Sukenari Hairline AS Gyu-hiki 240mm, Ebony Spacer Handle

Yes, carbon steels such as Aogami (Blue) and Shirogami (White) are not stainless steels, so left wet they can spot or rust.

Care is simple once it becomes habit. Wipe the blade as you go, especially after acidic food like citrus, tomato or onion. When you finish, wash it in warm water with a little soap and dry it straight away with a towel. Do not let it air dry, leave it in the sink or the dish rack, or put it in the dishwasher. Store it dry: on a magnetic strip, in a saya or in a block in a dry spot.

If the knife will sit unused for a while, wipe on a thin layer of oil before storing. In our video James shows that mineral oil can stand in for camellia (tsubaki) oil, and he picks one labelled medical grade; we list the Kitchen Knife / Cutting Board Mineral Oil Medical Grade for this. If you find light rust, lift it with a rust eraser and a little water, then dry and oil the blade; we list the CKK and Naniwa rust erasers.

Aogami Super, also called Blue Super, is a knife steel made by Proterial. Proterial describes it as an alloy steel with tungsten, chromium and vanadium, to improve heat-treatment properties and wear resistance. In its Japanese materials, Proterial calls its Aogami (青紙) steels high-carbon steel (高炭素鋼). Its own figures put carbon at 1.40 to 1.50%, tungsten at 2.00 to 2.50%, vanadium at 0.30 to 0.50% and chromium at 0.30 to 0.50%.

Proterial gives it a hardening hardness of 60 HRC or more. The uses it lists for the grade are high quality planes, other tipped tools, kitchen knives and sickles. Proterial's hardening note calls for quenching in water, with oil as the alternative.

With chromium well under the 10.5% the Japan Stainless Steel Association sets for stainless, it is not a stainless steel. Wipe it dry after use, do not leave it wet or in the sink, and do not leave acidic food on it.

Proterial lists its Aogami (Blue) steels as alloy steels in its English steel table and calls them high-carbon steel (高炭素鋼) in its Japanese materials; they are not stainless, so water left sitting on the blade will spot it. Each Aogami grade lists only a little chromium, between 0.20 and 0.50% depending on the grade, and that is not enough to make the steel stainless.

Dry the blade as soon as you finish cooking and after washing, and store it somewhere dry. Lift any rust early.

Will a carbon steel knife rust? covers rust removal and everyday care.

The difference is in Proterial's own figures: Aogami Super has 1.40 to 1.50% carbon, 2.00 to 2.50% tungsten, 0.30 to 0.50% chromium and 0.30 to 0.50% vanadium, against Aogami 2 (Blue 2) at 1.05 to 1.15% carbon, 1.00 to 1.50% tungsten and 0.20 to 0.50% chromium.

Proterial describes both as alloy steels with tungsten and chromium to improve heat-treatment properties and wear resistance, and also names vanadium for Aogami Super. In its Japanese materials, Proterial calls its Aogami (青紙) steels high-carbon steel (高炭素鋼). Both pages list hardening hardness over 60 HRC, and Proterial lists Aogami Super for high-quality planes, other tipped tools, kitchen knives and sickles.