Aogami and Shirogami: Hitachi’s White and Blue Paper

Hitachi Steel Ltd. — or more precisely its knife division Yasuki Hagane, also known as YSS, Yasuki Specialty Steel — has supplied Japanese forges with steel in ingots for decades. The ingots arrive wrapped in paper, and different grades come in different colours of paper. White, blue, yellow.

That is where the names come from. Shirogami is “white paper,” Aogami is “blue,” Kigami is “yellow.” The English “Blue Paper Steel” means exactly the same thing: steel wrapped in blue paper. It has nothing to do with the colour of the metal, the shade of the edge, or a “bluish tint from the hardening.”

The colour of the paper is a way of not mixing up ingots in the warehouse. Everything else was added later by salespeople.

What is inside

Shirogami is carbon steel in the strict sense of the term: iron, carbon, a little manganese and silicon. The same category as 1095 and W1. The composition is very clean, and carbides are present only as small inclusions in the iron.

Aogami is the same steel with additions, above all tungsten and chromium. There isn’t much chromium: steel counts as stainless from 11% chromium, and blue paper falls short of that line. Hitachi honestly keeps both grades in the “not rust resistant” section.

The grades are numbered, and the lower the number, the higher the hardness ceiling.

• Aogami Super — 63–65 HRC
• Aogami #1 (Ao ko, Ao ichi ko) — 62–64 HRC
• Aogami #2 (Ao ni ko) — 61–63 HRC
• Shirogami #1 (Shiro ichi ko) — 60–64 HRC
• Shirogami #2 (Shiro ni ko) — 60–63 HRC
• Shirogami #3 — 58–62 HRC

Which one is sharper

The rule is short: Shirogami can be sharpened keener, Aogami stays sharp longer.

That follows from the composition, not from character. Clean carbon steel has no hard inclusions to resist abrasion: the edge comes out even and thin, it is easy to touch up and just as easy to dull. The tungsten and chromium in Aogami produce carbides — the edge dulls more slowly, but it is also harder to bring to a razor.

Hence the recommendations that are taken as self-evident in Japan. For traditional knives like the yanagiba and deba, Shirogami #1 is the usual answer: such a knife is touched up often and lightly, and its whole point is edge geometry. For more general work people take Aogami. It also costs more.

Hardness isn’t set by the grade alone

The same grade gives different hardness at different forges: the method of forging and tempering matters as much as the composition. Shirogami #1 is the textbook example. The steel isn’t especially tough, but it tolerates extremely aggressive hardening, and its hardness ends up wandering between 60 and 65 HRC depending on who did the heat treatment.

The practical conclusion is unpleasant: “White #1” stamped on a blade is a specification for the stock, not for the knife. What was done to that stock afterwards is known only to the forge. The grade, incidentally, is often written right on the blade, so working out what your knife is made of usually takes no expert analysis.

Aogami Super

This is a separate grade, not “blue, only better.” Along with a higher carbon content, it has more tungsten and chromium than Aogami #1, plus added molybdenum and vanadium. The result is a steel that is extremely hard and tough at the same time — the very combination that the whole of knife metallurgy exists for. On a short list of the best steels for a kitchen knife, Aogami Super sits next to ZDP-189.

Yellow paper

Kigami is the same Hitachi steel, only cheaper: #1 and #2 give 60–62 HRC, #3 gives 59–60. For the kitchen, #2 makes more sense, with its higher carbon content. A reasonable option for someone who wants to try carbon steel and doesn’t want to pay for the characters on the blade.

Rust

Aogami is known for being especially prone to rust. Not “may darken” — it rusts, if the knife is left wet after work.

The maintenance is not heroic, just daily: wash it, wipe it dry, wipe on a little unsalted food-grade fat, put it away. A cook who works with carbon steel does this automatically and doesn’t consider it an achievement. The difficulty isn’t the ritual, it is that at home the ritual is performed by someone other than the person who chose the knife in the shop.

Who needs this

Aogami and Shirogami were developed specifically for knives, and that is rare: most knife steels reached the kitchen from bearings, dies and aviation.

But it helps to know who actually cuts with them. In a real kitchen, in heat and damp, a cook doesn’t work with blue paper. He needs a knife he can drop, flood with water and leave unattended for ten minutes, and one he wouldn’t mind losing. Knives made of exotic Japanese carbon steel are a domestic luxury, and their purpose is to give the owner pleasure, not to earn a cook money.

This isn’t an argument against buying one. It is an argument against kidding yourself: with a Shirogami #1 you are buying not performance but a relationship with an object. Sometimes that is worth the money.