Once our ore reaches the surface (or is removed from its open pit) it is not immediately ready for smelting, but has to go through a series of preparatory steps collectively referred to as “dressing” to get the ore ready for its date with the smelter […]
Ore removed from the mine would need to be crushed, with the larger stones pulled out of the mines smashed with heavy hammers (against a rock surface) in order to break them down to a manageable size. The exact size of the ore chunks desired varies based on the metal one is seeking and the quality of the local ore. Ores of precious metals, it seems, were often ground down to powder, but for iron ore it seems like somewhat larger chunks were acceptable. I’ve seen modern experiments with bloomeries […] getting pretty good results from ore chunks about half the size of a fist. Interestingly, Craddock notes that ore-crushing activity at mines was sufficiently intense that archaeologists can spot the tell-tale depressions where the rock surface that provided the “floor” against which the ore was crushed have been worn by repeated use.
Ore might also be washed, that is passed through water to liberate and wash away any lighter waste material. Washing is attested in the ancient world for gold and silver ores (and by Georgius Agricola for the medieval period for the same), but might be used for other ores depending on the country rock to wash away impurities. The simple method of this, sometimes called jigging, consisted of putting the ore in a sieve and shaking it while water passed through, although more complex sluicing systems are known, for instance at the Athenian silver mines at Laurium (note esp. Healy, 144-8 for diagrams); the sluices for washing are sometimes called buddles. Throughout these processes, the ore would also probably be hand-sorted in an effort to separate high-grade ore from low-grade ore.
It’s clear that this mechanical ore preparation was much more intensive for higher-value metals where making sure to be as efficient as possible was a significant concern; gold and silver ores might be crushed, sorted, washed and rewashed before being ground into a powder for the final smelting process. Craddock presents a postulated processing set for copper ore for the Bronze Age Timna mines that goes through a primary crushing, hand-sorted division into three grades, secondary crushing, grinding, a winnowing step for the low-grade ore (either air winnowing or washing) before being blended into the final smelter “charge”.
As far as I can tell, such extensive processing for iron was much less common; in many cases it seems it is hard to be certain because the sources remain so focused on precious metal mining and the later stages of iron-working. Diodorus describes the iron ore on Elba as merely being crushed, roasted and then bloomed (5.13.1) but the description is so brief it is possible that he is leaving out steps (but also, Elba’s iron ore was sufficiently rich that further processing may not have been necessary). In many cases, iron was probably just crushed, sorted and then moved straight to roasting […]
Bret Devereaux, “Iron, How Did They Make It? Part I, Mining”, A Collection of Unmitigated Pedantry, 2020-09-18.
August 15, 2023
QotD: Iron ore processing in pre-industrial societies
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