Exotic: Yoshikazu Ikeda Blue #1 Mizu-Honyaki K-tip Yanagiba 300mm with Blood Groove Pau Rosa Set

Yes, carbon steels such as Aogami (Blue) and Shirogami (White) react with food, moisture, and acids, so left wet they can spot or rust. With normal use they develop a grey, blue and brown patina, and that patina is expected on a carbon knife rather than damage.

Aogami has only a little chromium, far below what a stainless steel needs, so water left sitting on the blade will spot it. A patina is a stable oxide layer that slows rust down; orange or red specks are rust.

Care is simple once it becomes habit:

• Wipe the blade as you go, especially after onion, citrus or tomato.
• When you finish, wash it in warm water with a little soap and dry it straight away with a towel. Don't let it air dry or leave it in the sink or the dish rack, and never put it in the dishwasher.
• Store it dry: on a magnetic strip, in a saya or in a block in a dry spot.
• If it's going to sit unused for a while, wipe on a thin film of food-safe mineral oil before storing it.

If you do find rust, lift light orange spots with a rust eraser and a little water, then dry and oil the blade. We stock the CKK and Naniwa rust erasers, and the Koyo Sunlite Polishing Paste (Blue for Steel) lifts light surface rust or refreshes a finish. Rust on the edge itself comes off the next time you sharpen. Deep pitting needs a professional sharpener. Avoid steel wool and harsh scouring on a polished or kurouchi finish, because they scratch it.

On a stainless-clad knife only the thin line of carbon steel along the edge can rust; the cladding stays clean. A carbon-clad or honyaki blade can rust over its whole face and needs the same care all over.

Carbon steel rewards this small amount of attention with a keener edge and easier sharpening than most stainless.

Proterial, which makes both, describes each the same way: an alloy steel with tungsten and chromium added to improve heat-treatment properties and wear resistance, hardening to over 60 HRC.

Its figures give Blue 1 (青紙1号) more carbon and tungsten, 1.25 to 1.35% carbon and 1.50 to 2.00% tungsten against Blue 2's 1.05 to 1.15% and 1.00 to 1.50%, with chromium of 0.30 to 0.50% against 0.20 to 0.50%.

Mizu honyaki (水本焼) is water-quenched honyaki, and Yoshikazu Ikeda's own page says it is one of the professional Japanese knives he mainly makes. The Ikeda honyaki we list under titles that say Mizu-Honyaki are water-quenched.

On hardening, Ikeda says on the City of Sakai film site that the fire's temperature is everything and that he judges it by the colour of the flame. Honyaki can run about 1 to 2 HRC harder than comparable non-honyaki knives, and how much varies by maker.

Yoshikazu Ikeda (池田美和) of Ikeda Tanrenjo (池田鍛錬所) in Sakai, who began making knives under his father, a blacksmith, in 1967 and went independent as Ikeda Tanrenjo in 1983.

The City of Sakai's register lists him under forging (鍛冶), certified in 1988.

Yes. His profile page records his certification as a traditional craftsman by Japan's Minister of Economy, Trade and Industry in 1988.

The same page dates his start in knives to 1967, working under his father, a blacksmith, and records the workshop's independence as Ikeda Tanrenjo in 1983.