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Sharpening stones are used to grind and hone the edges of steel tools and implements. Examples of items that may be sharpened with a sharpening stone include scissors, knives and tools such as chisels and plane blades.
Sharpening stones come in a wide range of shapes, sizes and material compositions. Stones 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.
Stones are usually available in various grades, which refers to the grit size of the particles in the stone. Generally, the finer the grit, the denser the material, which leads to a finer finish of the surface of the tool. Finer grits cut slower because they remove less material. Grits are often given as a number, which indicates the density of the particles with a higher number denoting higher density and therefore smaller particles.
Natural versus artificial Stones
Although there is a certain amount of romance associated with using stone which is found naturally, there are some problems with this. First, over hundreds of years, the best mines have given up much of their best stone. This scarcity causes high prices for a good quality consistent stone. Lesser quality stones have problems of consistency and may have occasional larger pieces of grit or soft spots. With this in mind, and with modern technologies, artificial stones came to the market. There have been a variety of formulations over the years and the quality of artificial stones continues to increase.
For most users artificial stones offer many improvements over the natural stones of the past. The high cost and difficulty of obtaining quality natural stones make them impractical for most.
Oil stones and whetstones
Oil stones, also known as whetstones, are composed of a variety of materials, natural and man made. These types of stone are usually lubricated with oil, hence the name, although water may be used instead. The purpose of the lubrication is to aid the cutting action and to carry away the swarf. They come in a variety of shapes. Some shapes are designed for specific purposes such as sharpening scythes or drills
One of the natural minerals commonly used in oil stones is Novaculite. Examples of stones made from this material include Arkansas, Ouachita or Washita stones from the Ouachita Mountains in Arkansas. These come in various grades and colors, with the finer stones being denominated "surgical black" or "transparent white". Novaculite is from the Devonian period and Mississippian periods. It is a sedimentary rock composed mostly of microcrystalline quartz and is basically a recrystallized variety of chert. It is also the primary material in Charnley Forest and Turkey oilstones.
Artificial oilstones are usually composed of a ceramic such as silicon carbide. These stones 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.
Japanese waterstones
The Japanese have traditionally used sharpening stones which are lubricated with water to sharpen their metal tools. As they have been doing this for many hundreds of years, it is obvious that the first stones were those which were found occurring naturally. The geology of Japan provided a type of stone which consists of fine silicate particles in a clay matrix. This is somewhat softer than Novaculite.
Japanese stones are also sedimentary. The most famous are typically mined in the Narutaki District just North of Kyoto.
Advantages and disadvantages
These softer Japanese stones have a few advantages over harder stones. First, because they are softer they do not become glazed or loaded with the material they are sharpening. New particles are constantly exposed as you work with them and thus they continue to cut consistently. Second, they can be lubricated effectively with water (rather than oil) so nothing but a bucket of water is required. Finally, because they are soft, the worn material and the water form a slurry which in conjunction with the stone, sharpens and polishes the blade.The disadvantage is that they wear out faster than other types of sharpening stone, although this makes them easier to flatten.
Grades of waterstones
Historically there are three broad grades of sharpening stones. The Ara-to, or "rough stone", The Naka-to or "middle/medium stone" and the shiage-to or "finishing stone". There is a fourth type of stone which is used, but not directly. That is the nagura which is used to form a cutting slurry on the shiage-to or finishing stone which is often too hard to create the necessary slurry. Converting these names to absolute grit size is difficult as the classes are broad and natural stones have no inherent "grit number". As an indication, Ara-to is probably 500-1000 grit. The Naka-to is probably 3000-5000 grit and the Shiage-to is likely 7000-10000 grit.Diamond plate
A diamond plate is used when sharpening tools or other implements with steel blades.
The plate consists of a plastic or resin base onto which a steel plate is bonded. The steel plate is impregnated with a coating of diamond grit. The purpose of the grit is to grind away material from the blade as it is rubbed back and forth on the plate. The steel plate has a matrix of holes cut in it which are designed to capture the swarf that is cast off as grinding takes place.
Diamond plates are available in various plate and grit sizes. A coarser grit is used to remove larger amounts of metal more rapidly, then the finer grits are used to remove the scratches of the larger grits and to refine the edge.
See also
External links
Woodworking hand tools | Grinding and lapping | Knives
砥石 | Schleifstein | Wetzstein | Galoda | Wetsteen
"Sharpening stone"