Hushing

Hushing is an ancient mining method using a flood or torrent of water to reveal mineral veins. The method was applied in several ways, both in prospecting for ores, and for their exploitation. Mineral veins are often hidden below soil and sub-soil, which must be stripped away to discover the ore veins. A flood of water is very effective in moving soil as well as working the ore deposits when combined with other methods such as fire-setting. It was used during the formation and expansion of the Roman Empire from the first century BC on to the end of the empire. It is now redundant except in a variant known as hydraulic mining, where jets or streams of water are used to break down deposits, especially of alluvial gold and alluvial tin.

History



The method is well described by Pliny the Elder in Book XXXIII of his Naturalis Historia from the first century AD. He distinguishes the use of the method for prospecting for ore and use during mining itself. It was used during the Roman period for hydraulic mining of alluvial gold deposits, and in opencast vein mining, for removal of rock debris, created by mechanical attack and fire-setting. He describes how tanks and reservoirs are built near the suspected veins, filled with water from an aqueduct, and the water suddenly released from a sluice-gate onto the hillside below, scouring the soil away to reveal the bedrock and any veins occurring there. The power behind a large release of water is very great, especially if it forms a single water wave, and is well known as a strong force in coastal erosion and river erosion. The method was most effective when used on steep ground such as the brow of a hill or mountain, the force of falling water lessening as the slope becomes smaller.

If veins of ore were found using the method, then hushing could also remove the rock debris created when attacking the veins. Pliny also describes the way hillsides could be undermined, and then collapsed to release the ore-bearing material. The Romans developed the method into a sophisticated way of extracting large alluvial gold deposits such as those at Las Medulas in northern Spain, and for hard rock gold veins such as those at Dolaucothi in Wales. The development of the mine at Dolaucothi shows the versatility of the method in finding and then exploiting ore deposits.



There are the remains of numerous tanks and reservoirs still to be seen at the site, one example being shown at left. It was a small tank built for prospection on the north side of the isolated opencast north of the main mine. It was presumably built to prospect the ground to one side of the opencast for traces of the gold-bearing veins extending to the north. It failed to find the veins here, so was abandoned. It probably precedes the construction of the 7 mile long aqueduct supplying the main site, and was fed by a small leat from a tributary of the river Cothi about a mile further north up the valley. The method could be applied to any ore type, and succeeded best in hilly terrain. The Romans were well experienced in building the long aqueducts needed to supply the large volumes of water needed by the method, and was probably directed by army engineers.

Earlier evidence

The earlier history of the method is obscure, although there is an intriguing reference by Strabo writing ca 25 BC in his Geographica, to gold extraction in the Val d'Aosta in the Alps. He describes the problem gold miners had with a local tribe because of the great volumes of water they had taken from the local river, reducing it to a trickle and so affecting the local farmers. Whether or not they used the water for hushing remains unknown, but it seems possible because the method requires large volumes of water to be operated. Later, when the Romans assumed control of the mining operations, the locals charged them for using the water. The tribe occupied the higher mountains and controlled the water sources, and had not yet been subdued by the Romans.

The historian Polybius, who lived from 220 to 170 BC was writing much earlier in The Histories, and he records that gold mining in the Alpine region was so successful that the price of gold in Italy fell by a third during this period. From his description of large nuggets, and the find being made only two feet below the ground level, with deposits reaching down to 15 feet, it is likely to have been an alluvial deposit where water methods such as hushing would have been very effective. Modern attempts to identify the mines point to one especially large ancient gold mine at Bessa in Northern Italy. It appears to have been worked intensively in pre-Roman days and continued to expand with Roman involvement. The scale of the aqueducts there seems to support Strabo's comments.

Later examples

The technique appears to have been neglected through the medieval period, because Georg Agricola, writing in the 15th century in his De Re Metallica, does not mention hushing at all. On the other hand, he does describe the many uses of water power, especially for washing ore and driving watermills.

However, the technique was used on a large scale in the lead mines of northern Britain from Elizabethan times onwards. The method is described in the Royal Commission on Children in Mines in 1842 in relation to children being used in the lead mines of the Pennines. The remnants of the "hush gullies" are visible at many places in the Pennines as well as at many other locations such as the extensive lead mines at Cwmystwyth in Ceredigion, and at the Stiperstones in Shropshire.

One famous and spectacular example is the Great Dun Fell hush gully near Cross Fell, Cumbria, probably formed in Georgian era in the search for lead and silver. The gully is about 100 feet deep, carries a small stream, and is a prominent landmark on the bleak moors.

Although the Cornish did not use the term "hushing", there is at least one reference to the technique being used at Tregardock in North Cornwall. Around 1580 mine adventurers used the method to work a lead-silver deposit, although lives were lost in the attempt.

From http://en.wikipedia.org/