When printing pigmented colloids in layers, it can be challenging to maintain clean highlights. To overcome this, make the negative denser at or near 100% and use well-sized paper. Together, these methods improve the release of pigment in the highlights during development. The principle is simple: the exposure will not harden the emulsion under the negative’s extra dense area’s and the sizing of the paper will prevent mechanical pigment stain because the paper fibers are sealed in hardened size.
Some colloids are more difficult to dissolve and sometimes persistent stains may survive attempts to clear the paper even by changing temperature or using friction by brushing or spraying during development.
To prevent unwanted pigment attachment in whites and highlights, add an extra clear layer before adding subsequent pigmented layers.
First, brush a clear light-sensitive layer on the sized and hardened paper and let it dry. Then, add the first pigmented layer, and after drying, expose both layers through the negative. The clear layer prevents pigment attachment in the highlights that will wash away completely during development, taking any pigment above with it.
We found this method when Simone Simoncini and I were researching our novel zerochrome-SbQ method described in this paper.
This method can be combined with a printing method where all layers are exposed on top of each other and developed in water only after all exposures are done. The idea behind this is that each next layer is more pigmented and exposed with a shorter time. This results in parts in each layer that act the same as a clear layer for the subsequent layers because of the shorter exposure. With the layering and pigmentation the tonal range can be very long for an otherwise short scale printing process. This works beautifully for the new zerochrome-SbQ technique but will work for any direct pigment process.
However, problems can arise when the brushing of the next layer dissolves the layer under it. Colloids that dissolve more slowly work better than those that dissolve more quickly. This method, though maybe older, was first described in a 2003 text by Belgian photographer Philippe Berger and was perfected by Calvin Greer for his Print Makers Friend emulsion.