Few weeks ago I was just checking out my shelves to pass my time nd heed some time on my home too,there I found set of old papers,books e.t.c, my parents had a collection of historic old, yellowed newspapers. For example, I distinctly remember an old URDU newspaper Ikhbar-e-mashrik sitting on a bookshelf from July 21 93 or 94. Unfortunately, they were also hard to read due to the yellowed, brown color and fading print. So why do old newspapers – and books – turn yellow? And is there any way to prevent this from happening?
Wood is primarily made up of two polymer substances – cellulose and lignin. Cellulose is the most abundant organic material in nature. It is also technically colorless and reflects light extremely well rather than absorbs it (which makes it opaque); therefore humans see cellulose as white. However, cellulose is also somewhat susceptible to oxidation, although not nearly as much as lignin. Oxidation causes a loss of electron(s) and weakens the material. In the case of cellulose, this can result in some light being absorbed, making the material (in this case, wood pulp) appear duller and less white (some describe it as “warmer”), but this isn’t what causes the bulk of the yellowing in aged paper.
Lignin is the other prominent substance found in paper, newspaper in particular. Lignin is a compound found in wood that actually makes the wood stronger and harder. In fact, according to Dr. Hou-Min Chang of N.C. State University in Raleigh, “Without lignin, a tree could only grow to about 6 ft. tall.” Essentially, lignin functions as something of a “glue,” more firmly binding the cellulose fibers, helping make the tree much stiffer and able to stand taller than they otherwise would, as well able to withstand external pressures like wind.
Lignin is a dark color naturally (think brown-paper bags or brown cardboard boxes, where much of the lignin is left in for added strength, while also resulting in the bags/boxes being cheaper due to less processing needed in their creation). Lignin is also highly susceptible to oxidation. Exposure to oxygen (especially when combined with sunlight) alters the molecular structure of lignin, causing a change in how the compound absorbs and reflects light, resulting in the substance containing oxidized lignin turning a yellow-brown color in the human visual spectrum.
Since the paper used in newspapers tends to be made with a less intensive and more cost-efficient process (since a lot of the wood pulp paper is needed), there tends to be significantly more lignin in newspapers than in, say, paper made for books, where a bleaching process is used to remove much of the lignin. The net result is that, as newspapers get older and are exposed to more oxygen, they turn a yellowish-brown color relatively quickly.
Today, to combat this, many important documents are now written on acid-free paper with a limited amount of lignin, to prevent it from deteriorating as quickly.