Finding ore in the pre-modern period was generally a matter of visual prospecting, looking for ore outcrops or looking for bits of ore in stream-beds where the stream could then be followed back to the primary mineral vein. It’s also clear that superstition and divination often played a role; as late as 1556, Georgius Agricola feels the need to include dowsing in his description of ore prospecting techniques, though he has the good sense to reject it.

As with many ancient technologies, there is a triumph of practice over understanding in all of this; the workers have mastered the how but not the why. Lacking an understanding of geology, for instance, meant that pre-modern miners, if the ore vein hit a fault line (which might displace the vein, making it impossible to follow directly) had to resort to sinking shafts and exploratory mining an an effort to “find” it again. In many cases ancient miners seem to have simply abandoned the works when the vein had moved only a short distance because they couldn’t manage to find it again. Likewise, there was a common belief (e.g. Plin. 34.49) that ore deposits, if just left alone for a period of years (often thirty) would replenish themselves, a belief that continues to appear in works on mining as late as the 18th century (and lest anyone be confused, they clearly believe this about underground deposits; they don’t mean bog iron). And so like many pre-modern industries, this was often a matter of knowing how without knowing why.

Once the ore was located, mining tended to follow the ore, assuming whatever shape the ore-formation was in. For ore deposits in veins, that typically means diggings shafts and galleries (or trenches, if the deposit was shallow) that follow the often irregular, curving patterns of the veins themselves. For “bedded” ore (where the ore isn’t in a vein, but instead an entire layer, typically created by erosion and sedimentation), this might mean “bell pitting” where a shaft was dug down to the ore layer, which was then extracted out in a cylinder until the roof became unstable, at which point the works were back-filled or collapsed and the process begun again nearby.

All of this digging had to be done by hand, of course. Iron-age mining tools (picks, chisels, hammers) fairly strongly resemble their modern counterparts and work the same way (interestingly, in contrast to things like bronze-age picks which were bronze sheaths around a wooden core, instead of a metal pick on a wooden haft).

For rock that was too tough for simple muscle-power and iron tools to remove, the typical expedient was “fire-setting“, which remained a standard technique for removing tough rocks until the introduction of explosives in the modern period. Fire-setting involves constructing a fuel-pile (typically wood) up against the exposed rock and then letting it burn (typically overnight); the heat splinters, cracks and softens the rock. The problem of course is that the fire is going to consume all of the oxygen and let out a ton of smoke, preventing work close to an active fire (or even in the mine at all while it was happening). Note that this is all about the cracking and splintering effect, along with chemical changes from roasting, not melting the rock – by the time the air-quality had improved to the point where the fire-set rock could be worked, it would be quite cool. Ancient sources regularly recommend dousing these fires with vinegar, not water, and there seems to be some evidence that this would, in fact, render the rock easier to extract afterwards.

By the beginning of the iron age in Europe (which varies by place, but tends to start between c. 1000 and c. 600 BC), the level of mining sophistication that we see in preserved mines is actually quite considerable. While Bronze Age mines tend to stay above the water-table, iron-age mines often run much deeper, which raises all sorts of exciting engineering problems in ventilation and drainage. Deep mines could be drained using simple bucket-lines, but we also see more sophisticated methods of drainage, from the Roman use of screw-pumps and water-wheels to Chinese use of chain-pumps from at least the Song Dynasty. Ventilation was also crucial to prevent the air becoming foul; ventilation shafts were often dug, with the use of either cloth fans or lit fires at the exits to force circulation. So mining could get very sophisticated when there was a reason to delve deep. Water might also be used to aid in mining, by leading water over a deposit and into a sluice box where the minerals were then separated out. This seems to have been done mostly for mining gold and tin.

Bret Devereaux, “Iron, How Did They Make It? Part I, Mining”, A Collection of Unmitigated Pedantry, 2020-09-18.