Why have your knives professionally sharpened?
All knives lose their edge over time, even if you use a sharpening steel on a regular basis. Periodic sharpening by professionals will ensure that your knives have the proper angle and will help the blades maintain their edge longer.
Knives are sharpened by grinding against a hard rough surface, typically stone, or a soft surface with hard particles, such as sandpaper. Sandpaper is a form of paper where an abrasive material has been fixed to its surface.Sandpaper is part of the "coated abrasives" family of abrasive products. It is used to remove small amounts of material from surfaces, either to make them smoother , to remove a layer of material.
For finer sharpening, a leather razor strop, or strop, is often used. 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. Very sharp knives sharpen at about 10 degrees (which implies that the knife's edge is a 20-degree angle). Typical knives are sharpened at 15 degrees. Knives that require a tough edge (such as those that chop) sharpen at 20 degrees or more. For an extremely durable edge (such as a chisel or drawknife), blades can be sharpened to 30 degrees. In general, but not always, the harder the material to be cut the higher the angle of the edge.
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).