Andrey,(posted 8087 days ago)Pyro developers need alkaline conditions for tanning and stain development. The sodium metaborate in the PMK formula, and sodium carbonate in other pyro formulas provide the pH needed. Offhand I don’t remember the exact pH required, but it is strongly alkaline. See the "Book of Pyro" for details.
Tanning does harden the gelatin and you very clearly see a texture on the negative when it is still wet. The raised relief image is less apparent when the negative dries. It is recommended to use a non- hardening fixer because hardener is said to interfere with the desirable stain formation. Acid stop bath is to be avoided also. I sometimes use Kodak Rapid fix (without adding the hardener) and despite the fact that this fixer is formulated slightly acidic, I still get good stain. In fact, I avoid the alkaline after bath, which gives me too much stain. I print Pd/Pt and very heavily stained negatives have outrageously long print times. For silver printing this may not be a problem because stain is what gives pyro negs their magical properties.
If you want all the details you really should get Gordon Hutchings excellent “Book of Pyro”. Also consider trying Pyrocat-HD, a catechol based staining/tanning developer which I have grown to like better than pyrogallol based developers. For the Pyrocat formula see www.unblinkingeye.com.