Bog-Iron Ore
Bog iron could never compete with hematite (iron oxide, Fe3+2O3) and magnetite (iron oxide, Fe2+Fe3+2O4) as a major iron ore because there simply wasn’t enough of it. Today, bog iron is little more than a mineralogical and historical footnote. Nevertheless, because it could be “mined” using just rakes and shovels, and smelted at low temperatures, it was for centuries a vital source of iron for many cultures. All it took to produce metallic iron was a supply of bog-iron ore and charcoal, a clay oven, and a rudimentary knowledge of smelting. And that profoundly impacted history.
An indeterminate mix of iron oxides and hydroxides bog iron occurs as rust-colored nodules in wetland environments. Although individual deposits are always small, they number in the thousands, especially in northern temperate zones where retreating Ice Age glaciers left behind countless wetlands or bogs.
Bog iron forms from the chemical and biochemical oxidation of dissolved iron. The countless peat bogs in the northern regions of Russia, Europe, and North America are ideal environments for concentrating bog iron. Ironrich water from surrounding hills collects in these acidic, oxygen-deficient, lowland bogs where the dissolved iron (ferrous, Fe2+) converts to insoluble iron (ferric, Fe3+) and precipitates as yellow-brown or rust-colored nodules.
In a simultaneous biochemical reaction, the anaerobic siderophilic (iron-loving) bacteria that thrive in peat bogs also digest (oxidize) dissolved ferrous iron to supplement the production of insoluble ferric iron compounds.
Bog-iron ore consists primarily of limonite, a general term for a mix of hydrated ferric oxide-hydroxides. The most abundant limonite mineral is goethite, an iron oxy-hydroxide with the formula FeO(OH). Also present are silica and traces of magnetite and hematite. Noticeably heavy in hand, bog-iron nodules consist of nearly 50 percent iron and are thus a high-grade iron ore.
Commonly only an inch in size but occasionally larger, bog-iron nodules concentrate in layers just below the bog surface and are easily “mined” with rakes and shovels.
Iron-rich bogs are identified by a yellow-brown limonite stain and an oily, iridescent surface film, the latter produced by the action of the siderophilic bacteria. Bog iron’s color and the nature of its occurrence are reflected in such alternative names as “yellow iron ore,” “brown iron ore,” “meadow iron ore,” and “swamp iron.”
Bog iron is a rare example of a renewable mineral resource. With a continuous inflow of iron-rich water and ongoing, rapid chemical and biochemical oxidation, a mined-out bog-iron deposit can replenish itself in as few as 20 years.
Early metalworkers roasted bog-iron ore to drive off moisture, mixed it with charcoal, and loaded it into small, drafted clay furnaces called “bloomeries.” After ignition, carbon monoxide produced by incomplete charcoal combustion reduced the ferric oxides and hydroxides to metallic iron. Given the low bloomery temperatures, neither the ore nor the metallic iron ever actually melted.
This “solid-state” smelting process produced a “bloom”—a solid mass of metallic iron and silica-rich slag. Repetitive heating and hammering removed the slag, leaving behind hard, tough iron—actually high-carbon steel—suitable for making everything from nails and spikes to tools and weapons.
Archaeological evidence indicates that bog iron was first smelted in bloomeries in central and eastern Europe as early as 600 BCE, where the availability of iron contributed to the end of the Bronze Age in that region. Historians believe that bog-iron ore reached its peak of economic and societal importance with the Vikings from 800 CE to 1150 CE. Much later in North America’s mid-Atlantic and New England colonies, bog-iron ore was provided much of the iron needed during the American Revolution.
The importance of bog iron finally ended in the mid-1800s. At this time, the mass-mining of huge deposits of hematite and magnetite ores began providing far more abundant iron supplies. But by then, bog iron, that nondescript chemical and biochemical precipitant had already earned its place in history.