Split-toning is a generic, somewhat mercurial term used to describe the efforts at tonal splits in the darkroom. From the very beginning of photography, split-effects have been used and has formed a historical precedent for several contemporary practitioners of these processes. The question to be considered is when were these visual effects thought of and attempted? The first of the published works were Olivia Parker’s fascinating book, Signs of Life, in 1978 which now out of print and difficult to find.
The processes involved in achieving split effects are extremely diverse and constantly change as paper manufacturers keep reformulating their papers. The processes followed by every practitioner also varies: Olivia Parker, for example, printed on Kodak Azo paper and achieved artistic splits with strong, warm selenium. A book was written by Tim Rudman about the benefits of printing with lith developers and split-toning effects which can be achieved. Practitioners like Philip Borges have used special bleaching and toning methods which tones only certain areas while the remaining photograph remains unaffected. Linda Butler has achieved her split-effects as a result of combining Kodak Polytoner and Forte’s Fortezo paper, while Thomas Joshua Cooper uses a gold-based split-process. To conclude there is no thumb rule experimenting with the processes you might be able to create your own unique style.
Mordançage is a different name for the historic "etch/bleach" stated in vintage references. Besides, these old references form an excellent reference for tweaking your work of the mordançage process.