These flexible, paper-friendly, acid-neutral weights come in many forms. They are used to hold open the pages of rare books so that human hands do not have to touch the pages as often.
Below is an example of a rare book in a custom cradle with a book snake holding its pages open, at the Harry Ransome Center, U. Texas, Dallas. Notice the narrow opening!--the researcher takes pains to read without flattening the spine rather than breaking the spine or otherwise paining the book by expecting it to open flat for the researcher's convenience.
Image borrowed from : Jane Beeson Boyd, "An Adjustable Book Cradle Design," The Book and Paper Group Annual: The American Institute for Conservation. IV (1985) Available: http://cool.conservation-us.org/coolaic/sg/bpg/annual/v04/bp04-01.html