Originally Posted by Donald Miller
More on Donald's comments re: the reasons for using Pyro.
1. Adjacency effects -- Adjacency effects result from local developer exhaustion and can be obtained with other high definition developers, Rodinal for example. High definition developers are highly dilute solutions that contain about one gram or less of reducer per liter of working solution. Note that not all Pyro developers are high definition developers. PMK with 1.0 g of reducer per liter at the 1:1:100 dilution, and Pyrocat-HD, with 0.5 gram of reducer per liter, are high definition developers. ABC Pyro and Rollo Pyro, which contain from 4.5 g to 6.0 g, are not high definition developers because they will not normally produce adjacency effects.
2. Irradiation minimized -- There is yet another reason why Pyro negatives are sharper, and this applies to all Pyro developers, not just high definition ones. With Pyro developers the gelatin is tanned or hardened in the first minute or so of development, and this restricts most of the development action to at and near the surface of the emulsion. This fact minimizes the effects of irradiation, i.e. the scattering of light deep in the emulsion that results from light reflecting off silver grains.
3. Stain -- The role of stain in a Pyro developer is to mask film grain and boost effective printing contrast. Low-staining Pyro developers, ABC Pyro for example, produce very large grain. Pyrocatechin in non-staining formulas produces smaller grain than Pyrogallol but it too benefits from grain masking in a staining formula. PMK, Pyrocat-HD and Rollo Pyro are all staining formulas. Staining formulas give provide the sharpness inherent in tanning developers with the added benefit of less graininess.