This process could be considered as a reversal gum printing process in that an emulsion composed of a sensitizer, gelatin (or gum arabic), and a pigment (water soluble drawing ink or watercolor) becomes an insoluble colloid when coated and dried, except where exposed to actinic light.
Development and washing will prove to be much cleaner and easier if the paper has initially been sized with gelatin or acrylic, as in gum printing.
Emulsion: 1 to 2 inches of tube watercolor, or 2 to 4 cc of liquid drawing ink, should first be mixed well with 10cc of a solution composed of 100 grams of Sesquichloride of Iron (Ferric Chloride), 30 grams of Tartaric Acid, and 1000 cc of water.
Coating: should be as with gum printing:smoothly, evenly, not too thick, and onto a smooth-surfaced paper. Allow to dry completely. Mixing, coating, and drying should be done under safelight or buglight.
Expose: under UV by contact printing. Remember - this is a positive-to-positive process. Not only will you use a positive transparency for a final positive image, but the longer the exposure, the more the exposed emulsion will dissolve and wash away in the development. Exposures normally range from 15 to 25 minutes.
Develop: in warm running water until the image and excess chemical stains have been cleared (20 to 60 minutes); gentle hosing with warmer water may be necessary to clear the highlights. Final drying returns the image to permanent insolublility. The print may be coated and processed again and again to add other layers of color or information as in gum printing.