One of the oldest and most sophisticated methods was that of the Haya people. They're an African tribe in what is Tanzania today. The Hayas produced high-grade carbon steel for about 2000 years. The Hayas made their steel in a kiln shaped like a truncated upside-down cone about five feet high. They made both the cone and the bed below it from the clay of termite mounds.
Termite clay makes a fine refractory material.
The Hayas filled the bed of the kiln with charred swamp reeds. They packed a mixture of charcoal and iron ore above the charred reeds. Before they loaded iron ore into the kiln, they roasted it to raise its carbon content. The key to the Haya iron process was a high operating temperature. Eight men, seated around the base of the kiln, pumped air in with hand bellows.