From Japan · Updated 2026-07-03
Nagura and flattening: stone maintenance the Japanese way
What nagura actually does (it's not soap), why dished stones ruin edges, and the flattening methods Shapton officially endorses — and warns against.
What a nagura is actually for
Per Suehiro and the specialist shop HATOGI屋, a nagura has two real jobs: raising an abrasive slurry (tojiru) that speeds up polishing on finishing stones, and dressing a glazed or clogged stone surface back to a fresh, cutting state. The counterintuitive part for newcomers: don't rinse the slurry away — it contains the abrasive doing the work.
Suehiro even matches its synthetic nagura to stone grits: the NGR-10 (#1000) is meant for #1000–#2000 stones, while the NGR-50 (#5000) serves #3000–#8000 finishing stones.
Sources: スエヒロ — 人造名倉シリーズ (Suehiro: synthetic nagura series) · HATOGI屋 — 名倉とは (HATOGI-ya: what is a nagura?)
Flattening: the pencil-grid check
Stones dish in the middle with use, and a dished stone rounds the very edge you are trying to sharpen. The standard Japanese verification is the pencil grid: draw a grid across the stone, flatten, and stop when every line has vanished.
Shapton sells dedicated correction tools for this (the Kuuboo 'aircraft carrier' and glass lapping plates) and — notably — its manual advises against flattening on sandpaper. Rubbing two same-grit stones together works in a pinch; a coarse diamond plate like the Atoma #140 is the common professional pick.
Sources: シャプトン — FAQ (Shapton official FAQ) · 研匠光三郎 — 砥石の面直し (Mitsusaburo, professional sharpener: flattening)
These summaries extract facts from Japanese primary sources and rewrite them in our own words, with attribution. For the full detail (and the original nuance), follow the source links — they deserve the traffic.