The Bromoil process is an interesting mix of media that incorporates elements from the traditional wet darkroom, carbro printing, lithography, and a few alternative processes. Simply, it is primarily a technique that results in the transformation of a silver gelatin print into one where the original silver gelatin image is replaced with pigmented ink. This results in a permanent, flexible, and expressive (in the painterly sense) image.
The process begins with a traditional silver gelatin print on one of several non-supercoated printing papers, i.e., Ilford Multigrade FB matte, Luminos Charcoal, Kodak Ektalure G. The exposed paper should be developed in a non-hardening developer, such as Amidol, and fixed in a non-hardening, sodium thiosulfate, fix and washed well.
The completed print is then introduced to a bleaching / tanning solution that consisting of copper sulfate, potassium bromide, potassium dichromate, and distilled water. In this bath, the silver in the original print is bleached while simultaneously; the gelatin is tanned, or hardened. Both bleaching and tanning is accomplished in direct proportion to the amount of silver in the print. This is sometimes referred to as “differential hardening” and is related to the carbro process. The darkest values in the print will be most hardened, while the lighter values less hardened. The gelatin that is tanned, or hardened, the most absorbs less water and thus will more readily accommodate the oil-based ink. The highlights will absorb more water, thus repelling the oil based ink in proportion to the swelling... resulting in a replication / translation of the original but in ink rather than silver.
The inks used in the Bromoil process are both hard and soft lithography inks and they are applied to the matrix (a term used to describe a bleached and hardened print that has been soaked and ready to accept ink) with a specialty brush that resembles an old fashioned shaving brush. The print is soaked for 5 to 20 minutes depending on your image, and technique, and the ink is skillfully applied. This takes a good deal of practice and involves a delicate application of inks to the shadows and mid-tones while keeping the highlights clean. After the print is finished, it should dry for several days in a clean environment.