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Blade sharpness can be checked in multiple ways.

Visually, a very sharp knife has an edge that is too small to see with the eye; it may even be hard to focus in a microscope. The shape near the edge can be highlighted by rotating the knife and watching changes in reflection. Nicks and rolled edges can also be seen, as the rolled edge provides a reflective surface, while a properly straightened edge will be invisible when viewed head-on.

By touch, a blade can be checked by running a thumb across the blade (perpendicular to the edge – not along, which will cut it). A sharp blade will have a distinct edge, like a corner, and may sing slightly from vibration, while a dull blade will have a round edge and the thumb will slip over it.

A blade's sharpness may be tested by checking if it "bites"—begins to cut by being drawn across an object without pressure. Specialized sticks exist to check bite, though one can also use a soft ballpoint pen, such as the common white Bic Stic. A thumbnail may be used[2] at the risk of a cut, or the edge of a sheet of paper. For kitchen knives, various vegetables may be used to check bite, notably carrots, tomatoes, or cucumbers. In testing in this way, any nicks are felt as obstacles.

"Biting" sharpness is considered ideal for kitchen knives, but sharper blades are desired for shaving and surgical scalpels, which must cut without side-to-side slicing of the blade, and duller but tougher blades are more suitable for chiseling and chopping wood.

For testing the sharpness of a straight razor, a traditional though dangerous test is to place a moistened thumb on the edge, and feel if it catches.[3] If a thumb is actually drawn along or across a properly sharpened straight razor it will cut into the skin, drawing blood.

more information like this can be found on Wikipedia.