As a starting point in understanding Japanese tools, it’s probably best to examine how they are made. And even before looking at how they are made, it’s good to look at steel, and how steel works. In its simplest form, steel is a combination of iron and carbon. Pure iron consists of atoms of iron that arrange themselves naturally into a cubic structure. This cubic structure is not rigid, however. The cubic structure can move, which is why pure iron is malleable, and can be hammered and shaped into forms that are useful for us.
Carbon happens to be just the right size so that if it is able to get into the gaps of the cubic structure of iron, it can stiffen the structure so that its ability to move is greatly restrained. The addition of just 0.5-2 percent of carbon to iron can cause this to happen, creating carbides that are small and hard. These carbides are suspended in a matrix that ultimately makes steel. This is why steel is much harder than pure iron. This hardness is what allows steel to take an edge, and have that edge hold up to abrasive forces, such as being pushed through a piece of wood.
In terms of history, this was first developed in China around 500 B.C., and spread throughout the rest of Asia from there. The oldest examples of ironware in Japan appear around 300 B.C. In Japan, the raw materials to make steel came from iron-rich sand deposits, and charcoal, which supplied both a heat source and carbon.
The Japanese method of making steel starts with a clay furnace called a tatara. After constructing the tatara, iron-rich sand and charcoal are loaded into it. The charcoal is set on fire, and the fire is tended for three days. At the end of that process, chunks of steel, looking like miniature meteorites, can be retrieved from the tatara. These pieces of steel vary in their properties. Some are softer, and can be used like wrought iron or cast iron. But some of the steel pieces are very hard, and can be used for tool steel, or for swordmaking. This process of making steel was fairly well standardized by the 8th-9th century in Japan.
–Wilbur Pan, excerpt from “Forging Traditions: The Common Ancestry of Japanese & Western Edge Tools,” in Issue Six