I haven't tried any of this. I have made etched printed circuit boards using ammonium persulfate to etch away the copper where there is no resist. Perfulfate is also used in a reducer for silver images. I don't remember if that reducer also uses ferricyanide. The bleach of chromium intensifier converts silver into a compound that can be dissolved in a sulfite solution. That is the reason for the admonition to use a low sulfite developer when intensifying. I'd better quit. I don't know what I'm talking about.