The best material for this purpose is the red, well-burnt sand from the core of a brass casting, the sand being sifted, before using, through fine wire gauze. It will cut more truly and smoothly than any other material, and should be used with water, and not oil. After the cock is sufficiently ground, wash it and the barrel with clean water; and after wiping them comparatively but not quite dry, revolve the plug backward and forward in its place under a slight pressure, and the surface will assume a rich brown color, showing very distinctly the precise nature of the fit, and leaving a smooth surface, free from grit and not liable to either jam or abrade. Valves may be ground to their seats in a similar manner.