There are quite a lot of ores of iron, but not all of them could be usefully processed with ancient or medieval technology. The most commonly used iron ore was hematite (Fe2O3), with goethite (HFeO2) and limonite (FeO(OH)·nH2O) close behind. Rarer, but still used was magnetite (Fe3O4) and siderite (FeCO3). All of these can occur in big rock deposits, but may also occur as “bog iron” where oxidation occurs in acidic environments (in swamps and bogs) leading to the formation of small clumps of iron-rich material. Many of these ores can be spotted visually by someone who knows what they are doing; hematite can be blackish to reddish-brown but leaves tell-tale red streaks (of rust); goethite’s black-brown color is also fairly recognizable, as is limonite with its burnt yellow-orange hue. We’ll come back to these ores a few times both this week and next, because while they can all yield iron, some of them yield that iron easier than others.
One distinction here is between bog iron and iron in ore deposits. Bog iron is formed when ground-water picks up iron from iron-ore deposits, where that iron is then oxidized under acidic conditions to form chunks of iron minerals (goethite, magnetite, hematite, etc.), typically in smallish chunks. Bog iron is much easier to smelt because it contains fewer impurities than iron ore in rock deposits, but the quantity of iron available from bog iron is relatively low (although actually renewable, unlike mines; a bog can be harvested for iron again after a few decades as the processes which produce the bog iron continue). Because of its low output, bog iron tends to be an important part of the iron supply only when production is relatively low, such as during the Pre-Roman Iron Age in Europe, or the early medieval period.
But what I want to stress here at the outset is that while the local variety of iron may vary based on conditions, iron ores are sufficiently common that prior to the industrial revolution, it wasn’t generally necessary to trade or transport them over long distances because most areas have deposits. There are some exceptions (Japan is notoriously mineral poor – my limited geological understanding is that this is common in volcanic land formations – and while it does have some iron deposits, they are few and relatively small), but for the most part, getting iron ore was not hard. As we’ll see, timber availability was actually often a more pressing limitation on iron exploitation than the ore itself […]
Bret Devereaux, “Iron, How Did They Make It? Part I, Mining”, A Collection of Unmitigated Pedantry, 2020-09-18.
April 19, 2022
QotD: “Bog iron” in ancient and medieval society
Filed under: Europe, History, Quotations, Technology — Tags: BretDevereaux, Chemistry, Materials, Metalworking, MiddleAges, Mining — Nicholas @ 01:00
Comments Off on QotD: “Bog iron” in ancient and medieval society
No Comments
No comments yet.
RSS feed for comments on this post.
Sorry, the comment form is closed at this time.
