I. What Paper Is
Paper is a thin sheet made from plant fibers that have been separated, suspended in water, and then collected on a screen so that they lie across one another and bond as they dry. The principle is simple, and it has changed very little in nearly two thousand years. A papermaker begins with a material rich in cellulose, the long, chain-like molecule that gives plants much of their strength. The material is softened and broken down until its fibers come apart. Those fibers are mixed with a large quantity of water to make a thin, milky suspension known as pulp or stock. When the suspension is spread over a fine screen and the water drains away, the fibers settle into a loose mat. Pressed and dried, the mat becomes a sheet.
What holds the sheet together is not glue but the fibers themselves. Cellulose fibers carry many small chemical groups that attract water. While the sheet is wet, a film of water lies between neighboring fibers. As the water evaporates, the surfaces of the fibers are drawn closer and closer, until they form weak bonds directly with one another. Each bond is slight, but a single sheet of ordinary writing paper contains an enormous number of them, and together they give the sheet its strength. This is why paper weakens when it becomes thoroughly wet: water slips back between the fibers and loosens the bonds that drying created.
The character of a paper depends largely on its fibers. Long fibers, such as those from cotton, linen, or certain softwoods, tend to make strong, flexible sheets that resist tearing. Short fibers, such as those from many hardwoods or from grasses, make sheets that are smoother and more even but less strong. Papermakers have long blended fibers of different lengths to balance these qualities. A sheet meant for printing needs a smooth, even surface. A sheet meant for wrapping parcels needs toughness. A sheet meant for wiping hands needs softness and the ability to absorb water quickly. Each calls for a different recipe.
The way the fibers are treated before the sheet is formed matters as much as their source. Fibers that are beaten for a long time become frayed along their surfaces. The frayed ends, called fibrils, increase the area over which one fiber can bond with another, so heavily beaten pulp makes a dense, strong, and somewhat translucent sheet. Lightly beaten pulp makes a bulkier, softer, and more opaque sheet. Traditional papermakers judged the right degree of beating by eye and by touch, taking a little pulp between the fingers and watching how it drained. Modern mills measure the same property with instruments that record how quickly water passes through a sample of pulp.
Water is the other essential ingredient. Very large volumes of it are needed, both to carry the fibers and to wash the raw material clean. Early mills were therefore built beside rivers and streams, where clean water was plentiful and where the flow could also power machinery. The purity of the water affected the color and quality of the paper, and certain districts gained reputations for fine paper partly because their water was soft and free of iron. A trace of iron in the water could leave faint brown spots in finished sheets, a flaw that careful mills worked hard to avoid.
Most paper also contains materials other than fiber. Sizing is added to make the sheet resist the spreading of ink. Without it, paper behaves like blotting paper, and a line drawn with a pen will bleed outward into a fuzzy band.