Knife made of aluminum foil


The Japanese traditionally used natural sharpening stones lubricated with water (using oil on a waterstone reduces its effectiveness.) They have been doing this for many hundreds of years. The geology of Japan provided a type of stone which consists of fine silicate particles in a clay matrix, somewhat softer than novaculite.
Japanese stones are also sedimentary. The most famous are typically mined in the Narutaki District just north of Kyoto.

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 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 micrometer abrasive particle size). [Via: wikipedia]

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