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Dennis W.Actually rehydrating vintage paper is good for it. Document/book restorers do it all the time. The Conservation Services at the South Carolina Department of Archives and History call their water-based treatment "aqueous buffering". Their treatment consists of a distilled/deionized water soultion with a dash of other chemicals they use to deacidify. I certainly don't condone using any chemicals on cards. I also want to add that essentially any chemical used other than water will glow under the blacklight.
To answer your question Hal - I have not noticed any increase or decrease in the thickness of a card after the soaking process. One might assume it to be thicker as it absorbs water but I believe the pressing process neutralizes it.
The card will warp if not left between the books to dry long enough but does maintain it's rigidity. I wanted to add that I use linen paper as opposed to regular printer paper. I find that it absorbs faster.
I've had many cards graded over the years after going through this process and all have graded.
edited to correct typo