Why is Paper White When Trees Are Brown?

Most paper--but certainly not all--comes from trees. A variety of woods are used for paper, but all trees are made of cellulose, the basic building fiber that makes up most of the plant kingdom. The cellulose in trees is held together by a naturally produced adhesive known as lignin. It's in breaking down and reorganizing the fibers that all varieties of wood-originated paper are made. With non-wood sources, which include plants such as cotton and linen, whiter paper can be made more naturally. Sometimes these plants are blended with trees in the paper-making process to create lighter paper, which may be bleached further later in the process.

Most trees for paper making are harvested from tree farms. New trees are planted after the cut ones are taken to a mill. After dirt and other impurities are washed off, and the bark is removed, the recently harvested trees are run through the pulping process. Mechanical pulping chops and crushes the logs into very small chips of wood, separates the fibers in the chips from one another, and then stirs them into a slurry with water. Chemical pulping, on the other hand, uses chemicals, heat and pressure to dissolve the lignin in the wood and free the fibers. One of the chemicals used in this process is bleach, which whitens the fibers as it dissolves the lignin. The lignin and chemicals are then washed out of the pulp and burned, often as a fuel source.

Once the pulp has been made, different types of wood pulp can be mixed together. The mixing can be done for smoothness, strength, whiteness or other desirable qualities of the paper. Generally, this is the point at which any additional bleaching is done to make paper whiter. Dyes may be added to bleached paper to turn it different colors. Some varieties of paper are not made white, but rather keep the natural colors of the process (recycled paper, for instance, is often dingy brown or gray because it contains many dyes from different paper fibers being mixed together). After this final mixing, the pulp is sprayed into sheets on wire mesh, and the sheets are dried and pressed until they become paper.