It's a process similar to that used to photographically create screen prints (serigraphs).
A UV-photosensitive water-soluble emulsion (typically gelatin sensitized with potassium dichromate) containing a pigment (in this case, carbon or lamp black) is used to coat a surface. The emulsion hardens under exposure to strong UV, becoming more-or-less insoluble in proportion to the amount of UV exposure. The emulsion is exposed to a UV source through a negative image, meaning that the parts that are supposed to be white in the print are dark/opaque in the negative, and the areas that are supposed to be black in the print are clear/transparent, allowing the UV to strike the emulsion and harden it.
After the exposure, the print is washed (carefully) in warm water (usually around 40 degrees Celsius, or 100 degrees Farenheit) dissolving the unhardened gelatin and taking the excess carbon pigment with it.
Carefully working with negatives exposed using strong single-colour filtration and multiple thin emulsions containing different colours of pigments, it is possible to create full-colour pictures using this technique. That often includes a lot of hand-masking of the negatives so that colours can be layered in different orders in different parts of the picture. For complex prints like that, one usually starts with a full-colour traditional photographic image, and the colour-separated negatives are made from the full-colour positive image. (Although as a specialty hand-crafted printing technique, it has largely been replaced by dye transfer printing, since the transparency of dyes means that there are far fewer exposures and emulsions required, and following a simple yellow-magenta-cyan-black order will produce the desired colours.)
It's often difficult to tell what artsy types are yammering on about when they wax rapsodic about their techniques. It is quite possible that the artist used many different negatives, emulsions, exposures and washings in his process, but I'd take the "evaporation trails" part with a grain (or, rather, a large block) of salt. Unless you use a high-enough drying temperature to cause bubble damage to the water-expanded gelatine, evaporation does nothing for the image except to make it a whole lot less damp.
