Naniwa's "Dressing Stone" is traditionally known as 'Nagura.' This stone is used for cleaning all types of water stones. When you rub the Nagura stone on the surface of the whetstone, dirt/impurities are removed from the stone, and at the same time, clay-like material is removed from the surface of the Nagura. This cleaning process is often done on whetstones of finer grits, i.e., from 1000 grit upwards, depending on the stone's quality. Another role of Nagura is to improve the gloss of the knife surface. In this case, the cleaning is done at the very end of the sharpening process so that the clay-like material that comes out of the Nagura makes sharpening more pleasant and, at the same time, gives the knife a shiny surface.
The surface-forming stone must be thoroughly immersed before use.