- Paper is made out of wood fiber, which, in its turn, primarily consists of two polymer substances – cellulose and lignin.
- Cellulose is somewhat susceptible to oxidation, but not nearly as much as lignin, which functions in wood as a “glue” that binds the cellulose fibers.
- Lignin is a dark in color (like firm brown paper or cardboard), so the paper made for books must be bleached in order to remove most of the lignin. The paper for newspapers is usually made with a more cost-efficient process, so it has more lignin. And as newspapers get older and are exposed to more atmospheric oxygen, they turn yellow relatively quickly.