The Book Switch Theory

[Jorge_Stolfi]

Could it be that Wilfried simply wanted to make it visible using chemicals, but the ink did not react properly due to low iron content?

I think it is definitely possible.

There is no good reason to write an ex-libris (or a quire mark, or a tentative alphabet substitution table) with iron-gall ink, even on a vellum book.  They are not critical info that must survive centuries.  Making iron-gall ink is a non-trivial task.  You may use it you have it already at hand, because you have been writing other stuff on vellum.  Otherwise you would use china (lampblack) ink, or pencil.  And neither would react with the chemicals that one would use to enhance iron-based ink.

By the way, among those Jacobus supposed ex-libris on Rene's page, sample #4 -- the one that is identical to the You are not allowed to view links. Register or Login to view. "signature" -- looks like it was written with light gray ink, very unlike what I expect of iron-gall.  It may be diluted china ink.

All the best, --stolfi