Sharpening stone
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, razors 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 also some drawbacks. First, over hundreds of years, the best mines have given up much of their best stone.[citation needed] 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.
One of the most revered natural whetstones is the yellow-gray Belgian coticule, which 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 natural elegance and beauty, as well as for providing both a fast-cutting surface for establishing a bevel and a higher-grit surface for refining it.
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.[citation needed]
Whetstones and oilstones
Whetstones may be natural or artificial stones. Artificial stones usually come in the form of a bonded abrasive composed of a ceramic such as silicon carbide (carborundum) or of aluminium oxide (corundum). Bonded abrasives provide a faster cutting action than natural stones. 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.
When the block is intended for installation on a bench it is called a bench stone. Small, portable stones (commonly made of bonded abrasive) are called pocket stones. Being smaller they are more portable than bench stones but difficulty in maintaining a consistent angle and pressure when drawing the stone along larger blades can make them harder to use. However, they are still very capable of forming a good edge. Frequently, fine grained pocket stones are used for honing, especially "in the field". Despite being a homophone with wet, whetstones need not be lubricated with oil or water though it is very common that one of these is used. Lubrication aids the cutting action and carries away swarf.
Finer grades of whetstones intended for use with oil (though, again, water can also be used) are called oilstones. Oilstones usually come in the form of bench stones. One of the natural minerals commonly used in oilstones 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 "pink translucent". Novaculite is from the Devonian 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.
Extremely fine grades of stone that remove very little metal may be called polishing stones.
Japanese waterstones
The Japanese have traditionally used sharpening stones which are lubricated with water to sharpen their metal tools. (Using oil on a waterstone is deleterious to the stone.) 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, which can ruin the stone) so nothing but 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 Japanese 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, the nagura, which is not used directly. Rather, it is used to form a cutting slurry on the shiage-to, 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 (using a non-Japanese system of grading grit size) 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 a steel plate sometimes with a plastic or resin base. The plate is coated with diamond grit, an abrasive that will grind material from the blade. The plate may have a series 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 sizes (from credit card to bench plate size) and grades of grit. A coarser grit is used to remove larger amounts of metal more rapidly, such as when forming an edge or restoring an edge which is damaged. A finer grit is used to remove the scratches of larger grits and to refine an edge. Two-sided plates exist where each side is covered by a different grit.
Unlike stones which can become rounded—decreasing their effectiveness—diamond plates remain flat. They do not require the use of a lubricant.
Diamond plates, being made from diamond—even though of industrial grade—tend to be more expensive than stones.