We already reviewed how miners managed without explosives in the Prehistory and beginning of History and how efficient was the firesetting technique. Let’s continue the journey with an overview of another important mining technique in use during Classical Antiquity.
Describing how gold is found, Pliny the Elder wrote: "Gold is found in our own part of the world; (…) Among us it is procured in three different ways; the first of which is, in the shape of dust, found in running streams, the Tagus in Spain, for instance, the Padus in Italy, the Hebrus in Thracia, the Pactolus in Asia, and the Ganges in India; indeed, there is no gold found in a more perfect state than this, thoroughly polished as it is by the continual attrition of the current. A second mode of obtaining gold is by sinking shafts or seeking it by causing the collapse of mountains [Ruina Montium] (…)." (Pliny the Elder, Natural History XXXIII, 21,4)
The first step consisted in "bringing rivers from the more elevated mountain heights, a distance in many instances of one hundred miles perhaps, for the purpose of washing these debris. The channels thus formed are called "corrugi," from our word "corrivatio", I suppose; and even when these are once made, they entail a thousand fresh labours."
Altitude rivers were diverted to a large retention basin ("a couple of hundred feet in length and breadth, and some ten feet in depth", wrote Pliny), communicating with a network of vertical shafts and long tunnels regularly drilled and dug in very high bench of alluvial ground. We can evaluate the height of the bench through Pliny’s comment: "the person (…) [is] hanging suspended all the time with ropes, so that to a spectator who views the operations from a distance, the workmen have all the appearance, not so much of wild beasts, as of birds upon the wing. Hanging thus suspended in most instances, they take the levels, and trace with lines the course the water is to take; and thus, where there is no room even for man to plant a footstep, are rivers traced out by the hand of man".
"In the reservoirs there are generally five sluices left, about three feet square; so that, the moment the reservoir is filled, the floodgates are struck away, and the torrent bursts forth with such a degree of violence as to roll onwards any fragments of rock which may obstruct its passage.", wrote Pliny.
Recovering Pure Gold
"Trenches—known as "agogæ"—have to be dug for the passage of the water; and these, at regular intervals, have a layer of ulex [Note: Probably Ulex Europaeus] placed at the bottom. This ulex is a plant like rosemary in appearance, rough and prickly, and well-adapted for arresting any pieces of gold that may be carried along. The sides, too, are closed in with planks, and are supported by arches when carried over steep and precipitous spots."
"The gold found by excavating with galleries does not require to be melted, but is pure gold at once."
Tailings Disposal Techniques"The earth, carried onwards in the stream, arrives at the sea at last, and thus is the shattered mountain washed away; causes which have greatly tended to extend the shores of Spain by these encroachments upon the deep (…)" This gives also an idea of the (huge) scale of the operation. Las Médulas is a World Heritage Site since 1997. Some questions remain nevertheless pending. Amongst many others: why is Pliny the only available source about the application of this amazing technique in Spain? Was it applied in other parts of the world?