刃こぼれ

A chipped knife: what caused it, and what actually repairs it

A chip is not the end of the knife. It is metal that has to be removed — the edge is reshaped below the damage, on a coarse stone, and then rebuilt.

The repair is subtraction

Wikipedia’s sharpening-stone article gives the repair directly: a coarser grit is used to remove larger amounts of metal more rapidly, such as when forming an edge or restoring a damaged edge, and a relatively coarse stone is used to reshape a damaged blade edge, which is then refined by finer grit stones. Synthetic grits run from about 120 at the coarse end upward. That is the whole procedure — grind the edge back past the deepest point of the chip, then work up through the grits.

Why thin edges chip

The cause is in the same literature and it is geometric rather than a fault. The smaller the angle between 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. A Japanese edge is thin because the steel is hard enough to hold a thin edge — and that thinness is the same property that makes lateral force dangerous to it. Twisting through something, prying, and hitting bone are all side force.

What it costs, and when to stop

Two consequences worth stating plainly. Repair costs blade height: the metal removed to reach past the chip is gone, and a knife repaired several times is measurably narrower than it began. And a chip is worth fixing rather than ignoring, because the edge on either side of it is unsupported — but a very deep chip is a job for a coarse stone and patience, not for the finishing stone you sharpen with.

Where to look

If you are buying a stone for this rather than for maintenance, the coarse end is the part that matters — the grit page below explains why the number on it is not comparable between makers. And if the chipping keeps happening, the cause is more likely to be what the edge is meeting than the knife: the board guide and the hardness guide are the two worth reading next.

No shop link here. Whether a chip is worth grinding out yourself or taking to a sharpener depends on how deep it is and how much you value the blade, and that is not a judgement a page can make for you.