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The backing material in Japanese blades is often a source of curiosity. Why are the blades made this way, with a thin layer of hard tool steel welded by forge (or bonded where the tool steel cannot be welded) to a thicker main body of soft steel or wrought iron?

Historically, steel suitable for blade edges was scarce and very expensive. Using only enough of this valuable material to accomplish the task made good economic sense. Over time and as technology progressed, steel suitable for blade edges became more readily available and less costly, to a point where making the entire blade by stamping to shape, material removal or die forging from the solid in this once expensive material became less costly than the ages old method of using only ‘good stuff’ for the edge material.  

Tools in most of the Western world took on this ‘all tool steel’ method of blade making, whereas the Japanese tool makers stuck with the old method of laminating their blades.

Why?

The answer is actually quite simple, and is at the heart of what Japanese tools are all about.

“Japanese blades, known in general as hamono, should be pleasant and efficient to use at all times”.

Remaining sharp for a long time is not enough, although Japanese blades are renowned and revered for this. They should also be easy to keep in good working order which extends to sharpening and general care. To that end, making the bulk of the blade body from softer, more easily abraded material and keeping the hard, more difficult to abrade steel to a minimum naturally allows the blade to be more easily sharpened.

The backing material used varies, but generally falls into three distinct types.

(Left to right, watetsu, 19th century British wrought iron, modern production low carbon steel <background>)

#1- Indigenous iron, watetsu, often used in hinges, old laminated blades, etc. This is produced as a by product during the smelting of traditional Japanese edge steel.  

#2- Reclaimed wrought iron, imported primarily from England in the 19th century and used for ship’s chains, bridges, railways tracks and building girders before modern steel was available. It offers a measure of rust resistance and at the forming stages of the blade, it is easier to shape by forging and allows easier forge welding. But because wrought iron is no longer made, the base material is relatively expensive and need to be worked into an appropriate shape.

(Mr. Uozumi forging old wrought iron into flat bar.)

#3- Softer, low carbon steel, available in a wide range of sizes and types, is relatively inexpensive but is a little less readily abraded and has virtually no resistance to corrosion.

By using these softer metals to make up the bulk of the blade, the blade is far easier to sharpen especially when working on the bevel side.

What is kamaji?

Often the wrought iron used with Japanese blades is called ‘kamaji’, and broken down the word translates as ‘pot base (metal)’. Interestingly, this word is not often used in Japan, where the term ‘jigane’ (base/ground metal) is more commonly used, although kamaji  is also an accepted and appropriate word.

The origins of the word kamaji are not clear, and there are two potentially correct theories. One is that the original process for making iron and steel was known as ‘puddling’ where the molten, high carbon cast iron was melted in a large pot and stirred continuously until the iron was decarburized (excess carbon burned off). The other theory related to one source of wrought iron being old boilers for heating and power generation, these being thoroughly decarburized during their service life and being ideally suited for laminated blades.

We use the term kamaji as it is a generally accepted term in the English speaking world for reclaimed wrought iron used in Japanese blades.

There is yet another reason why a backing of kamaji is used in Japanese kanna blades. By making the blade thick and heavy, valuable mass is added to it which can reduce vibration. As the kamaji is very soft and possesses very different characteristics to the hard hagane (edge steel) it acts as an additional damper for vibration, effectively eliminating the ‘chatter’ that can occur with thinner bladed planes.  

An example of wrought iron jigane in a Tsunesaburo blade. Note the small inclusions of slag and the thin layer of hagane.