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Grinding maize in a working watermill, Thethi, northern Albania.

Millstones or mill stones are used in windmills and watermills, including tide mills, for grinding wheat or other grains.

Millstones used in Britain were commonly of two types:

• Derbyshire Peak stones of grey millstone grit, cut from one piece, used for grinding barley; imitation 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.

Making furrows on a millstone

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 or "cracking". The furrows and lands are arranged in repeating patterns called harps. A typical millstone will have six, eight or ten harps. 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 come in pairs. The base or bedstone is stationary. Above the bedstone is the turning runner stone which actually does the grinding. The runner stone is supported by a cross-shaped metal mill rynd fixed to a "mace head" topping the main shaft or spindle leading to the driving mechanism of the mill (either water or wind powered). The pattern of harps is repeated on the face of each stone, when they are laid face to face the patterns mesh in a kind of "scissoring" motion creating the cutting or grinding function of the stones.

The interior of a functional water mill

The basic anatomy of a millstone. Note that this is a runner stone. A bedstone would not have the "Spanish Cross" into which the supporting millrynd fits.

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.

• Millstone Dressing Tools
• European millstone quarries : a data base