Knife sharpening

a hard, rough surface, typically stone, or a soft surface with hard particles, such as sandpaper. Additionally, a leather razor strop, or strop, is often used to straighten and polish an edge. The smaller the angle between the blade and stone, the sharper the knife will be, but the less side force is needed to bend the edge over or chip it off. The angle between the blade and the stone is the edge angle Ц the angle from the vertical to one of the knife edges, and equals the angle at which the blade is held. The total angle from one side to the other is called the included angle; Ц on a symmetric double-ground edge (a wedge shape), the angle from one edge to the other is thus twice the edge angle. Typical edge angles are about 20∞ (making the included angle 40∞ on a double-ground edge). The edge angle for very sharp knives can be as little as 10 degrees (for a 20∞ included angle). Knives that require a tough edge (such as those that chop) may sharpen at 25∞ or more. Different knives are sharpened differently according to grind (edge geometry) and application. For example, surgical scalpels are extremely sharp but fragile, and are generally disposed of, rather than sharpened, after use. Straight razors used for shaving must cut with minimal pressure, and thus must be very sharp with a small angle and often a hollow grind. Typically these are stropped daily or more often. Kitchen knives are less sharp, and generally cut by slicing rather than just pressing, and are steeled daily. At the other extreme, an axe for chopping wood will be less sharp still, and is primarily used to split wood by chopping, not by slicing, and may be reground but will not be sharpened daily. In general, but not always, the harder the material to be cut the higher (duller) the angle of the edge. The composition of the stone affects the sharpness of the blade (a finer grain produces sharper blades), as does the composition of the blade (some metals take and keep an edge better than others). For example, Western kitchen knives are usually made of softer steel and take an edge angle of 20Ц22∞, while East Asian kitchen knives are traditionally of harder steel and take an edge angle of 15Ц18∞. Knife sharpening proceeds in several stages, in order from coarsest (most destructive) to finest (most delicate). These may be referred to either by the effect or by the tool. Naming by effect, the stages are: sharpening Ц removing metal to form a new edge rough sharpening (using either water stones, oil stones, or medium grits of sandpaper in the scary sharp method of sharpening) fine sharpening (using the same tools as above, but in finer grits) straightening Ц straightening the existing metal on the blade, but not removing significant quantities of metal polishing (also called stropping)Ц giving a mirror finish, but not significantly altering the edge. polishing may also be achieved by buffing a blade: instead of moving the knife against a flat leather strop loaded with fine abrasive, the knife is held still and a powered circular cloth wheel is moved against the knife. Named by tools, the same three stages are: grinding (on a grinding wheel) or whetting (on a whetstone) steeling, using a honing steel stropping, on a razor strop or buffing on a wheel The word "honing" is ambiguous, and may refer to either fine sharpening (step 1.2) or straightening (step 2). The finest level of sharpening is done most frequently, while the coarser levels are done progressively more rarely, and sharpening methods differ between blades and applications. For example, a straight razor used for shaving is stropped before each use, and may be stropped part-way through use, while it will be fine sharpened on a stone a few times per year, and re-ground on a rough stone after several years. By contrast, a kitchen knife is steeled before or after each use (and may be steeled during heavy use, as by butchers), and sharpened on a stone a few times per year.