Making a Masterpiece
Removing impurities
The best pieces of tamahagane are sent to a swordsmith, who heats,
hammers, and folds the steel repeatedly in order to further combine
the iron and carbon, and to draw out any remaining undissolved
impurities, or "slag." This step is as vital as it is tedious,
because if other elements besides iron and carbon remain in the
resulting sword, they will weaken it. Once the skilled smith has
removed all of the slag, he can judge the carbon concentration of
the tamahagane by the degree to which it yields to his constant
pounding. One expert has likened eliminating slag from steel to
squeezing liquid from a very hard sponge.