Paper is felted or matted sheets of cellulose fibers, formed on a wire screen from a water suspension, and used for printing or writing on. Rags and cloth – still used for special high-grade papers – were the raw materials used until generally replaced by wood pulp processes developed in the mid 19th-century. Logs are now pulped by three methods. Mechanical pulping normally uses a revolving grindstone. In full chemical pulping, wood chips are cooked under pressure in a solution that dissolves all but the cellulose: the kraft process uses alkaline sodium sulfide solution; the sulfite solution uses various bisulfites with excess sulfur dioxide. Semichemical pulping employs mild chemical softening followed by mechanical grinding. The pulp is bleached, washed, and refined – i.e., the fibers are 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 water resistance, and dyes and pigments as necessary. A dilute aqueous slurry of the pulp is fed to the paper machine, flowing onto a moving belt or cylindrical drum of fine wire mesh, most of the water being drained off by gravity and suction. The newly-formed continuous sheet is pressed between rollers, dried by evaporation, and subjected to calendering. Some paper is coated to give a special surface.