Hushing

Hushing is an ancient and historic 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. Hushing was used during the formation and expansion of the Roman Empire from the 1st century BC on to the end of the empire. It was also widely used later, and apparently survived until modern times where the cost of explosives was prohibitive. It was widely used in the United States, where it was known as 'booming'. 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, is commonly used. The method is well described by Pliny the Elder in Book XXXIII of his Naturalis Historia from the 1st 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. Such a wave could be created by a sluice gate covering one end of the reservoir, possibly a permanent fixture such as a swinging flap or a rising gate. The size of the tank controlled the height of the wave and its volume. Hushing 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. The rate of attack would be controlled by the water supply, and perhaps more difficult the higher the deposit to be cleared. 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 right. 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 construction was probably directed by army engineers. The earlier history of the method is obscure, although there is an intriguing reference by Strabo writing ca 25 BC in his Geographica, Book IV, Chapter 6, 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: