The wood may have long ago off-gassed processing chemicals, but wood is inherently acidic and as the cellulose continues to break down over time, more acids are released. Oak is the most acidic wood, and given the time period of your shelves, I’d guess they’re either walnut or oak. Anything enclosed inside the wooden cases will suffer from the acidic byproducts.
A couple of years ago Conservator Marc A. Williams allowed us to post his recommendations for sealing wood on our blog. http://collectionsconversations.wordpress.com/2011/10/11/sealing-wooden-storage-equipment/
Hope this is helpful!
Adrienne Berney, NC C2C