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A bole hill is, or rather was, a primitive type of lead smelting facility commonly used in parts of Britain prior to the 17th century. These smelters were built near the top of hills to harness the stronger air currents experienced at elevated altitudes and made use of locally-felled timber to fire their furnaces. The bole hill smelting process was fairly unsophisticated, with lead-bearing ore laid on top of layers of furnace fuel packed into a simple walled hearth. Excavated channels directed the molten lead down the hill from the bole to casting points where it was cast into ingots known as sows. The bole hill smelter was replaced by the more productive smeltmill towards the end of the 16th century.
Prior to the discovery of its potential to cause severe poisoning, lead was used extensively to produce a wide variety of industrial and domestic items, including pipes, crockery, and paints. In Britain, prior to the 17th century, lead ore was generally processed in fairly primitive smelters known as boles. These facilities were located on high ground, typically on the top of hills, where the stronger winds encountered at higher altitudes aided in force ventilating the furnace. Consisting of little more than a simple walled furnace hearth, the bole hill utilized local timber as a source of furnace fuel.
To fire a bole hill furnace, the hearth was first laid with large timbers called blocks. These blocks, often up to 20 feet (6.1 m) long, formed a stable base for the rest of the firing material. Blackwork, or partially-melted ore, was then packed on top of the blocks, followed by a layer of slightly smaller timbers known as shankards. Several courses, or layers, of smaller fire trees completed the fuel pile in readiness for firing. Unprocessed ore was was then simply laid out on top of the fuel pile, which was then ignited.
A series of lined channels dug into the hillside led down from the hearth to casting points at the base of the hill. These channels directed the molten lead from the hearth to where it was cast into sows, or ingots, weighing approximately 1,100 lbs (500 kg) each. On average, a typical bole hill firing utilized 30 tons (30.5 tonnes) of wood, 40 tons (40.6 tonnes) of ore, and produced around 18 tons (18.3 tonnes) of ingot lead.