Knives are sharpened by grinding against a hard rough surface, typically stone, or a soft surface with hard particles, such as sandpaper. 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. Typical knives are sharpened at 20 degrees. Knives that require a tough edge (such as those that chop) sharpen at 25 degrees 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. Straight razors used for shaving must cut with minimal pressure, and thus must be very sharp and have a thin angle, and often a hollow grind, and are stropped daily or more often. Kitchen knives are less sharp, and generally cut by slicing rather than just pressing, and are steeled daily.
The composition of the stone affects the sharpness of the blade (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 traditionally of softer steel and take an edge angle of 20–22°, while East Asian kitchen knives are traditionally of harder steel and take an edge angle of 15–18°.
