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The term “grindstone” is used to describe two different things, both made from stone. In one sense, a grindstone is a stone which revolves on a central axis at varying rates of speed, designed for grinding and polishing. In another sense, a grindstone is a millstone, a specialized stone which is designed specifically for grinding grain. The grindstone in question is usually clear from the context; both are also used to describe a sense of burden or work, as in “keep your nose to the grindstone.”
In the first sense, a grindstone is a disc of thick, porous stone with a hole in the middle. A rotating axle can be mounted through the hole to turn the stone; it is common to mount a grindstone over a tub or basin, so that filings are ground into an easily cleaned container, rather than all over the floor. Using an electric motor or foot pedal, the grindstone can be turned at varying rates of speed, which are altered depending on the project underway.
Grindstones are often used to sharpen knives and other tools. They can also be used to create a polish on something like stone or wood; grindstones used for polishing typically have a very fine grain, as coarse grains would gouge the project, rather than buffing it. In many shops, several grindstones are kept, allowing people to work through progressively finer-grained grindstones to achieve the desired finished look.
A millstone is actually one of a pair of large circular stones. Millstones are stacked on top of each other for the grinding process; typically the bottom stone is rotated while the top stone remains stationary. Grain is poured through a hole in the upper millstone and then ground between the two, trickling out through grooves in the stone as flour. The grain of the flour can be varied by using millstones of various sizes and different speeds during the grinding process.
Many millstones are quite large, designed for grinding massive quantities of grain at once. Early millstones were powered either by animals harnessed to the stone or a waterwheel, which is why many historical flour mills are located near rivers. Archaeological digs have uncovered early versions of millstones, suggesting that humans have been acquainted with the basic steps for making flour for thousands of years. Most modern flours are steel cut, rather than stone ground; some people feel that stone ground flours are superior, and actively seek them out.
"Keep your nose to the grindstone" refers to the time when the men grinding the stone had to, literally, put his nose close to the grindstone to ensure proper spacing between two stones.
Too much space and the grain would not be properly ground, and too little space would give of a burning smell from the stones being rubbed against each other.