From Japan · Updated 2026-07-07
Blue steel vs white steel, and what “kasumi” really means
The Sakai cutlery association's own FAQ answers the two most-argued Japanese-knife questions with real numbers — including that at the same grade number, blue and white reach the same hardness.
Blue and white are Hitachi steel grades, and the number is carbon
“Blue” (Aogami, 青紙) and “white” (Shirogami, 白紙) aren't descriptions — they're Hitachi Metals' Yasugi Steel grade names. The Sakai Cutlery Federation's FAQ lays out the actual specs: white comes in #1–#3, blue in #1, #2, and Super, and the number is the carbon content — #1 is 1.2–1.4%, #2 is 1.0–1.2%, #3 is 0.8–0.9%.
The part that settles a lot of Reddit arguments: hardness tracks carbon content, so at the same grade number, blue and white reach the same hardness. Blue #2 is not inherently harder than White #2. What differs is something else entirely.
What blue steel actually adds (and why it costs more)
Per the same FAQ: very pure steel above 1% carbon is hard to heat-treat well, so white #1/#2 gets roughly 0.3–0.5% chromium and 1.5–2% tungsten added to improve hardenability — and that alloyed version is what's sold as blue. Aogami Super is a 1.4–1.5% carbon blue with extra tungsten plus about 0.4% molybdenum and vanadium.
Those alloying elements are expensive, which is why blue steel and blue-steel knives simply cost more. The familiar rule of thumb — blue holds an edge and heat-treats more forgivingly, white takes a keener, easier-to-refresh edge — follows from this chemistry, but the maker's own framing is hardenability and cost, not raw hardness. (As an aside, the same FAQ notes Japanese swords are a different material lineage entirely: tamahagane smelted from iron sand, not tool steel.)
“Kasumi” and “honkasumi” are shop labels, not a spec
The other question the FAQ quietly demolishes: kasumi grades. In Sakai usage a honyaki (本焼き, through-hardened mono-steel) knife is finished with a wooden-whetstone finish, while a clad/laminated knife (合わせ) gets the whitish kasumi finish from stone powder. So “kasumi” fundamentally signals a clad-construction knife, not a honyaki one.
But the ladder people obsess over — nami-kasumi, chū-kasumi, jō-kasumi, hon-kasumi (並霞・中霞・上霞・本霞) — is each shop's ranking of its own lineup, not an industry standard. The association is blunt about it: one shop's “chū-kasumi” can be another shop's “hon-kasumi.” If someone asks whether their knife is “really honkasumi,” the honest answer is that the word only means something relative to the shop that sold it.
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.