Thank you for your reply. I really appreciate it. I have extensively researched this before posting my question. Go search on "us postage stamps foxing rusting images" on google and you get about 10 to 20 pics of examples. I've read the posts on this site and others related to this for which I have posted the same question. You are the only person who replied so thanks again. I only want some feeedback if these specific stamps look like they are foxed/rusted. Since no one yet can answer that I will assume that most of the pro's are crying wolf and determing if a stamp is foxed etc. is highly subjective.
Take your pick. It's all in the eye of the beholder/buyer, especially with slight changes such as you depict.
To my eye, there are dark marks on the backs of both stamps suggesting remnants of a cancellation. And, the stamp on the left appears to have spent part of its life in contact with a grid of some kind, which left behind some oxidation.
It seems that what you're wanting is some kind of grading or evaluation scale. Not sure such a thing exists. At some level, though, it's moot. Because, a degree of deterioration is to be expected in 100+ year-old paper.
This is one of the reasons I don't collect Mint stamps. Some 'battle-damage' is to be expected in Used stamps, not usually preserved with the utmost of care throughout their lives. To me, these insults add 'character' and are part of the story each stamp tells about its history.
As far as "the utmost of care", that may or may not be economically warranted in non-archival contexts.
This is a version of the "rust never sleeps" truism.
