Marco
Thanks for the valuable clarification.
Now I'm anxious to see the test results for direct thiourea toning.
Ralph, Olli J pointed in one his posts here in this thread (see page 8) to possible work of Istvan Kecskemeti from Finland. I have tried to search it on the internet using Google, but came up empty handed up to now. Maybe you could PM Olli J asking for it...
By the way, here is a presentation by Istvan about archival boxes and different types of paper, although I must admit I find the graphs created for this presentation not the best... It is not obvious in all graphs what is actually depicted, and that is unfortunately a basic error I all to often see...
Anyway:
http://www.echn.net/enviart/Portals/0/Madrid2010/3 KecskemetiCondSurvey.pdf
Interestingly though, he ranks "chemical wood pulp, bleached", as is used for most or all modern photo papers as I understood, as susceptible to degradation, while only pure rag papers (cotton, hemp, flax), are ranked "most archival"...
Of course, this is already partly known from the excellent state in which some medieval books from pure rag survive.
Marco