Other than Western-style kitchen knives generally made in Western countries and Honyaki knives in Japan, most of Japanese kitchen knives are often composite knives forged with Hagane(steel) steel and Jigane(soft iron). This composite structure is ideal for knives that will be sharpened on a whetstone.

But one specific problem – which does not occur with a double-edged knife – occurs with only thin & long sashimi knives like Yanagiba, and thin & wide Usuba knives, all of which have single-edged thin blade.

Figure (a) is a concept drawing of a Usuba knife. By heat treating this, the hagane becomes martensitized, increasing in length as it cures. Thus, as in Figure (b), it’s typical for the knives to warp upwards when viewed horizontally. To correct this, the knife is placed on an anvil as in Figure (c) and the Jigane is struck with a hammer, extending it and straightening the knife.

          Figure 1. Why Thin-Bladed Composite Usuba Knives Warp

After the product is ready, but as time passes after production the steel side shrinks as Figure (d), warping in the opposite direction as shown in Figure (b). Heat-treated steel cannot be lengthened and there is no way to straighten out and fix the knife.

Whether or not the Forging process that starts the mounting of steel is done carefully, is said to greatly affect the deformation of the product. Fundamentally, the reason behind this warping is the pasting of two materials, a soft material in Jigane and a hard material in steel. This being the case, the solution would be to make a knife out of a single piece of metal, which was the Honyaki.

[Referenced from Hamonoarekore by Kato and Asakura(2013)]

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