Welcome to brain Stuff from house stuff Works dot com where smart happens. Hiem Marcia Brain with today's question why do newspapers turn yellow over time? Paper is made from wood, which is made up mainly of white cellulos. In the ideal case, paper would contain nothing but white cellulos, But wood also has a lot of a dark substance in it called lignant, which ends up in the paper despite
the best efforts to keep it out. The exposure of this lignant to air and sunlight is what turns paper yellow. Lignin makes wood stiff and helps trees to stand upright. You could say that it acts like a glue to bind the cellulose fibers together. Lignant is a polymer, a substance that's formed by joining together simple molecules into giant
molecules that act differently than the simple molecules would. According to Dr Who Men Chang, a professor of wood and paper science at NC State University, a tree could only grow about six ft tall if it didn't have lignant. Lignant also helps to protect the wood from pests and other damage. Newsprint, which must be produced as economically as possible, has more lignant in it than finer papers. At the mill, the wood that will be turned into newsprint is ground
up lignant and all. That's not uncommon because paper manufacturers use the benefits of lignant in some types of paper. For example, brown craft paper, which is the dark brown paper used in grocery store bags, and cardboard are stiff and sturdy because they have more lignant in them, and because those kinds of paper aren't treated with bleaching chemicals. It doesn't matter how dark they are because the printing
on them is limited. To make a fine white paper, the mill puts the wood through a chemical solvent process which separates and discards the lignant. Pure cellulose is white, and the paper made from it will be white and it will resist yellowing. Lignant eventually turns paper yellow because of oxidation. That is, the lignant molecules, when exposed to oxygen in the air, begin to change and become less stable. The lignant will absorb more light, giving off a darker color.
Chang says that if newsprint were kept completely out of sunlight and air, it would remain white. After only a few hours of sunlight and ocygen, however, it will start to change color. Be sure to check out our new video podcast, Stuff from the Future. Join how Stuff Work staff as we explore the most promising and perplexing possibilities of tomorrow. The House I Find app has a Rye down at it today on items