Casein is a colloid derived from milk. For those already versed in gum printing, casein printing is done much the same way: a colloid is mixed with a light-sensitive dichromate and watercolor, brushed onto paper and exposed under a negative to UV light. Where the light hits the most, the casein hardens the most. Where the light hits the least, the casein and pigment wash away in a simple water bath, and thus an image emerges. For each print, this exposure and development process is done multiple times, particularly for a tricolor print, which will require a red, yellow and blue exposure to complete it.
Casein printing differs from gum printing in a number of ways however.
• It is quicker to expose, from as little as 15 seconds to four minutes
• A lesser-strength dichromate is best such as 10% potassium dichromate
• It is quick to develop, in as little as several minutes to 15 minutes
• It is very hardy, and alkalies such as ammonia or sodium carbonate as well as rougher methods of brushing can be used during development
• The layer is thinner, resulting in a faster layer, a sharper layer, and a finer-grained layer, but one that still carries a high pigment load
• Casein is dead-matte, not glossy, when powdered pigments are used
• Ammonium caseinate powder kit from Photographers’ Formulary or fat-free cottage cheese and nonsudsy ammonia 10% solution
• Potassium (or ammonium) dichromate
• Powdered pigments or tube watercolors (suggestions: Daniel Smith or M. Graham quinacridone rose, thalo blue, nickel azo yellow, lamp black and/or burnt sienna)
• Gamblin PVA (Polyvinyl Acetate) Sizing
• 140lb watercolor paper like Fabriano Artistico, Arches Aquarelle or watercolor paper sold in convenient watercolor pads