Every edge dulls. A knife's edge is a thin strip of steel; in use, it bends over to one side and wears down. Honing and sharpening fix those two problems.
Key idea
Honing (on a steel or ceramic rod) pushes a bent edge back into line; it removes little or no metal. Hone often, even every day. Sharpening (on a whetstone or a sharpener) grinds away metal to form a new edge. Sharpen when honing no longer brings the edge back, perhaps every few weeks to months at home. The angle matters most: many Western knives are sharpened at about 20 degrees per side; many Japanese knives at about 15 or less.
Why it works
Holding an angle by feel is hard at first. Simple trigonometry helps: if a blade is 1½ inches tall and you want 15 degrees, the spine should sit about $1.5 \times \sin 15^\circ \approx 0.39$ inch above the stone, about the height of a stack of five or six U.S. quarters.
Worked example: How high?
A chef's knife blade is 2 inches tall. How high should the spine be for 20 degrees?
- $2 \times \sin 20^\circ \approx 2 \times 0.342 = 0.68$ inch.
Answer: About 0.68 inch.
Worked example: Total angle
A knife is sharpened at 15 degrees per side. What is the total (inclusive) angle of the edge?
- Two sides: $15 + 15 = 30$.
Answer: 30 degrees.
Common mistake
Thinking the steel sharpens. A knife that still drags after honing has worn out its edge; it needs a stone.
Practice at home
- Test sharpness on a sheet of paper held by one corner: a sharp knife slices cleanly; a dull one tears or catches. Hone and test again.
- Hone your chef's knife with light, even strokes, alternating sides, five or six on each. Lighter is better than harder.
The science behind it
- Sine, cosine and tangent: the trigonometry behind the spine height
In Meal Prep
- Knives: honing, sharpening and storing: the home guide