In its descriptions of these stamps, Michel often includes the phrase "auf gestrichenem Papier" (e.g., 1926, Mi 385ff; SC 351ff). I've done enough research to learn that different materials can be used in the production of coated papers, at least one of which, chalk, is water-soluble. Neither Michel nor Scott says that these issues are on chalky paper. Yet I'm wondering are they safe to soak?
EMaxim, "Gestrichenem" denotes coated paper but I do not think it includes chalky paper. In German chalky paper is Kreidepapier. I have a lot of used stamps off-paper from this era and they don't look damaged. But! It is not always easy to tell the coated paper from the ordinary paper version and it is easier if they are mint, so maybe water does affect them to some degree.
Danke sehr! That's the answer I was hoping for. I'd figured that if Michel didn't say Kreidepapier that the coating wouldn't be chalky. Just wasn't sure that whatever Stoff was used would still be safe. Most of my used copies of these stamps don't seem much worse for the soaking, so if there is an effect at least it hasn't been fatal (so far!).
Is it possible that it is easy to recoat the paper and fool everybody ,so the catalogs find no reason to identify the difference .Then you got to expertize the paper with some ability to identify real coatings .
I think Germania is right to suspect that even non-chalky coatings might still be affected by soaking. Here are two mint copies "auf gestrichenem Papier" with two that have been soaked (Mi 537, SC 430).