Steel production accounts for around seven per cent of humanity’s greenhouse-gas emissions. There are two reasons for this startling fact. First, steel is made using metallurgic methods that our Iron Age forebears would find familiar; second, it is part of seemingly everything, including buildings, bridges, fridges, planes, trains, and automobiles. According to some estimates, global demand for steel will nearly double by 2050. Green steel, therefore, is urgently needed if we’re to confront climate change.
To understand steel, you need to think at the level of high-school chemistry—even the chemistry you learned on the first day will suffice. Basically, steel is iron, with a little carbon added in to increase strength: tiny carbon atoms nestle between the larger iron ones, making the steel denser and more ductile. In a sense, iron isn’t so hard to find—it makes up five per cent of the earth’s crust, by weight—but metals in rock are mixed with other elements. You must get them out, in pure form, before you can build that sword or Eiffel Tower. In this respect, iron presents a particular challenge: iron atoms bind tightly with oxygen atoms, like complementary pieces in a jigsaw puzzle. Two irons and three oxygens make ferric oxide, or Fe2O3—a complete picture that’s hard to pull apart. Ferric oxide forms easily—so easily that, in the presence of water, naked iron will stick to oxygen in the air, creating rust.
For most of human history, therefore, the problem of iron extraction was unsolvable. Five thousand years ago, the ancient Egyptians made beads out of iron—but they got their metal from meteorites, in which it had already been split from oxygen by some unknown extraterrestrial process. Another thousand years would elapse before making usable iron became possible, through a process called reduction. Sometime around 2000 B.C.E., it was discovered, possibly by accident, that iron-heavy rock, or ore, became malleable when it was heated over charcoal fires. Today, we can explain why this happens: at high enough temperatures, iron atoms loosen their grip on oxygen atoms. The oxygen binds to the carbon in the charcoal, forming CO2, which flies off into the air. What’s left behind is purified, or “reduced,” iron. The process of reduction allowed the Iron Age to begin.