Japanese chisels and plane blades are made of mild steel with a high carbon steel welded on the back to provide a hard cutting edge. Japanese cutting edges are harder than those made almost anywhere else, and that creates a unique problem for the makers and the users. The hollow grind is put in by the maker before the tool is heat treated. It's done then to remove as much steel in that area while it is relatively soft, making it much easier to grind the back flat after the tool has been hardened. If you flatten a blade before hardening it, it may warp some in the process, and undo all the work done, so that's why they don't do much flattening before heat treating. The practice of grinding a hollow in the back was started ages ago when they didn't have power grinders like we have now. They could flatten them today with power equipment and not have to put the hollow in them at all. But they continue to put the hollow in the back to help the user with routine sharpening of the tool. That hollow leaves much less steel around the edges that needs to be removed when performing routine sharpening. Hollow grinds are not a thing in tools from other countries because nobody else uses steel as hard for the edges. But very hard steel is also more brittle, so they need to be used with some care, especially around hard knots. As a blade is used over the years, the cutting edge will get nearer to the hollow from thousands of resharpenings. When sharpening the back, very little material is usually removed, but when the edge gets near the hollow, the user needs to apply more pressure near the cutting edge area of the back. That will thin the hard steel on the back and the hollow will get shallower. That increases the distance between the hollow and the cutting edge. That process needs to be done carefully to keep the back surface flat around the whole perimeter.
Japanese chisels and plane blades are made of mild steel with a high carbon steel welded on the back to provide a hard cutting edge. Japanese cutting edges are harder than those made almost anywhere else, and that creates a unique problem for the makers and the users. The hollow grind is put in by the maker before the tool is heat treated. It's done then to remove as much steel in that area while it is relatively soft, making it much easier to grind the back flat after the tool has been hardened. If you flatten a blade before hardening it, it may warp some in the process, and undo all the work done, so that's why they don't do much flattening before heat treating. The practice of grinding a hollow in the back was started ages ago when they didn't have power grinders like we have now. They could flatten them today with power equipment and not have to put the hollow in them at all. But they continue to put the hollow in the back to help the user with routine sharpening of the tool. That hollow leaves much less steel around the edges that needs to be removed when performing routine sharpening. Hollow grinds are not a thing in tools from other countries because nobody else uses steel as hard for the edges. But very hard steel is also more brittle, so they need to be used with some care, especially around hard knots.
As a blade is used over the years, the cutting edge will get nearer to the hollow from thousands of resharpenings. When sharpening the back, very little material is usually removed, but when the edge gets near the hollow, the user needs to apply more pressure near the cutting edge area of the back. That will thin the hard steel on the back and the hollow will get shallower. That increases the distance between the hollow and the cutting edge. That process needs to be done carefully to keep the back surface flat around the whole perimeter.
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