Why Does Paper Turn Yellow with Age?
Oxidation causes paper to turn yellow with age. Most paper is made of plant fibers that have been bleached white. After a time, the fibers, made of cells that once were living, slowly begin to break down. As they weaken, oxygen and carbon dioxide from the atmosphere join with them to form new compounds, some of which have a yellowish color.
Sunlight also plays a part in this yellowing process. Ultraviolet rays in sunlight cause chemical changes in the paper fibers, just as these rays cause changes in our skin when we sunbathe to get a tan.
It mostly depends entirely on what fibers the paper is made of, the processing and purification steps it has gone through, and the additives or residues that are left in it.
Asian-style papers are made from the core of tall grasses (bast fibers) and Western-style papers are made from 100% un-dyed cotton and linen rags or cast-off fibers from spinning.
After being picked or washed clean of impurities, the long fibers that are left are pure cellulose, which is actually colorless, but reflects light opaquely and we see the color white. Newsprint and some other papers are made from ground wood or straw, also cellulose, but which have more non or hemi-cellulosic compounds by weight, including lignin.
Lignin and other non-cellulose compounds are subject to oxidation – and when they pick up extra oxygen it alters their molecular structure (becoming chromophores) in a way that absorbs and reflects wavelengths of light differently – and changing the colors we perceive with our eyes to yellow and brown in visible light.
Pure cellulose papers do not oxidize as much as impure ones, but they do tend to become warmer in tone than when first made. This may be because of additives (sizing, fillers or coatings) aging, impurities in the water they were made in, or picking up and adsorbing pollution that affects the color of the cellulose.