Why Paper Yellows

[A] Paper made before the middle of the nineteenth century is often in better condition than paper made after it, which is the reverse of what age alone would predict. [B] The explanation lies in what the sheet is made from and in what was added to it. Earlier paper was beaten from linen and cotton rags, which are almost pure cellulose. [C] Wood pulp, which replaced them once demand outran the supply of rags, carries a second substance that binds the fibres in the living tree, and that substance darkens as light and air reach it. [D]

The second cause was deliberate. To stop ink spreading, papermakers treated sheets with a size that left a weak acid in the finished paper, and acid attacks cellulose slowly and without pause. A book printed in 1890 may therefore be brittle while one printed in 1590 turns its pages without complaint. Conservators can wash the acid out, but the treatment is expensive, must be done sheet by sheet, and cannot restore what has already been broken. The practical response has been to change what is manufactured rather than to rescue what exists, and paper sold for archival use is now made without either the pulp or the acid.