If frequent use has worn a hollow in an oilstone, regrind the stone flat, using silver sand and an old piece of glass, about 45 cm/18 in square. Lay the glass on a flat surface and sprinkle a thin layer of silver sand over it. Wet the stone and, keeping it damp, rub it in circular movements over the sand until the stone is flat.
If working on a very smooth surface, such as laminate, lay sheets of newspaper beneath the glass, dampening the top sheets. Otherwise, the pressure being exerted on the stone may cause the glass to slide about.