Limestone Products
Why do old books turn yellow?
The paper was destroying itself from the day it was made.
Dr. Lime Explains
Because of what is in the paper, and how it was made.
Cheap paper contains lignin, the part of wood that darkens in light. That is why newsprint yellows in a day on a windowsill.
The deeper problem is acid. For more than a century, paper was made in acidic conditions. The acid stayed in the sheet. It broke the fibres apart slowly, until the pages went brittle.
Modern paper is made in alkaline conditions instead, with ground limestone in the sheet. Limestone and acid cannot sit together, and the limestone keeps neutralising acid the paper takes up from the air.
Worth knowing
Libraries call this permanent paper. A book printed on it is expected to last several hundred years, while one from the acid era may not survive its first. The filler chosen at the mill decided that.
Source: Library of Congress.
