The Bromoil Process
 
 

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To explain the bromoil process, it is helpful to look at the oil print first. The prints are made on paper with a thick gelatin layer that has been sensitized with dichromate salts. Exposure using a negative for contact-print leads to hardening of the dichromated gelatin, in direct relation of the amount of light received. After exposure, the print gets soaked in water. The non-hardened parts absorb relatively more water than the hardened parts, so after sponge-drying the print, while still moist, one can apply a lithographic ink to the oil-base. The non-mixing character of oil and water results in a coloring of the exposed parts of the print, creating a positive image. The ink application requires considerable skill, and as a result no two prints are alike.
Bromoil prints are a direct variety of this process:  One starts with a normally developed print on a silver bromide paper which is then chemically bleached and hardened. The gelatin which originally had the darkest tones, is hardened the most, the highlights remain absorbent to water. This print can then be inked like the oil print.
Long-term effects on stability:
Inadequate rinsing of the chrome salts can lead to discoloration of the prints under influence of light in the longer term. The irregular thickness of the gelatin layer can, in unfavourable conditions, lead to stresses in the pictorial layer, which can be damaged this way.
This technique was used to produce color prints into the 1930s before the color-film was developed: Three identical black & white photographic takes of an object were made on Ilford Hypersensitive Panchromatic film with the corresponding filters (blue, green, red) The developed negatives were enlarged and transposed on bromide-silver photographic paper. The bromide-silver layer was bleached and tanned as described above. Rather hard bromoil ink was applied, yellow on the blue-filtered, red on the green-filtered and blue on the red-filtered matrice. The fine-graded colored matrices, exactly fitted one above the other, are passed alternately through an etching press. Thus a transferred color picture on absorbing and fibre-free paper or even cloth is produced. A picture appears that reminds the beholder of pastel-paintings, which however show the distinctivenes of detail and contour of photography.