A paper mill is a factory that produces paper, usually using vegetable fibers or wood pulp. Wood and other raw materials are cut into small chips and cooked with chemicals in large vessels. The chemicals separate a substance called cellulose from the wood fibres. The cellulose is then added with other chemicals and additives, and pressed into paper in large machines called paper machines. Most paper is made from pine, spruce and eucalyptus. Other things often used include rags.