What is the difference between staining and tanning?
There seems to be confusion among some photographers regarding the terms staining, and tanning, and the terms are often used interchangeably, probably because most staining developers are also tanning developers, and most tanning developers are also staining developers, but there are exceptions, and the two mechanisms perform very different, but highly complimentary functions in negative development.
Stain is the rock star of the staining/tanning duo, and gets most of the attention and press in the ultra-geeky photo community. The oxidation products of staining developers stain the film emulsion in proportion to exposure, meaning; more stain in the densest parts of the negative (highlights) than in the thinnest parts (shadows). The stain is useful because of its color as it relates to the spectral sensitivity of the printing paper. There are basically three sensitivity-types of printing papers: Graded silver papers, sensitive primarily to blue light, Variable Contrast papers, sensitive to Blue and Green light in various proportions, and Contact papers, which are sensitive to UV light. The effect of a stained negative on Graded silver papers, and Contact papers is essentially the same; to increase contrast. The effect of a stained negative on VC papers is more complicated, due to the Yellow Filter Effect of the stain image that creates an automatic split-filter effect with simultaneously reduced highlight contrast and increased shadow contrast.
A stained negative is a composite, composed of a silver image and a superimposed stain image that combine to create print density, and therein lays the magic! To achieve a given print density, less silver density is required, meaning less grain, because the stain has no grain. And since the stain density is added to the silver density, the total contrast potential of the negative is increased, meaning greater expansion potential, which is critical for long scale and self-masking UV contact printing processes. Stain density is also immune to the Callier Effect produced by condenser equipped enlargers, so stained negatives benefit all formats and printing processes.
Tanning is the unsung hero of acutance. Tanning developers harden the emulsion in proportion to exposure, creating a relief image, and inhibiting the migration of the developer to the depths of the emulsion, and between areas of high and low densities, simultaneously reducing the appearance of grain and increasing apparent sharpness. Hardening is proportionally greater in the highlights than in the shadows, so shadow areas are developed to a greater depth in the emulsion, increasing emulsion speed by the compensating effect.
Staining and tanning each deliver great advantages in film development, but in combination represent an almost magical set of characteristics that seem to defy conventional wisdom on the subject of developer formulation. It’s important to remember that conventional wisdom is based almost entirely on non-staining/tanning developer formulation, and very little has been written on the subject of staining/tanning developers over the last century, and much that has been written is ill informed or just plain nonsense, so when you’re told staining/tanning developers produce coarse grain, or give away emulsion speed to produce enhanced sharpness and tonality, so they’re only really good for large format contact printing, consider the mechanisms of staining and tanning and the benefits they confer to all formats.