Paper, flat sheet, usually made of plant fibers, used for writing and printing, probably invented in China c.A.D. 105, using bark and hemp. Cotton rags and cloth—still used for special high-grade papers—were the raw materials most often used until generally replaced by wood-pulp processes developed in the mid-19th century. In chemical pulping, wood chips are cooked under pressure in a solution (soda, sulfate, or sulfite) that dissolves all but the cellulose. The pulp is then bleached, washed, and refined, the fibers being crushed, frayed, and cut by mechanical beaters. This increases their surface area and bonding power. At this stage various substances are added: fillers (mainly clay and chalk) to make the paper opaque, sizes (rosin and alum) for resistance to water, and dyes and pigments as necessary. The pulp is fed to the paper machine, where it flows onto a moving belt or cylindrical drum of fine wire mesh, and most of the water is drained off by gravity and suction. The newly formed continuous sheet is pressed between rollers, dried by evaporation, and subjected to calendaring. Some paper is coated to give a special surface.