To John S.
You can intensify the stain part of the image by the bleach-redevelopment process. When you bleach the negative (probably not a good term to use) in a rehalogenating bleach such as is used in sepia toning, the stain image from development in Pyrocat or PMK remains. Redeveloping restores the original silver and adds to it more stain. Be sure that the negative is well fixed and washed before you bleach it the first time. Any remnant of fixer will combine with the ferri to remove silver permanently.
Each redevelopment cycle hardens the emulsion a little more.