The Gadget
Japanese Whetstones
The Japanese traditionally used sharpening stones lubricated with water (using oil on a waterstone reduces its effectiveness[citation needed]). They have been doing this for many hundreds of years, and the first stones were naturally occurring. 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 that in conjunction with the stone, sharpens and polishes the blade.A disadvantage is that they become uneven faster than other types of sharpening stone, although at the same time this makes them easier to flatten. Grades of waterstonesHistorically, 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. Current synthetic grit values range from extremely coarse, such as 120 grit, through extremely fine, such as 30,000 grit (less than half a micron abrasive particle size).
Natural and artificial stones
Natural stones are less common than they used to be. Historical demand has exhausted most known natural quarries and little effort has gone into discovering new ones due to, and since, the concurrent rise of gunpowder and industrial chemistry: the former obviating the military importance of whetstones, and the latter allowing for the mass production of artificial ones.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. This scarcity causes high prices for a good quality natural stone. Lower quality natural stones have problems with consistent grain sizes, low abrasive particle content leading to "slow" sharpening, inconsistency in hardness, inclusions of large foreign particles of other stone materials, and cracks and other such natural imperfections.
Modern synthetic stones are generally of equal quality to natural stones, and are often considered superior in sharpening performance due to 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 slower, as desired Natural stones are often prized for their natural beauty as stones and their rarity, adding value as collectors' items. Furthermore, each natural stone is different, and there are rare natural stones that contain abrasive particles in grit sizes finer than are currently available in artificial stones.
Source: Wikipedia
Japanese sword polishing, particularly in the finishing stages, is still most commonly performed with extremely costly and rare natural polishing stones, as the inconsistent grit sizes of the abrasive particles in natural stones will often produce visual results of polishing that reveal characteristics of the steel that the uniform grain particles in artificial stone will not.Some Japanese waterstones are known to reach 50,000+, 75,000+, or even 120,000+ grit in abrasive particle size, while the finest artificial stone sold is the Shapton 30,000 grit stone (though diamond abrasive particles of several hundred thousand grit are available, typically as a suspended liquid or dry powder).