It's a STEEL - Knives & Sharpeners
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The art of knife sharpening

An art, not a chore: what goes into a good edge, what can go wrong, and what cheap sharpeners do to a blade.

Sharpening looks simple: rub the edge on something hard until it cuts. It is not. A good edge is two flat bevels meeting cleanly at the right angle for the knife, with the burr taken off and no more steel removed than the edge needs. Getting there takes an understanding of the steel, a steady angle and practice, and plenty can go wrong along the way.

What goes into a good edge

Angle and apex

An edge is two bevels that meet at an apex. Sharpening means removing steel from each bevel until they meet along the whole length of the blade, then refining that apex with finer abrasives until it is smooth. If the angle drifts from stroke to stroke, the bevels are rounded rather than flat; if they never quite meet, the knife feels sharp in places and dull in others.

The burr

As the bevels meet, the thin steel at the apex bends away from the stone and forms a burr, a fine lip along the edge. The burr tells the sharpener that the bevels have met, and then it has to go: left on, it makes the edge less keen.

The right angle for the knife

There is no single correct angle. A thin kitchen slicer is happiest at a low angle, where it glides through food; a hunting or bushcraft knife wants a more robust edge that will not chip on bone or wood. Makers' own angles run from about 9 degrees a side, the low end of the range Zwilling gives for its Miyabi knives, to the 20 degrees a side at which our own handmade knives leave the bench. We choose the angle for the knife and the work it does.

The steel decides the stone

Steels differ in how they sharpen. Stainless steels such as 440C carry large chromium carbides, often more than 10 microns across; simple carbon steels such as 1095 have small iron carbides, usually under 2 microns, and chromium carbides are the harder of the two. In steels rich in vanadium carbide, the carbides are harder than aluminium oxide, the most common abrasive, while diamond and CBN are harder than any of them. The stone has to suit the steel.

What can go wrong

Cheap sharpeners can do more harm than good

Many cheap pull-through sharpeners have a V of tungsten-carbide blades that scrape steel off the edge instead of grinding it. When Science of Sharp used one on a blade of S110V, a hard steel rich in carbides, and examined the edge under an electron microscope, the pull-through had deformed the apex, left cracks and voids around the carbides, and taken nearly 0.2 mm off the height of the blade. America's Test Kitchen found that another, though quick, left shreds of steel along the cutting edge, and a lecturer in materials science at MIT told them that carbide and ceramic tend to grab at the blade and are more prone to leaving the edge rough.

These gadgets also sharpen at one fixed angle, whatever the knife. Electric sharpeners can take a lot of steel off: Global Knives Australia advises against them for that reason, and in one of America's Test Kitchen's tests an electric model heated the blade and ground a dip into its edge. Not every cheap sharpener is useless, and America's Test Kitchen says a good electric sharpener removes no more steel than a whetstone. But the fixed angle, the speed at which some of them take steel off and the damage the worst of them do are why a good knife is better off on a stone or a guided system.

Heat that ruins the temper

A knife's hardness comes from its heat treatment: the steel is hardened, then tempered at a set temperature, around 200 °C for many knife steels. Knife edges are easily overheated on powered grinders, because there is so little steel at the edge, and if the steel goes past its tempering temperature it softens. In measurements by Roman Landes, reported by Knife Steel Nerds, edges sharpened on a belt grinder with 220-grit belts were up to 5 points softer on the Rockwell C scale. Hand sharpening and water-cooled sharpening are the best ways to keep an edge from overheating.

Too much steel taken off

Every sharpening removes a little steel, and Victorinox points out that this gradually thins the blade. Sharpen more often or harder than the edge needs, and a knife wears out sooner than it should. Careless sharpening can also wear a hollow into the edge near the handle, and round off the tip.

Scratches that start cracks

Scratches left in the edge by sharpening can act as starting points for cracks, and cracks are where chips begin. Finishing with finer grits leaves finer scratches.

Two methods: the Wicked Edge and stones by hand

The Wicked Edge WE60 precision sharpener
The Wicked Edge holds the blade in a vice and works both sides at a fixed angle: the same angle for every stroke, and the same angle next time.

We sharpen on the Wicked Edge precision system. Because its guide rods set the angle, not the hand, we can work both sides of the blade evenly through the grits and onto the strops.

Not every knife goes on the Wicked Edge. Depending on the type of steel and the type of knife, some call for traditional sharpening by hand on stones. Wicked Edge itself says, for instance, that some daggers and stilettos may not be possible to clamp, and that on small knives the vice leaves too little clearance to reach the lower angles.

A woman sitting in the forest, working on the edge of a knife

The short version

Sharpening is an art, not a chore to be rushed. It rewards knowledge, patience and the right tools, and it punishes shortcuts. Whoever does the sharpening, the rules are the same: keep the angle, take the burr off, keep the edge cool, and take off no more steel than the edge needs.

If you would rather have it done, we sharpen knives in Potchefstroom from R50.

Sources and further reading

Knives and sharpening equipment are dangerous if used carelessly. Follow the maker's instructions, keep fingers behind the edge, and if in doubt bring the knife to us. See our terms.

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