Also called a refiner, a beater is an older, mechanical refining device for paper pulp. It is an oval-shaped tank containing metal bars mounted on a rotating beater roll in the center of the tank and stationary metal bars mounted on a bedplate attached to the wall of the tank. The fibers, suspended in a water slurry, are circulated through the tank and drawn between the two sets of bars, the resulting abrasion alters the fibrous structure in such a way as to make them suitable for forming into a paper web. This process, known as beating or refining, is the most crucial stage of the papermaking process. Various fillers, coatings, or sizing can also be added to the pulp fibers at this point, as can dyes used to color the paper. (Paper which has had its sizing added during refining is known as beater-sized, while paper that has had its colorant added during refining is known as beater dyed.) The fibers are pumped from the beater to a conical refiner—also called a jordan—for further beating.
Newer pulp refining methods employ continuous disk refiners which take the place of the beater system. (See Refining and Paper and Papermaking: Papermaking.)