In the Bromoil process a silver gelatin print is created. After drying overnight, the paper is bleached and tanned - the tanning process hardens the emulsion relative to the amount of exposure received. After drying again overnight, the paper is soaked and then stiff ink is applied to the paper by striking it with a stiff brush. The areas that received less light during exposure were hardened less during the tanning process, thus the swelling of the gelatin rejects the ink to a greater degree than the areas which received more exposure.
The Cyanotype process was discovered by Sir John Herschel in 1842. Two chemicals: Ammonium iron citrate and Potassium ferricyanide are dissolved in water and applied to paper. The paper becomes light sensitive, and after drying can be contact printed from a large format negative. The paper is developed in water, where the iron salts are washed away and the ferric ferrocyanide remains as a blue color.
These images are basic photograms, where items are placed on expired photographic paper, and the paper is then exposed. However, instead of exposing the paper under an enlarger and developing it with the standard chemicals, the paper is exposed to sunlight until it changes color. Fixing the paper will cause it to lose its color, so the results are scanned. Curves and levels are digitally altered to bring out the contrast and enhance the coloration.
The Van Dyke process requires that paper be coated with a sensitizer, then contact printed under ultraviolet light. There are numerous formulas that can be employed, but the common element to all is silver nitrate. After exposure the paper is fixed and washed, which not only makes the paper no longer light sensitive, but also enhances the brown color.