Old paper has secrets... if only you shine a light through it
I didn't know old books could get so forensic. The last thing I expected was for them to hold details—images—hidden to the naked eye.
The pronounced vertical lines are chain lines, and the horizontal ones are from the screen from the paper-making process.
And then, if you can make it out, the paper has a watermark which tells you who milled it (and thus where and when), and it's called a watermark because the paper was wet when it was imprinted!
Where the watermark is and the direction of the chain lines (vertical/horizontal) tells you how the paper was folded.
Here's another example from the same book I was looking at:
This second image has the book's most repeated watermark, the bull's head. It's in the center of the page with vertical chain lines, so this book is classified as a folio, where each sheet was folded once to make four pages. (Other common folding styles are quarto and octavo.)
We can turn to Charles-Moïse Briquet's Les Filigranes, a multi-volume watermark dictionary. (This is also available on the Internet Archive.)
With a little French, we can find pages that match our watermark. Here we have "Tête de bœuf," which Google Translate says is "Beef head". (It's a bull head.) Then we play a game of spot the difference. The second watermark looks similar to 14183 (of the 4th volume), so for the sake of the exercise, we'll look at that one. The entry contains "Belfort, 1458" or a variation from "Marbourg, 1459".
For the other page, with the crest, I found "Armoiries Bande," or the band version of a coat of arms.
The closest match I found is entry 995 from Vol. 1.
We get dates and locations ranging from 1586-1609 in Strasbourg and the surrounding area. From a quick search of the web, the book specifically has the Strasbourg Bend watermark. Already, we can begin to place where the paper to print the book was sourced from and at about what time, revealing a partial history of the book from just these details hidden in the pages. If I had my own rare books collection, I'd be shining a light through every one, looking at the different watermarks and noting new ones I see in a sort of curator's Pokédex.
