Iron Oxide in it’s Hematite Mineral form
By Rose Schreiber
Across the world, on every continent, there are ancient rocks striped red and ashy grey, marked by alternating bands of iron oxide and silica. Iron, silicon, oxygen: by far, the three most abundant elements that make up this planet. Taken together, these elements account for 80% of Earth’s chemical composition. The striped rocks are old—some of Earth’s oldest—and they were formed, layer by sedimentary layer, as much as 3.7 billion years ago. For roughly one and a half billion years, these bands accumulated: the iron that was dissolved in seawater precipitated out, settling on the ocean floor. Then, on a geologic timescale, seemingly overnight, the markings disappeared. What happened? These banded iron formations—formations that supply most of iron ore mining today—are the likely record of an utterly transformational planetary event: the oxygenation of Earth’s atmosphere. Discrete bands of iron oxide could only have formed in the early, anoxic days of Earth’s existence, when free oxygen was scarce—their disappearance marks the biological entrenchment of photosynthesis and a profound chemical revolution on the planet we call home.
Geologically ancient and abundant, yes, and ancient and abundant for us, too. Human beings have been mining iron ore, all over the world, for tens of thousands of years. Hematite, the mineral we crush to form red iron oxide, is found at some of our earliest archeological sites. As body pigment, as cave paint, hematite has left some of the oldest known records of symbolic human behavior—as far back as 164,000 years. Use of iron in ceramics—as both accidental and deliberative colorant in our clays, slips, and glazes—is young by comparison. Iron ore metallurgy, a mere infant.
The images in the caves at Lascaux were painted roughly 17,000 years ago, with pigments including Iron Oxide in its Hematite form
These days, the red iron oxide in our ceramic studios—when it is not synthetic—is mined in Spain: the Santa Rosa mine just west of Zaragoza, in Tierga. This particular mine has been active for over 100 years. By now, 200 meters deep, the mine consists of a rhizomatic network of tunnels spanning 37 kilometers, over 20 miles. It is a room-and-pillars system, where the underground cavities are upheld by ruddy pillars of untouched rock. The hematite from Santa Rosa is not banded, rather, it is remarkably pure. Ore from this corner of the globe provides the bulk of iron oxide used in pigments around the world.
Fe-2-O-3, such a vibrant powder. Hematite, where hema– means blood, and little wonder: the very entrails, the very innermost place, the very core of Earth is iron. Think: wine-red, rust-red, crimson. Think: maroon, chestnut, scarlet, saffron. Versatile hue, shapeshifter, begetter of watery greens, arctic blues, syrupy yellows, honey, amber, gold, the richest striations of red-brown-black. It is our most common colorant, adroit and changeful. Of our colorants, only iron and copper are responsive to reduction. On its own, the iron-oxygen bond begins to decompose at cone 4; reduction allows that process to happen sooner, so that oxygen can travel freely through open and porous bodies. In oxidation, in high-iron glazes—oil spots, tenmokus—my, what an interesting thing: bubbles surging inside the glaze, swelling, popping, leaving a footprint, testament to a phase separation, distinct molten chemistries. Red iron oxide: a marker always, brilliant signpost of time and change.
