Why did sand fall out of this old book?

"Today I looked at a handwritten account book from 1717. It listed a series of expenses paid by the city of Leiden (the Dutch city where I live) to various suppliers - of books, papers, pens. Being a medieval book historian, any source made after 1500 is alien. Because I am used to handling parchment books, it was odd to handle a book that was made out of paper - and a lot of it, for that matter. Also new to me was the fact that related materials were held together by needles and to see dozens of rare actual receipts, small slips that were crossed out when paid. The biggest surprise, however, was the material that came falling out of the account book: sand..."

Some readers here will already know the explanation for the sand; others can find the answer at Erik Kwakkel.