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 refer to the grit size of the particles in the stone. Generally, the finer the grit, the denser the material, which then 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 vs. Artificial Stones
Manmade or Synthetic Stones are manufactured with abrasives ground to a consistent particle size, blended with selected bonding agents, molded to shape, and finished to exacting surface configurations.
- Aluminum oxide, which has a relative hardness of 9.2, is bonded to form reconstructed stones, including modern Japanese water stones (resin bond) and India stones (vitrified bond). Originally this material was from natural sources (emery and corundum), but manufactured abrasives have dominated since the early 1900s.
- Crystolon stones manufactured with silicone carbide abrasives, are the best choice for quick sharpening where the speed in producing the edge is more important than the fineness of the cutting edge. Usually gray in color, they are available in coarse, medium and fine grits.
- Ceramic stones are made from alumina (aluminum oxide) or silicon carbide in a ceramic bond. Silicon carbide has a hardness of 9.5 and will sharpen anything except carbide tipped tool bits. Cceramic stones are offered in a wide variety of sizes and grits.
- Industrial diamonds are made into stones by bonding them to steel and are therefore also called diamond files. Diamond has a relative hardness of 10. Two very different types of diamonds are used in diamond hones. Monocrystalline diamond stones last longer because the diamonds do not fracture readily. Polycrystalline diamond is less expensive but wear faster.
- India Stones are quarried from the finest Ozark Novaculite (silicon quartz) deposits, precision sawn to size, and finished to exacting surface configurations. Although there are no industry established standards for classifying hardness, particle size, or color of natural stones, the standards established by Norton nearly 100 years ago are accepted by most knowledgeable sources as industry standards. However, all suppliers do not adhere to these standards, some choosing to classify and label sharpening stones at their own discretion. This has resulted in much confusion for users, who experience widely varying performance levels between similarly identified stones of different manufactures.
- Arkansas Stones, because of their dense construction, absorb oil at a much slower rate then man-made stones. Hence, these stones do not require oil pre-filling during manufacture. However, a few drops of oil should be applied prior to use. When sharpening, Arkansas stones do not remove as much metal as man-made stones; hence tools and knives last significantly longer. Due to their unique crystalline structure, Arkansas stones polish as they sharpen, imparting a keen, smooth edge/surface.
- Hard Translucent Arkansas (ultra fine) stones are the finest grained and most dense natural stone available. Used to produce the keenest, most precise finish possible. They impart polished, razor-like edges. Translucent white in its purest form, hard translucent Arkansas stones may contain shades of red, yellow and grey. Translucent stones are named due to their ability to let light pass through. Superfine Arkansas stones are coarser and less dense than hard Arkansas stones. Used for imparting the finest edges necessary for most cutting tools and knives. Typically, they are opaque milky white in color.
Soft Arkansas (extra fine) stones are the coarsest-grained and least dense of the natural stones. Used primarily to sharpen and upgrade tool and knife edges to an even, polished surface. Frequently after sharpening with synthetic stones. They are also typically opaque milky white in color.
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 re-crystallized 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.
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 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.
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 may have 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.