Quote Originally Posted by Alessandro Serrao View Post
then why with permanganic acid a metabisulfite clearing bath is used?
I ask myself: what's the purpouse of the clearing bath?
The clearing bath is used to insure that all chromium or manganese compounds are removed from the emulsion. Manganese easily forms manganese dioxide, a black, insoluble compound. The clearing bath must be acidic to prevent this compound from forming. Once formed manganese dioxide would be impossible to remove without destroying the emulsion.

I believe the reason why people have difficulty with permanganate bleaches is because they compound them improperly. Both the bleach itself and the clearing bath must be acidic.

When using a permanganate bleach it is important that the manganese goes from Mn (VII) to Mn (II). If the bleach process should be stopped at manganese dioxide Mn (IV) then roughly half the oxidizing power of the bleach is lost.