Extra to add to previous e-mail on pyro:
The statement to compare with Barry's observations concerning Pyro negs and VC papers can be found in:
'The Book of Pyro', Gordon Hutchings (1992), pp44+45.
To summarise, Gordon states that the yellow-green pyro stain is in proportion to the silver content and thus acts as a colour mask to reduce printing contrast esp. in the print highlights. Thus shadow/mid contrasts can be adjusted with VC filters but the highlights will always print with good tonal separation.