And regarding,That is certainly my understanding as well. I've transferred magenta dye to a fixed-out (non-hardening) FB paper and the density is very good, but diffusion is terrible! The mechanism of imbibition though, relies on this pH gradient and not a mordant.I thought a Thorium salt was needed just to enhance sharpness not to actually hold the dye in the receiving gelatin?"
Speaking of which... you can buy thorium nitrate on eBay for fairly low prices. Not that I'm even considering such an endeavor..