Ralph, in your second last paragraph, this rings a bell to me: it was described (oxides of nitrogen from environmental pollution) as REDOX attack (red oxide "rust" in mono prints, typically resin-coated papers, much less so with fibre-base). Several of my early resin-based framed prints were tarnished by REDOX around the same time you mentioned a decline in the incidence was noticed (1990 onward, to say around 1995). It was an exasperating period of many ruined prints living, as I did, right on a main heavy traffic road. But the jury was always out whether this was a contributor or something else. Selenium toning was cited as a reasonable safeguard against REDOX, but never a guarantee. Mono prints I've had produced on RC paper since 1997 and given MGCF treatment have been entirely free of any malaise.

I think some dry mount tapes have removal-grade adhesive; my framer has on occasion disassembled frames and backing and removed prints to realign them in the mat (a fault of rushed jobbing at that time). Nowadays I don't see any dry mount taping. All I get is fancy stuff...