Mill stones are used in windmills and watermills for grinding wheat or other grains.
In Britain there are two types of mill stone:
- Derbyshire Peak stones of grey millstone grit, cut from one piece, used for grinding barley; disused Derbyshire Peak stones are used as decorative signposts at the boundaries of the Peak District National Park. Derbyshire Peak stones wear quickly and are typically used to grind animal feed since they leave stone powder in the flour making it undesirable for human consumption.
- French burr stones, used for finer grinding. Not cut from one piece, but built up from sections of quartz, cemented together with plaster, and bound with iron bands. French Burr comes from the Marne Valley in northern France.
The surface of a millstone is divided by deep grooves called furrows into separate flat areas called lands. Spreading away from the furrows are smaller grooves called feathering. The grooves provide a cutting edge and help to channel the ground flour out from the stones. When in regular use stones need to be dressed periodically, that is, re-cut to keep the cutting surfaces sharp.
Millstones need to be evenly balanced, and achieving the correct separation of the stones is crucial to producing good quality flour. The experienced miller will be able to adjust their separation very accurately.
Not all windmills were used for grinding corn; many were used for drainage purposes and referred to as windpumps; they were not equipped with mill stones.
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