Mnišovský's report

[Jorge_Stolfi]

Why would he photograph it and then erase it??

Because he could show the photograph to prospective buyers, to bolster the "Bacon" claim; but the erasure would avoid expert examination of the original "signature", which (even with the technology available at the time) could have identified it as a forgery.  The erasure left just enough of a trace to "prove" that the photo was legitimate.

But, again, I think this is only a possibility; not very likely.  The most likely theory, in my view, is the "consensus" one: that indeed there was a faint signature on f1r, and he honestly believed that the chemicals he rubbed over it would bring it out.

My guess is that he guessed that the signature was in iron-gall ink, so he blotted it with acid to release the iron and then with ferrocyanide to turn the iron into prussian blue.  But, if that is what he did, it seems it was not iron-gall; maybe India ink (soot + gum arabic), or lead pencil...