Paper has been traced to China in about AD 105. It reached Central Asia by 751 and Baghdad by 793, and by the 14th century there were paper mills in a number of places in Europe. The invention of the printing press in about 1450 markedly increased the demand for paper, and at the beginning of the 19th century wood and other vegetable pulps began to replace rags as the foremost source of fibre for papermaking.

Before 1798, Nicholas-Louis Robert created the first paper-making machine. Using a moving screen belt, it was made one sheet at a time by dipping a frame or mould with a screen bottom into a vat of pulp. Some years later the brothers Henry and Sealy Fourdrinier improved Robert’s machine, and in 1809 John Dickinson invented the first cylinder machine.

Although almost all steps in papermaking have become highly mechanized, the basic process has remained mostly unchanged. First of all, the fibres are separated and wetted to create the paper pulp, or stock. The pulp is then filtered on a woven screen that forms a sheet of fibre, which is then pressed and compacted to squeeze out most of the water. The remaining water is removed by evaporation, and the dry sheet is further compressed and, depending upon the intended use, coated or impregnated with other substances.

Differences among the grades and types of paper are determined by a number of factors: the kind of fibre used; the preparation of the pulp, which is either by mechanical (groundwood) or chemical (primarily sulfite, soda, or sulfate) methods, or by a combination of both; by the adding of more substances to the pulp, the most common being bleach or colouring and sizing, the latter to retard penetration by ink; by conditions under which the sheet is formed, including its weight; and by the physical or chemical treatment applied to the resulting sheet.

Although wood has become the principal source of fibre for papermaking, rag fibres are still used for paper of maximum strength, resistance to mould, and permanence. Recycled wastepaper (including newsprint) and cardboard are also important sources. Other fibres used include straw, bagasse (residue from crushed sugarcane), esparto, bamboo, flax, hemp, jute, and kenaf. Some paper, particularly specialty items, is created from synthetic fibres.

Weight or substance per unit area, called basis weight, is measured in reams (now commonly 500 sheets). Paper is also measured by caliper (thickness) and density. The strength and durability of paper is determined by factors such as the strength and length of the fibres, as well as their bonding ability, and the formation and structure of the sheet. The optical properties of paper include its brightness, colour, opacity, and gloss. Among the most important paper grades are bond, book, bristol, groundwood and newsprint, kraft, paperboard, and sanitary.

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