This blade has undergone a traditional Nugui treatment (拭い)—a polishing method that involves the application of a specially prepared paste made from finely ground iron oxide (such as magnetite or hematite) mixed with oil (commonly clove or sesame oil). The paste is hand-rubbed into the blade’s surface, typically over the ji (blade body), to subtly adjust the tone, colour, and contrast of the steel.

Nugui is used to enhance the visibility of grain patterns (jihada), deepen the background tone, and create contrast against the hamon or other polished elements of the blade. Depending on the type of iron oxide used, the resulting colour can range from bluish-black to reddish-brown or even warm bronze hues.

Unlike chemical etching or artificial colouring, Nugui is a natural, non-destructive finish used in traditional togishi (sword polishing) and high-end Honyaki kitchen knife finishing. It brings out the intrinsic beauty of the steel and reflects the polisher’s judgement and aesthetic intent.

This level of finishing is typically found only on collector-grade or artisan-crafted blades, where steel structure, not just edge performance, is part of the knife’s visual identity.