I googled dry mounting and archival, and I found two things:
1) There's a "define your terms" thing that goes on, and some people insist (somewhat pedantically, I think) that nothing that can't be totally reversed can be termed "archival," because "archival" in the their dictionary means totally reversable -- being able to return the art work to what it originally was. I think of that as pedantic because it also depends on your definition of what the art work was -- if you define it as printing on paper, then dry mounting can't be archival, because it's not totally reversible (you can't remove the adhesive, whatever it is, where it's penetrated the paper.) But if you define the artwork as printing on paper on board...then the situation is different. If you say your definition of your artwork is "absolutely flat," then the dry mounting becomes part of the artwork, and ther's nothing to reverse.
But that's for pedants to worry about.
The more interesting thing was,
2) That some products use acid-free and buffered tissue to support acrylic-based dry-mount adhesive, which, whether you want to define it as "archival," or not, would be permentent and non-degradable. I have some experience with modern acrylics, and you could take a photo out of its frame and put it under water and when all the paper and image washed away, you'd still have the acrylic left...it's a permanent, dry, non-degrading plastic that will last longer than your image. If somebody put a gun to my head and said I had to dry-mount, this is what I 'd use.