On the stone floor of d'Olde Zwarver there are 4 pairs of mill stones.These are natural stones from Germany. A pair of mill stones consist of "ligger" (="lying") and "loper" (="walking"). The stones wear out during grinding. Therefore do they have to be restaurated regularly. Sharpening the stones is called "billen" of the stone. When the stones are sharpened, the upper stone needs to be pulled up by means of a crane. The stone weights about 1200 kg. Sharpening the stones takes a very long time. Sjaak was busy for hours and hours.