shreds off short wood fibbers from the block. The fibbers produced by this process are short and are used only in the production of cheap newsprint and used to be added with other types of wood fibber in the making of high-quality paper. Another technique uses a chemical-solvent processes where wood chips are treated with solvents that remove resinous material and lignin from the wood, leaving pure fibbers of cellulose. The oldest of the chemical-solvent processes, the soda process, introduced in 1851, uses a solution of caustic soda (sodium hydroxide) as a solvent. The wood is cooked or digested in this solution under steam pressure. The fibbers produced by this process do not have great strength but are used in mixtures with other wood fibbers. Pulps produced by any of these processes are washed then passed through a series of screens to remove knots, debris, and other unwanted material. Some pulps are bleached to produce a whiter sheet of paper. Most paper today is made on Fourdrinier machines patented after the first successful papermaking machine, which was developed in the early 19th century. It is capable of making a continuous sheet of paper up to 33 feet wide, at speeds faster than 3,000 feet per minute. Some machines are more than 350 feet long. The Fourdrinier machine has an endless belt of wire mesh that moves horizontally. A flow of watery pulp is spread on the level belt that passes over a number of rolls. A shallow wooden box beneath the belt catches most of the water that drains off, leaving a matt of fibbers on the surface of the wire. Air suction pumps beneath the belt help to drain the water through the wire, and the belt itself is moved from side to side to aid the felting of the fibbers. Once the sheet of fibbers is strong enough it is then passed between large press rolls that squeeze out most of the remaining water from the sheet. At this point a watermark may be produced by pressing a wire pattern into ...
