1951 Bowman Mantle PSA 7(OC) on PWCC tonight

[steve B]

Originally Posted by Goudey77

I heard that the old slabs had some issues with their seals. So if you lived in a humid climate the cards could shrink up to %5
If you study paper fibers used in the 1950's you'd understand the risks involved with post war era cardboard.

So all the cards that measure full size were oversize when they were made?

Also- That card is most likely more than 5% small both ways.

The fibers are mostly wood pulp, maybe with some rag content and additives like clay, plus some seizing. Except in the cards that are multiple layers, where the white front surface is either bleached wood fiber, or may have a high rag content, linen, cotton, maybe some silk or wool Plus some seizing.

What are todays cards made of?
The exact same stuff, except that the white paper/cardstock is now the entire card since 1992 for Topps, and earlier for the other companies.
30's cards?
Guess what?
yeah, the same stuff.

Paper shrinkage generally happens soon after the printing process. And it's less normal on things produced by offset lithography.
Stamps which are intaglio printed do have shrinkage immediately after printing under some conditions, which is why the BEP used experimental papers in the 1920's as well as different spacing on some sheets a bit earlier. Then changed to a "dry" printing process in the 50's. (The sheets are printed "wet" to help the paper get forced into the recesses in an engraved plate and then to retain ink. The edges shrunk faster than the center making the spacing and thus perforations uneven. With more force, less wetting was needed. )