A chisel should be sharp, says the woodturner Joost Kramer. Sharpening a chisel is possible only with something that is even harder. Therefore his grindstone is covered with diamond grit. If the last burr after polishing seems to be off, a ultimate test takes place to prove that the chisel is sharp. If he can shave the hairs on his arm with the chisel, only then the sharpening is considered successful. His chisel is now sharp.