Sharpening stones, or whetstones, are used to sharpen the edges of steel tools such as knives through grinding and honing. Such stones come in a wide range of shapes, sizes, grits and material compositions for varied use. They may be flat, for working flat edges, or shaped for more complex edges, such as those associated with some wood carving or woodturning tools. They may be composed of natural quarried material or from man-made material. They come in various grades, which refer to the grit size of the abrasive particles in the stone. (Grit size is given as a number, which indicates the spatial density of the particles; a higher number denotes a higher density and therefore smaller particles, which give a finer finish to the surface of the sharpened object.) Stones intended for use on a workbench are called bench stones, while small, portable ones, whose size makes it hard to draw large blades uniformly over them, especially "in the field", are called pocket stones.
Often whetstones are used with a cutting fluid to enhance sharpening and carry away swarf. Those used with water for this purpose are often called water stones or waterstones, those used with oil sometimes oil stones or oilstones. Whetstones will wear away with use, typically in the middle. Tools sharpened in this groove will develop undesirable curves on the blade. In order to prevent this, a whetstone may be levelled out with sandpaper or a levelling or flattening stone.
Terminology
The term is based on the word whet, which means to sharpen a blade, not on the word wet. The modern verb to describe the process of using a sharpening stone for a knife is simply to sharpen, but the older term to whet is still sometimes used, though so rare in this sense that it is no longer mentioned in, for example, the Oxford Living Dictionaries.
Natural stones
Natural whetstones are typically formed of quartz, such as novaculite. The Ouachita Mountains in Arkansas are noted as a source for these, giving them the name "Arkansas stones". Novaculite is very hard and has small crystals (3-5 microns), making it suitable for the later fine stages of knife sharpening. Novaculite and other stone formations are found around the world such as in Eastern Crete which produces a stone known as the Turkish Stone, mined in the Elounda mountain but sold all throughout the Levant (hence its name) since antiquity. Similar stones have been in use since antiquity. The Roman historian Pliny described use of several naturally occurring stones for sharpening in his Natural History. He describes the use of both oil and water stones and gives the locations of several ancient sources for these stones. One of the most well-regarded natural whetstones is the yellow-gray "Belgian Coticule", which has been praised for the sharpness of the edge it can give to blades since Roman times, and has been quarried for centuries from the Ardennes. The slightly coarser and more plentiful "Belgian Blue" whetstone is found naturally with the yellow coticule in adjacent strata; hence two-sided whetstones are available, with a naturally occurring seam between the yellow and blue layers. These are highly prized for their appearance, and for providing both a fast-cutting surface for establishing a bevel and a finer surface for refining it. Different veins of this stone are suitable for knives, tools, and razors respectively. Certain versions (such as La Veinette) are very sought after for razor honing. The hard stone of Charnwood Forest in northwest Leicestershire, England, has been quarried for centuries, and was a source of whetstones and quern-stones. Natural stones are often prized for their natural beauty as stones and their rarity, adding value as collector's items. Furthermore, each natural stone is different, and there are rare natural stones that contain abrasive particles with different properties than are currently available in artificial stones. Two common stones in the UK are the Water of Ayr stone and the speckled Tam'o Shanter stone, both forms of slate used as razor oilstones.
Artificial (synthetic) stones Artificial stones usually come in the form of a bonded abrasive composed of a ceramic such as silicon carbide (carborundum); aluminium oxide (corundum, also known as water stone or India stone); and CBN (cubic boron nitride). They provide more aggressive cutting action than natural stones, and are used for the middle stages of knife sharpening. Traditionally, natural stones were used for the later, finer stages, but today very fine artificial stones are available. They are commonly available as a double-sided block with a coarse grit on one side and a fine grit on the other enabling one stone to satisfy the basic requirements of sharpening. Some shapes are designed for specific purposes such as sharpening scythes, drills or serrations. Modern synthetic stones are generally of equal quality to natural stones, and are often considered superior in sharpening performance because of consistency of particle size and control over the properties of the stones. For example, the proportional content of abrasive particles as opposed to base or "binder" materials can be controlled to make the stone cut faster or more slowly, as desired. The use of natural stone for sharpening has diminished with the widespread availability of high-quality artificial stones with consistent particle size. As a result, the legendary Honyama mines in Kyoto, Japan, have been closed since 1967. Belgium currently has only a single mine that is still quarrying Coticules and their Belgian Blue Whetstone counterparts.
Japanese natural and synthetic waterstones
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