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May, 1939

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Although the mining and smelting of lead can be traced back in Britain to the pre-Roman period, i.e., for over 2,000 years, the science of dressing and cleaning the ore, prior to smelting it, has only a short history in comparison. Systematic washing of ores was only introduced here during the sixteenth century and until about 1800 A.D. little advance or change in methods was made. The modern period of mechanical treatment can be taken approximately as dating after 1850.

The earliest detailed account of ore treatment we have is that of Agricola in his great work "De re Metallica," printed and published posthumously in 1556, and fully illustrated with woodcuts. He describes in detail the art and practice of mining and metallurgy as he saw it in Saxony and its neighbourhood at the beginning of the 16th century, and in the case of the treatment of copper, lead, and tin ores, states that many of the methods he described were then of fairly recent introduction.

Ore treatment of necessity falls under two general headings (a) picking, breaking and grading of ore-bearing material and (b) cleaning and concentrating the ore by washing. In the latter, two principles are implied though not explicitly stated-the separation of ore from stone by settlement through water, and the removal of stone from ore by a stream of running water.

It may be convenient to recall that the basic principle of all methods of ore washing is the fact that equalised particles of materials of different densities will settle at different rates through a column of water, and also that streams of water having different velocities will be required to pick up and carry the same sized grains of materials of very different densities. Hence if a mixture of ore and stone is crushed and graded to a uniform size, then stirred well in water and allowed to settle, the ore will tend to settle first, and the stone later, giving a partial stratification with a concentration of ore towards the bottom of the settled material — or, a stream of water may also be arranged that its velocity is sufficient to carry off particles of stone, but leave behind similar sized particles of ore. It is clear from Agricola that in practice these principles were then being applied, and empirically it had been ascertained that a benefit accrued when material to be separated was graded to one size. Until very recent years, all methods of ore concentration have been based upon these principles.

The processes of ore separation as given by Agricola can be simply summarised as follows. Ore plus rock from the mine (house, as it is called here) was picked over, lumps of pure ore sent to the ore heap, lumps of stone with no visible ore sent to the "dead" heap. The remaining mixed rock and ore was reserved for treatment. This mixed stuff was broken down with hammers, then sent to the stamp mill to be crushed, and finally sieved to a size "not larger than hazel nuts." The stamp mill is fully described by Agricola as something fairly "modern" and new to him; from other sources these early stamps as we know them were introduced at Joachimsthal in 1519 and, a year or two later, sifting and wet stamping were added. The inventor is presumably unknown.

The "nuts" from the stamp mill were washed by shaking vigorously on a sieve in a tub of water, this being repeated two or three times to get the stuff really clean. The cleaned "nuts" were again picked over, ore lumps sent to the ore heap and deads thrown out, the remainder being sent hack to the mill. Fine material that settled in the washing tubs was taken out and treated in a simple buddle or strake. In each case a running stream of water was used, flowing down an inclined shallow box ; in the buddle the fine material (our term would be "slimes") was placed on the head board of the buddle, and moved across the stream of water, with a rake or shovel, the water carrying off the fine stone and dirt, leaving the ore particles behind, In the strake, action was rather more vigorous, the slimes being raked up and tossed against the current of water, not across it. For extra refinement, a gentler stream of water was used, running over a canvas-lined strake, the finest powder ore settling into the canvas texture, to be washed off at intervals, while the mud was carried away in the stream. In each case, the concentrates were returned many times to be rewashed, before a good concentrate was obtained. The buddle and strake are described as the old and well established methods in general use, but a refinement recently come into use (i.e. in about 1500 A. D.) was the jigging sieve. A shovelful of small mixed stuff was put on a fine sieve held in a tub of water and, by skilful shaking and tossing, the finer lighter mud washed off some of the finest ore settled through the mesh of the sieve, but what remained on the sieve gradually shook down into a layer of rich ore at the bottom, a layer of mixed poor quality ore and stone, and a layer of waste at the top that was skimmed off and rejected. The middle layer was returned to be treated again and again. (Fig. 1.)

In Britain we have no evidence for treatment other than breaking and rough washing in a stream, using the simplest possible buddle or strake. The methods described by Agricola probably came into this country by two routes. The German miners in the Lake District, who opened up the copper mines near Keswick about 1564, built and used much of the apparatus described by Agricola, and by them its use was introduced into Wales a few years later. In 1565 William Humphrey, Assay Master of the Mint, introduced to Derbyshire a German skilled in mining, partly to develop the making of brass. Shortly after that, "Dutchmen" were working at the copper mine near Richmond in Swaledale, Yorkshire. It seems fairly certain that these Germans introduced the buddle and strake as well as the jiggling sieve, in the forms described above, and later we have the authority of Pryce who says "There can be no doubt that the Cornish were almost entirely obliged to the Derbyshire and other lead miners for the best methods of dressing ore in the first place." That was written in 1778 when Pryce described the methods of ore dressing then in use in Cornwall. They are almost identical with Agricola's descriptions, the apparatus and processes having suffered little change in their passage via Derbyshire, the only important addition being that of the keeve.

In mine accounts in Yorkshire during the early 18th century, there is sufficient evidence of these methods of washing ore at the buddle and tub, to suggest that they had already passed into fairly general use. Farey (1811), writing of Derbyshire mining, makes the point that the finest slime ores (locally called "belland" or "smitham") washed by the buddle and keeve, were very little suited to smelting in the old type of furnace (the ore hearth or simple blast furnace) and could only have been generally used and smelted successfully after the introduction of the "cupola" or reverberatory furnace, after 1720. The ore hearth and blast furnace both use a powerful blast brought by the tuyere straight on to the hearth, and it would have been very difficult to hold the fine dusty ore on the hearth until it began to paste up ; in the reverberatory furnace, of course, the blast is nothing like so strong, and does not in any case bear directly on the ore.

Pryce describes the "spalling" of ore to the size of a man's fist, followed by picking, then stamping. The stamped house was passed over a fine grate so that the slimes were carried off to a slime pit where an ore concentrate tended to settle near the inlet of the pit, the stone being carried further towards the overflow. The buddle was a pit 7 in. by 2¼ in. by 2 ft. deep, with an inclined plane or "jagging board" at its head, on which the house was put a shovel full at a time in ridges parallel to the run of the water. The ridges of house were moved from side to side across the stream of water and back again, until every part has been washed. Material carried down by the stream settled in the long trough of the buddle and, when the latter was full, was sorted into three fractions. Nearest the jagging hoard most of the fine ore particles bad settled to form the "head" or "crop" ; near the tail of the buddle the material was mostly waste and these "tails" were sent to the waste heap, while the middle section or "crease" contained sufficient ore to repay a second treatment. Crop and crease were in fact usually re-buddled. The keeve seems to be the chief addition made to Agricola's methods and is the most characteristic of the contributions of the northern mining fields where it was known as the wash tub, and later the Dolly tub.

The keeve is described by Pryce as a large vat, one-third filled with water, which the dresser stirs round vigorously with a shovel. A boy acts as server, and puts fine house into the keeve, a shovelful at a time, scattering it into the stirring water bit by bit, until the keeve is nearly full. The stirrer keeps the mixture tossing about, then finally lets all settle. Two boys hammer the sides of the tub with mallets for about quarter of an hour after the tossing this helps the house to "pack." The water is then poured off, and the top surface of the settled material, which is waste, is taken off. The remainder is mixed material with increasing concentration of ore near the base. These settlings are taken out and given a final washing in water on a fine hair sieve used as a jigging sieve. In some cases the Cornish ore dressers turned the poured-off muddy water from the keeve and the slimes from the buddle, into a trunk buddle, 10 ft. by 3 ft. by 9 in. deep, which was exactly like the buddle in principle, but with greater width and length its use with a relatively slow water current effected a slight further concentration even in the slimes.

In Derbyshire and the Yorkshire Dales the processes of lead ore dressing were very similar. Bouse from the mine was picked and roughly broken with hammers, mainly wielded by women. The smaller mixed house was broken down on the "knock-stone" by girls using a bucker, comprising a plate of iron, about 3 in. sq. and to I in. thick, with a back loop by which it was hafted with a stick about 10 in. long. (Fig. 2.) Finely broken material, called "knock bark," was washed in the keeve, girls acting as servers to the keevers. The house, after rejection of the tail, or waste, was washed on a jigging sieve ; material passing through the sieve was washed over again two or three times. A rough first washing was occasionally carried out on a strake. Some of the mine accounts for Swaledale include payments of 6s. a month, and a heavy blue skirt yearly, to "washer women" on the dressing floors.

When the reverberatory furnace was really proving successful, between 1760 and 1800, it was possible to smelt much finer ground ore than ever before. This resulted in an attempt to concentrate more of the slimes at the dressing floors, and a tremendous activity, by independent miners and washers, in re-washing old waste heaps. Much of the waste from the hand picking and from buddle tails proved, on finer breaking, to contain a small percentage of ore, and one observer estimated that, by 1810, nearly all the waste heaps of Derbyshire had been turned over, broken small, and buddled, The London Lead Company, in the North Pennines, and the agents of the Duke of Devonshire in Wharfedale, Yorkshire, began the reorganisation of their washing departments during that period. It had previously been the custom for most miners, working on "tribute" in small partnerships, to get the ore, and to put out the dressing to groups of boys and women who were paid a percentage on the ore produced, the miners then selling the ore to the smelt mills.

Drawings and Photographs accompanying the article