Potential Varieties 1890 Bureau Series

Hey guys. I purchased an album recently where some of the stamps were sealed in plastic then placed in the album. They would look like the 2c, 3c, and 4c as shown below:




I'm assuming they have foxing, so the seal would help prevent the spread of any contaminates.
The book is in chronological order from Scott#1-802, is about 75% complete, and doesn't have any doubles with the exception of color variations...
I don't know of any notable varieties within Scott 220-224 (aside from the 220 "cap above 2" which are both already accounted for), so I was wondering why the original owner would bother putting doubles of these in an otherwise meticulously crafted album, and damaged ones at that.
Does anyone know of any additional varieties to be had here?

The Scott U.S. specialized catalogue does list shade varieties of most of these. As the color differences are not deemed particularly significant, they are not assigned separate catalogue numbers. A catalogue value is given for each shade listed even though these values are almost always identical.
It is possible that the original collector may have been collecting multiple shades of some denominations. But this is just a guess.

Foxing is a broad term without a universally accepted definition; it is NOT always a some spreading fungal spores scrouge that can infect other stamps. In fact, more times than not it is formed by oxidation or acid hydrolysis (if you search scholarly archive written on paper foxing over the last few decades you will see that most of the research is centered on the oxidation and acid hydrolysis aspects of this issue.
Don

Edit: For me, I would never spend time treasure hunting 100+ year USED stamps unless I has several thousand of the same stamp. As the BEP pressman posted here, even the pigment in stored cans of inks would change sitting sealed on the shelf. Add 100+ years, and then Lord knows what environmental conditions/baths/treatments the stamps have seen and...
well, never mind.

Excellent! Thanks JL. I think you are spot-on.

Don, I believe you. But if I have a nice collection that needed a filler, and the filler looks like it has some kind of disease, I wouldn't waste any time putting a hazmat suit on that bad boy just in case.

This collection was very well-kept, but I do also enjoy buying old, decrepit albums and preserving whatever I can from them. Many times you can see slow, microscopic red bugs crawling around between the pages, so I always wear gloves and use plastic covering when doing it.
Then! I take extra pleasure in burning those musky old stamp graveyards

I totally agree and even avoid ugly looking otherwise very nice stamps.
Don