The earliest paper, made of white cotton rags beaten by hand consisted almost entirely of long cellulose fibers. As the demand for paper increased, paper makers used chlorine to bleach colored rags. Later, heavy machinery made it possible to turn wood into paper pulp. But any chlorine bleach left in the paper eventually turns into hydrochloric acid. And so, too, does the lignin, which holds the cellulose fibers together in wood. Other acid-producing compounds were used to treat the paper, and even air pollution from coal-burning factories increased the acid-content of paper. All that acid breaks the chemical bonds between the cellulose molecules, making the paper brittle and turning it yellow. Today, many important documents are printed on acid-free paper, but in the meantime, library preservationists are working on ways to save millions of books before the acid in the paper turns them to dust.