Finishing whetstone” determines the sharpness of kitchen knives, razors, chisels, planes, and other cutting tools. Among them, “Kyoto whetstones” are regarded as “exceptional” by sharpeners and chefs around the world. The Nakayama, Daitori, Okudono, and Shobutani areas in particular have produced “legendary” whetstones. Why is it that only Kyoto has been able to produce grinding stones of such high quality?
The answer boils down to three main reasons
1. “Invisible abrasives” called radiolarians
The host rock of Kyoto’s finishing stones is a geological formation called “slate. It is made up of mud and planktonic carcasses that were deposited in the deep sea and hardened over a long period of time under the influence of geothermal pressure and heat. The slate contains abundant planktonic fossils called “Radiolaria,” which are microscopic silica skeletons.
These radiolarians are only 0.05 to 0.2 microns in diameter and are extremely fine and hard, acting as “natural ultrafine particle abrasives. Abrasive is the act of grinding a substance into smaller particles. The size of RASHIBARA’s particles is ideal for this purpose.
The Kyoto grinding stone layer contains an overwhelmingly large amount of these radiolarians, and this is what makes it possible for the grinding stone to be both smooth and abrasive at the same time. In fact, microscopic observation reveals many minute radiolarian fossils in star or reticulate shapes inside the grinding wheel. This is directly related to the “fineness” of the sharpening.
Major Distribution of Slate Rocks in Japan
| area | Distribution zone/zone | Features and Applications |
|---|---|---|
| Kyoto – Hyogo | Tamba Belt, Hokusetsu Belt | The most important natural grinding stone production area |
| Central Shikoku | Sambagawa Metamorphic Zone | Crystalline schistosity, very hard |
| Kyushu (Saga – Nagasaki) | Paleogene sedimentary rocks | Known as a low rainwater infiltration formation |
| Tohoku region (northernmost six prefectures of Honshu) | Abukuma Mountains, Dewa Mountains, etc. | closely related to landslides and faulting activity. |
| Chubu – Kanto | Iide belt and Chichibu belt | Mainly Paleozoic, little industrial use |
*However, “slate is available” does not necessarily mean “suitable for grinding stones.
2. strata that have undergone just the right amount of “metamorphism
Geologically speaking, radiolarian beds accumulated on the seafloor are then pushed deep into the earth by plate movement and undergo metamorphism. If this metamorphism is too weak, the result is simply mudstone or shale, which is too sticky to be used as a grinding stone. On the other hand, if the metamorphism is too strong, the stone becomes chert, which is extremely hard and difficult to break.
The slate of Kyoto undergoes low to moderate metamorphism, which is somewhere between low and moderate, so that recrystallization is not too advanced and the radiolarian structure remains to some extent. This is the main reason why the slate is in an “ideal condition for abrasive particles. The strength of metamorphism depends on volcanic activity and crustal deformation in the surrounding area, but Kyoto was in the right place. The balance is so exquisite that it is hard to believe it is a coincidence.
To use an analogy, it is like “al dente pasta” that is neither too soft nor too hard. The grinding stone layer is also difficult to grind if it is too hard, and too soft and the grain will become clogged. The stones of Kyoto have an amazingly excellent balance between these two.
*Why is it that only the suburbs of Kyoto have high quality?
| element (e.g. in array, in programming, in programming) | Features of Slate in Kyoto |
|---|
| particle size | Uniform and very fine |
| cleavage (in gems) | Easy to break straight (good workability) |
| inclusions | Contains moderate amounts of fine quartz and feldspar |
| geological environment | Combination of complex metamorphism and depositional environments |
| historical use | Accumulated technology and reputation since the Edo period |
3. geological structure + weathering + water + human eye
No matter how good the quality of a whetstone is, in order for it to be used as a whetstone, it must be “extractable,” “processable,” and “stably supplied. For this purpose, the formation must be easily fractured in a certain direction and the joints (natural fractures) must be fine.
In the Kyoto grindstone layer, these are in perfect alignment. There was the inclination angle of the strata, the direction of breakage, natural weathering due to moisture, and above all, the “craftsman’s eye” to discern it.
In the culture of sword smiths and woodworkers that has continued since the Edo period, rules of thumb such as “this is a good stone,” “this is too slippery,” and “this is too hard” have been accumulated, and in the quarries, experts who know “which layer and which direction is good” have dug out the whetstone layer like miners. In the quarries, experts who know “which layer is good for which direction” have been digging out the stone layers just like miners.
Today, there are areas where mining is prohibited or access is difficult due to land subsidence, but in those days, people and nature breathed in unison, and high quality whetstones were “selected” and dug out.
summary
Kyoto’s finishing stones are not the result of a series of coincidences, but a world-class material created by the trinity of “miraculous geology,” “just the right metamorphosis,” and “craftsman’s discernment.
The amount of mined materials will continue to decrease in the future. That is why we should use the whetstones we have now with care, and think about the wisdom of nature and people that lies behind them, for the best sharpening.
The Kyoto whetstone you see in front of you now is the fruit of hundreds of millions of years of life on the earth and hundreds of years of human observation.