Rotary digester to process rags by boiling caustic soda (sodium hydroxide, lye). Cotton paper is made from cotton linters or cotton from used cloths (rags) as the primary material source, hence the name rag paper. Cotton paper is superior in both strength and durability to wood pulp-based paper, which may contain high concentrations of acids. It was not until the introduction of wood pulp in 1843 that paper production was not dependent on recycled materials from ragpickers. To expedite the pulping process the rag engine was invented in Holland in the late 17th century. This device emerged as an alternative to mechanical stampers. The beater, also known as a "hollander," consisted of a tub filled with rags and water, which were beaten to pulp by a set of rotating blades powered by a windmill, or later a waterwheel.