Album paper.

There are pages on the internet,  useful from the US.

GB
www.amazon.com/The-Care-Preservation-Philatelic-Materials/dp/B000PG8RCC

My Xerox paper quotes 200 years.

Since it was published by the APS in 1989, maybe a current member of APS can obtain a copy and do a review?

Perfs, I have the book, and it is so.
The more you read on philatelic husbandry the more one can become obsessed,
Some mounts have doubtful longevity.
To keep things in perspective, I generally collect what my wallet dictates, mint lightly hinged to downright grungy.
So I have reasonable quality paper mount when possible and keep in dust covers,
Some collections I have demounted which look from the 1950's come up well,
the paper just a hint of toning and the stamps still appear pristine.

Interested in the cotton paper mentioned by Andy.

Ahem! cough cough.

"100% Cotton paper may contain small amounts of acids, and should be tested or certified before use for archival documents."

Was surprising to me too,  the above was quoted in your link to wiki.    Do we believe wiki?
Perhaps the process / bleaching during modern production?
 

I never use split-backs as, when I use them, there is a real risk of perf-damage (pun intended ). But, a friend of mine made a good point (Perfs12) - since one does not want the album to fall apart either, one should use archival paper regardless of mounts, which as rod222 points out may have a limited life.

I can't comment on what they'll be like 50 years from now but stamps I placed in black Showgard mounts 36 years ago are showing no discernible signs of deterioration or damage.

You can experience this, by purchasing old albums.
Some I have, the stamps are near pristine, albeit the hinge, others , in their death throws.

I think it depends mainly on the conditions the albums are kept in,  the humidity,  constant mild conditions, whether slip cases have been used,
and / or behind protective storage.
Stamp mounts will not protect against poor husbandry.

 

Crystal mounts were crystal clear mounts sold by H E Harris. It was basically a long, flat tube with the ends open. There was a strip across the top that would wet to secure the mount to the page. You would slide your stamp into this long mount and then trim it to size. The mounts were sealed on the top and bottom and open on the sides.

Over time, the adhesive strip would yellow the page. The mount itself would actually shrink and damage the stamp.

Chuck, for what it's worth, and very respectfully, I would suggest throwing those Crystal Mounts away. 

My mother used them in the 1960's and 70's, and I've inherited her collections.  I would say 60-70% of the stamps have been damaged.  Either (1) by the adhesive strips on the top turning yellow and staining the stamps (kind of like the way cellophane tape does), or else (2) by the clear plastic mount shrinking, and causing the stamps to get wavy and wrinkled.  Also, (3) the gum on the stamps often became kind of mottled opaque and rough.  Any one of those three types of damage seems to be irreversible.

I don't think it had anything to do with the atmospheric conditions of how the stamps or albums were housed, because the condition of her stamps varied from page to page in the same album.  I think there were good batches and bad batches of the mounts.

I haven't checked recently, but there used to be at least one dealer who sold Showguard 2nds who advertised in Linn's.  I'll see if I can find that, if no one else chimes in first.  Others here may know who has the best online prices for 1st-quality ones.  (I happened to acquire a bunch in a supplies lot at auction a few years ago, so I haven't had to go shopping for them in a while.)

I bought two albums a Harris and a Scott. The Harris is almost full the Scott was not and doesn't have any mounts in it all. All the stamps in the scott are hinged

Philatarium If I take the mounts out of the Harris album It will leave the residue from the mounts and unless I put them in Black mounts the glue will get on the stamps and or show through the mounts

When I removed the Crystals from my Bulgaria pages, the adhesive strip stuck fast, and the plastic part ripped away, leaving a plastic glued strip in place.
I printed fresh Steiner pages.
You need to remove the stamp first before attempting removal of any Crystal mounts, if you decide on that route.

That would seem to be a major decision, only you can make.
I'd appraise the collection for damage,  if there is none immediately recognisable then work on one page
to see if your intended route is workable and to your liking.
See if the mounts come off easily, if not page reprinting, or new Harris pages are required, this is a big decision,
one best tackled slowly, a page at a time.
You may need to try black mounts or clear, see which ones you prefer, once a decision is made, you have to stick with it.

If there is clearly no damage /staining /shrinkage,  then perhaps leave for a while, use your new additions with pages using 
mounts of your choice and wait to see how you like the result, no need to rush into a decision you'll regret.