Reading the remains of an iron furnace
On the Black Sea coast of Turkey, deposits of ancient slag record technological innovation from 3,000 years ago. This glassy, rock-like byproduct of metal production is all that remains of early Iron Age bloomery furnaces. Scientific analysis shows these structures, likely built of clay and stone, reached internal temperatures of 1,100–1,300°C. This was hot enough to smelt iron, but below the metal's true melting point of 1,538°C.
The process worked through solid-state reduction. Iron ore and charcoal were layered inside the shaft-like furnace. Bellows pumped forced air through clay pipes called tuyeres, raising the temperature. The burning charcoal produced carbon monoxide, which reacted with the iron oxides in the ore, chemically stripping the oxygen away to leave metallic iron. The result was not a liquid metal but a bloom: a spongy, porous mass of iron mixed with slag. This bloom would then be removed while still hot and hammered by smiths to consolidate the metal and expel the remaining molten slag.