Whetstones, also called oilstones, are rectangular blocks made of the extremely hard carborundum (a composition of silicon carbide). They are fine grained, often with one side slightly coarser than the other. Knives should periodically be honed on whetstones to keep them really sharp. This is done by first lubricating the stone with oil or water, then drawing the knife blade with slight pressure across the whetstone at about a 20-degree angle. Doing this 5 to 6 times on each side of the knife is adequate. If the whetstone's two sides are of differing textures, this activity should be performed first on the coarser side and finished on the finer-grained side. This will give the knife an even sharper edge. The sharpness of a knife's blade can be maintained by using a sharpening steel prior to each use.