Knife sharpening

[2] The smaller the angle between the blade and the stone, the sharper the knife will be, but the less side force is needed to bend the edge over or chip it off.

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.

The Western-style kitchen knives are generally in the range of 52–58 on the Rockwell scale, which denotes the relative hardness of a material.

Knife sharpening proceeds in several stages, in order from coarsest (most destructive) to finest (most delicate).

Blades may also be damaged by being corroded by acid (as when cutting lemons or tomatoes) or by high temperatures and corrosive chemicals in a dishwasher.

If a knife is used as a scraper, a pry-bar, or encounters hard particles in softer materials, there may be a sideways load at the tip, causing bending damage.

Visually, a very sharp knife has an edge that is too small to see with the eye; it may even be hard or impossible to focus in a microscope.

Specialized sticks exist to check bite, though one can also use a soft ballpoint pen, such as the common white Bic Stic.

For kitchen knives, various vegetables may be used to check bite, notably carrots, tomatoes, or cucumbers.

A blade polished to a finer edge (greater than 3000 grit or so), although technically sharper, can have a harder time initiating cuts on soft vegetables or fruits with a skin, such as tomatoes or bell peppers, due to the lack of micro-serrations on the apex of the knife.

These micro-serrations can decrease the effort needed to initiate cuts by tearing the food on a microscopic level.

A sharpening steel is a type of hardened cylindrical rod used similarly to honing stones.

Because steels have a small diameter they exert high local pressure, and therefore affect the knife metal when used with very little force.

This is often done with a leather strap, either clean or impregnated with abrasive compounds (e.g. chromium(III) oxide or diamond), but can be done on paper, cardstock, cloth, or even bare skin in a pinch.

This is important, since repeated sharpening will end up thickening the primary bevel and it will be harder for the knife to pass through material.

Knife sharpener in Kabul , Afghanistan (1961)
The Knife Grinder by Massimiliano Soldani (c.1700), Albertinum, Dresden
A railway camp cook sharpens a knife blade on a stone wheel, 1927
Electric knife sharpener
Two professional knife sharpeners in streets of Kathmandu
Knife sharpening with 14th century methods.
Edge of a knife after sharpening and stropping. Although this edge is sharp enough to bite a thumbnail, cut paper smoothly, or shave arm hair, the microscope plainly shows an edge which reflects light back into the lens. A truly sharp edge is too thin to reflect significant light.
Knife sharpening shop in Delhi
A clamp-on knife sharpener. The rod guides the sharpening stone to maintain a consistent angle. The angle can be adjusted by moving the guide posts up or down. This sharpener uses a diamond dust coated stone to remove metal from the knife blade.
A leather strop on a wooden handle, top. The leather is coated with green chromium oxide polishing compound. Bottom, one pattern of butcher's steel for knife edge maintenance.
Left side shows the approximate angle for sharpening. Right side shows the approximate angle for thinning.