Quote Originally Posted by Alan Johnson View Post
I wonder if the humic acids produced by pyro developers harden the emulsion.This might result in slower diffusion of developer in highly exposed areas so that they do not burn out when printed.That's just a guess by the way,not to be quoted as fact.
Pyro/Cat development tans (hardens) the emulsion. There is less silver developed and this is "made up for" by the stain which increases density proportionately with the amount of silver developed (ie the amount of exposure). This does not necessarily result in highlight compression in the negative, although there can be a compensation effect when printing on VC papers.

One thing tanning does not do, is prevent silver migration, because silver does not migrate to begin with. That is a non issue with modern hardened films.