Wood, chemical or mechanical, usually finds its way to the paper mills in the form of pulp boards, and is known as chemical or mechanical wood pulp. No boiling is necessary, but the boards are fed into the breaking engine, and reduced to half-stuff, a little bleach liquor added to chemical wood, and the contents of the engine, when sufficiently reduced, are let down to the draining tanks for the bleach to expend itself. Then the pulp is ready for the beating engine, where it is reduced to the necessary degree of fineness.
Some materials are more effectively reduced to pulp in the edge runner or kollergang. This machine is similar in appearance to a mortar mill, but the arrangement is slightly different. The pan of the machine is stationary, and the stones revolve and travel round the pan. Only a small quantity of water is used with the pulp, and waste papers which require rubbing apart only, and strong wood pulps of which the fibres are drawn out, and not in any way reduced in length, are treated in this machine more economically and more effectively than in the beating engine.
