Wiki-Abrasive grains

It refers to the hard particles used in grinding wheels for polishing. In the case of synthetic grinding wheels, materials such as carborundum, green carborundum, and white arundum (often referred to as C, GC, and WA, respectively) are used. Each of these materials has its own hardness and resistance to grinding, resulting in differences in grinding power, depth of scratches on knives, etc. Natural grinding stones are made from the remains of plankton called radiolaria. Compared to artificial whetstones, natural whetstones are less angular, so they are less likely to scratch knives and produce a clean finish on the knife blade after sharpening.

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Mastering Japanese Knife Sharpening: Techniques & Tips | KIREAJI