Making a Masterpiece
Dissolving carbon
While fired at high temperatures, the tamahagane is never allowed to
reach a molten state. This is to ensure that just the right amount
of carbon will dissolve into the steel, and that the percentage of
carbon will vary throughout the tamahagane (between 0.5 and about
1.5 percent). Katana-makers use two types of tamahagane:
high-carbon, which is very hard and allows for a razor-sharp edge,
and low-carbon, which is very tough and allows for shock absorption.
A sword composed simply of one kind of steel or the other would
either dull too quickly or be too brittle. On the third night of
smelting, when the tatara masters break open the clay furnace
to expose the tamahagane, they use the degree of ease with which the
pieces of newly made steel break apart to discern their carbon
content.