Nineteenth-century paper was often manufactured with material that was highly acidic. This produces today paper that is heavily toned or brittle. If the crease is heavy, you want to be careful folding the paper back to its original position, because you will highly likely break off the corner at the fold. Even with special tools.
Papers produced before 1800 (as a round number) tend to have been made with sturdier materials - or rather, less acidic materials - and many papers curled or folded can be easily manipulated back to their original position. If the covers are sturdy enough, and given light pressure, the pages will do well to be put back to their original position.Not only was I appreciative of the education but that's the kind of detail that sends my book-nerd heart palpitating exquisitely with pleasure. So to speak. And he also advised in case I ever want to undo the dog-ear:
Take a slip of paper, say 1/4 inch wide and 1 to 2 inches long. You can slide that little slip of paper under a dog-eared corner, and gently fold the corner back to its original position. Your thumb, finger, or fingernail will not be subtle enough to lift up the corner. But you can do it with a thin slip of paper.Helpful but also enchanting to learn that!
Okay, here are some recently-added old (but not ancient) books -- non-English (which I rarely do as I prefer to be fluent with what's in my library) additions to my Tiny Book Library: Spanish literature!