Response to Tanning by Pyro in neutral or slightly acid environment?

Pyrogallol is famous for its tanning/hardening properties - it was originally used to tam leather. In fact, pyro based developers were sometimes used to develop relief images for printing. Strictly speaking, I think its the oxidation product of pyro which provides the tanning action, whichi s why tanning is also proportional to exposure. The tanning produces two wonderful qualities to negatives. The first is a very unique adjacency effect. More importantly, the tanning prevents the developer from penetrating into the depths of the emulsion - this makes pyro/catechol developers surface developer, therefore very, very sharp i.e., high acutance. The reason for this is that developers that develop in the depths of the emulsion are typically developing irradiated grains. That is, the exposing light gets scattered within the emulsion. A developer developing in the depths of the emulsion will thus reduce the acutance of the resultant image. Surface acting developers will retain the highest acutance. This will be typically most apparent in the highlights. Small local contrast in the highlights will be preserved more accurately with a tanning developer (which is why most developers formulated for long luminance ranges tend to be based on tanning developing agents e.g., the famous Windisch formula). The reason has nohing to do with the stain but with tanning that allows even small local contrast differences in the highlights to be preserved - differences that would be lost to the reduced acutance of developers that develop in the depths of the emulsion. Irradiation in the emulsion is most problematic with areas of high exposure - so long range scenes are especially problematic and tend to be where tanning developers most readily reveal their magic. Hutchings book will provide chapter and verse on the properties of pyro. Cheers, DJ

[ Previous | Next ]