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Please anyone--Clearly state the damage which may occur, and the cause of this damage.
I've already stated 2 main causes of the damage above. For these 2, the actual damage is gum damage, and also graying of the stamps in areas where moisture has been trapped in the mount. Again, I'm a little tied down. I will try to find a few examples and post pics in another thread later. Please be patient, as it takes time to dig out and scan examples.
I find a little of this in 10-20% of the collections I examine, and major damage (exceeding 50% of the stamps in the lot) in about 5% of the collections I examine. So most collectors are careful enough not to have this happen. I'm glad that you are in the majority of collectors for which you have selected the proper brand mounts and have mounted them without running into problems.
However, when this happens to a collector, many never realized it was happening. Why? Because most collectors, once they mount their stamps, never bother to take them out to examine again. So they use the same method and same type mounts to continue to mount their collection, never realizing the damage that was occurring. This has even happened to some of my friends!
Secondly, not all mounts are "archival" (remember, that is a relative term, there is no standard by which a material is universally "archival"). Crystal mounts have already been mentioned in this thread as being problematic. Scott/Showgard/Hawid are all fine when used properly. However, there are other knock-offs and generics -- these should be avoided as some are known to interact with certain inks on a few stamp issues, and also adversely affect stamps in some way.
Finally, even the Scott..Hawid mounts don't mean your stamp is safe if your album is not stored properly. I've actually thumbed through one album that was completely mounted in Showgards, but most of the stamps were very lightly stuck down (easy to remove) and the gum blotchy in appearance. The owner was in a low humidity environment, but had stored the album laying down with other things on top and unmoved for over a decade. Remember, the stamps themselves are not "archival".
Anyway, I'll get around to that thread with lots of these examples, to encourage others to take good care of our collections. Little things can make a big difference decades later.
Sorry, it was not my intention to scare anyone. Just remember, "archival" is a sales term, not a standardized term.