A Hollander is a machine used to beat bits of fiber (here probably cotton rags) into a pulp, breaking the fibers down into small enough pieces to form a sheet of paper.

gif from Paperslurry

The name implies a Dutch origin, and indeed the machine was developed in the Netherlands in the late seventeenth century to replace the stamping mills which had previously performed this function, much more slowly.

Rotating metal blades within that black housing rotate like a mill wheel in the flow of slurry (just water and fiber) which circulates in the race, getting broken down more and more as time passes.

Here’s an unusually gigantic Hollander in operation at Papeterie Saint-Armand in Montreal.

Bear in mind that the materials used in paper making in the seventeenth and eighteenth centuries were primarily cotton and linen rags, so don’t think of a Hollander as chomping up bits of trees! The earliest recorded use of wood for paper making (apart from the action of wasps, which Réaumur hypothesized in 1719 might be adapted for our purposes) was in 1800 when Matthias Koops, an English papermaker, published a book part of which was printed on “paper made from wood alone”. It wasn’t till 1844 that Friedrich Gottlob Keller patented the first practical wood-grinding machine, thus enabling the development of the groundwood paper industry.