Ohiro Karasu Whetstone 6" x 1"

Ohira Karasu whetstones are used for finishing knives on sharpening systems such as Hapstone, TSPROF, and other systems compatible with the Edge Pro form factor. The stones are custom-cut from bench stones sourced from a reliable Japanese seller, then lapped, mounted on high-quality aluminum blanks, and laser-engraved. All operations are performed in-house at Gritomatic.

Name breakdown:

• 大平 (Ohira) — a western Kyoto mine (Nishi Mono) located near Mt. Atago. One of the few mines still occasionally active. Full designation: 大平山合砥石 (Ohira-kōzan gō-toishi = Ohira mine finishing/polishing stone).
• 烏 (Karasu) — visual/color attribute meaning "crow": a dark blue-black-and-white pattern within the stone resembling crows in flight against a darkening sky. Karasu indicates origin from the Aisa layer (合さ) of the Hon-Kuchi Naori strata. Karasu stones almost exclusively come from the Aisa layer, which sits at the boundary between Tomae and Namito. The Aisa layer has only four seams and is known for high hardness and abrasive force, but also for a higher probability of impurities.

Mining region: Ohira mine (大平山), western Kyoto (Nishi Mono), near Mt. Atago. One of the few Kyoto whetstone mines that is still occasionally active (along with Maruoyama). Stock is also available from material stored by major wholesalers (Tanaka, Imanishi, Hatanaka, Asano). The Hon-Kuchi Naori seam at Ohira connects westward to Yaginoshima and eastward toward Atagoyama, Ozuku, Okudo, and Nakayama.

Grit rating (JIS equivalent): Ohira stones in general are described as approximately 9,000–10,000 JIS. Karasu/Aisa origin implies harder, denser material which tends toward the upper end of the fineness range.

LV Hardness: LV 4–5 (hard to very hard). Ohira Karasu is documented as "comparatively hard" and requires nagura. The Aisa layer from which Karasu originates is known for hardness. Individual specimens on the market have been noted at hardness 3.5–5 depending on the specific seam, but Karasu from Aisa is typically on the harder end. Not recommended for beginners.

Recommended progression: After a synthetic 6,000–8,000 JIS stone. Intended for final finishing of high-quality knives. Nagura is required.

Composition: Siliceous shale, Aisa layer (合さ) of the Hon-Kuchi Naori geological formation. The Aisa layer sits below Tomae and above Namito, with only four seams. Karasu patterning (dark blue-black areas) is caused by carbonaceous or organic material concentrated during sediment compression. Aisa stones are noted for high abrasive force but also for a higher incidence of impurities compared to Tomae or Namito. The Ohira mine specifically produces a rare variant with Karasu layers distributed from the surface through to the bottom of the stone.

Steels to sharpen: Well suited for hammered blades and high-carbon Japanese steels and traditionally forged blades.

Usage recommendations:

• No soaking required — surface wetting only
• Nagura is required — the stone is hard and does not raise slurry easily on its own; use Mikawa Shiro Nagura (Botan or Mejiro) to develop working slurry
• Not recommended for beginners; requires experience and controlled pressure
• Maintain slurry on the surface throughout sharpening; do not allow the stone to run dry