Due to their form, dimensions, and the nature of the treatment of the surfaces, they reproduce precisely the most ancient implements used for grinding cereal grain into flour. The production of flour by rubbing wheat by hand took several hours. Quern stones were used in China at least 10,000 years ago to grind wheat into flour. In early Maya civilizations the process of nixtamalization was distinctive in that hard, ripe kernels of maize (corn) were boiled in water and lime, thus producing nixtamal which was then made into unleavened dough for flat cakes by grinding with a handstone on a quern ( metate). The use of grinding stones for vegetal food processing, and possibly the production of flour, was widespread across Europe from at least 30,000 years ago. However, in many non-Westernised, non-mechanised cultures they are still manufactured and used regularly and have only been replaced in many parts of the world in the last century or so. They were generally replaced by millstones once mechanised forms of milling appeared, particularly the water mill and the windmill, although animals were also used to operate the millstones. Quern-stones have been used by numerous civilizations throughout the world to grind materials, the most important of which was usually grain to make flour for bread-making. Revolving beehive quern-stones and a saddlestone on display at Cliffe Castle Museum, in Keighley, West Yorkshire. One class of upper quern-stones has from two to three sockets for the rod used to turn them and this is thought to reflect the need to reduce wear and tear by having alternative points of contact when in active use. Most handstones have a handle hole on the upper surface, but one class of quern-stones have a slot handle which indicates that a piece of wood was placed horizontally and protruded out from the edge so that the operator could turn the stone by standing and using a rod vertically. The upper stone sometimes had a cup-shaped area around the hopper hole with a raised edge. to be added but still acted as a centering device. Sometimes a millrind was present as a piece of wood (or other material), which allowed the cereal etc. Quern-stones are frequently identifiable by their grooved working surfaces which enabled the movement of flour. The upper stones were usually concave while the lower ones were convex. An old Gaelic proverb is "The quern performs best when the grindstone has been pitted." Design of quern-stones
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