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Bole hills were primitive lead smelting plants used in Britain before the 17th century. They were built on hills to take advantage of strong drafts and used local timber as fuel. Molten lead was directed down channels to casting points at the base of the hill. The process was replaced by more productive methods in the 16th century. Lead was extensively used for industrial and household items before its harmful effects were discovered.
A bole hill is, or rather was, a primitive type of lead smelting plant commonly used in parts of Britain before the 17th century. These foundries were built near the tops of hills to take advantage of the stronger drafts experienced at high altitudes and made use of locally felled timber to fire their furnaces. The bole hill smelting process was quite simple, with lead ore placed on top of layers of furnace fuel packed into a simple walled hearth. Dug channels directed molten lead down the hill from the bolus to casting points where it was poured into ingots known as sows. The Bole Hill foundry was replaced by the more productive foundry towards the end of the 16th century.
Prior to the discovery of its potential to cause serious poisoning, lead was used extensively to make a wide variety of industrial and household items, including pipes, kitchenware, and paint. In Britain before the 17th century, lead ore was generally worked in rather primitive smelters known as boles. These structures were located on high ground, typically on the top of hills, where the strongest winds encountered at the highest altitudes helped in force the ventilation of the furnace. Consisting of little more than a simple walled hearth, bolo hill used local timber as a source of fuel for the kiln.
To light a bolus furnace, the hearth was first laid with large beams called blocks. These blocks, often up to 20 feet (6.1 m) long, formed a stable foundation for the rest of the fire material. Blackwork, or partially smelted ore, was then packed on top of the blocks, followed by a layer of slightly smaller lumber known as a shankard. Several courses, or layers, of smaller fireshafts completed the fuel pile ready for firing. The raw ore was then simply laid atop the fuel pile, which was then ignited.
A series of aligned channels cut into the hillside led from the hearth to the casting points at the base of the hill. These channels directed the molten lead from the hearth to where it was poured into sows, or ingots, weighing about 1,100 kg each. On average, a typical bolo hill firing used 500 metric tons (30 metric tons) of wood, 30.5 metric tons (40 metric tons) of ore, and produced approximately 40.6 metric tons (18 metric tons) of lead ingot.
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