Look at a sheet of paper and you are seeing many thin fibers pressed together. Softwood fibers are about four times as long as hardwood fibers. That is why they make stronger paper. Cotton fibers are much longer than wood fibers. In the field they can be 3 to 7 cm long, which is why cotton paper is strong. Machine-made paper has a grain. It tears and folds more easily in one direction, because the fibers line up in the direction the paper moves.
Wood is made of cellulose fibers held together by lignin. For paper to last a long time it must be acid-free, and lignin helps turn paper acidic and brittle. Paper sizes follow a pattern. The A series starts with A0, a sheet that is one square meter. Cutting it in half makes A1, and halving again makes A2, so every size keeps the same shape. In the United States, paper weight is measured by the weight of a ream, which is 500 sheets.
Paper has a long history. A court official in China named Cai Lun is said to have made paper from tree bark, old rags and fishnets, more than 1,900 years ago. Before wood was used, paper was often made from old cloth rags, such as linen and cotton. The first paper mill in what is now the United States was built in Pennsylvania in 1690. In the 1800s, people learned to make paper from wood pulp, and wood became the main ingredient. Papermaking spread from China to the Middle East by the 700s and to Spain by about 1150.
The Fourdrinier machine is named for Henry and Sealy Fourdrinier, two London stationers who paid for its development in the early 1800s. The kraft process makes very strong paper, and its name comes from the Swedish word for strength. Wood chips are cooked in a huge pressure cooker called a digester. There, heat and chemicals dissolve lignin. Lignin is the natural glue that holds wood fibers together. A pulp mill can make more power than it needs by burning bark and the used cooking liquid. So every sheet of paper is a mat of thin fibers pressed together.
Words to know
- grain: fiber direction
- ream: 500 sheets
- digester: pressure cooker
