resulted in the same visual effect even when the same dye was used. The most commonly used mordant in photography is an insoluble metal ferrocyanide, but thiocyanates and halides [especially iodides] were also used in a two-stage process.
In Crabtree and Ives Toning method, the print is dyed in a single solution in an acetic acid/10% acetone solution containing the mordanting agent potassium ferricyanide and a basic dye. It is difficult to be sure how common this process was although it is widely mentioned in the literature.
In dye toning the image areas tones first and the process is "time critical" - the longer the film is left in the dye solution the more the highlight areas fill with dye, so test strips were used to estimate the time needed. Contrast is also controllable by concentration of the dyes -solution of high concentrations dye more quickly but produce higher contrasts. A wide range of colours are possible and the following table shows some of those known to be used in the US and in Europe in the late 1920's.
Dye toning processes were the basis of some stencil processes that were experimented with in the 20's, but did not succeed in becoming commercially successful. The silver image can be bleached out using fix solution but the image left is usually to thin to be of use.