Agricola, Georgius, De re metallica, 1912/1950
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              blow into the furnace in either a mild or a vigorous manner. </s>
              <s>For those ores
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              which heat and fuse easily, a low hearth is necessary for the work of the
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              smelters, and the pipe must be placed at a gentle angle to produce a mild
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              blast from the bellows. </s>
              <s>On the contrary, those ores that heat and fuse
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              slowly must have a high hearth, and the pipe must be placed at a steep incline
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              in order to blow a strong blast of the bellows, and it is necessary, for this
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              kind of ore, to have a very hot furnace in which slags, or cakes melted from
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              pyrites, or stones which melt easily in the fire
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              15
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              , are first melted, so that the
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              ore should not settle in the hearth of the furnace and obstruct and choke up
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              the tap-hole, as the minute metallic particles that have been washed from
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              the ores are wont to do. </s>
              <s>Large bellows have wide nozzles, for if they were
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              narrow the copious and strong blast would be too much compressed and too
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              acutely blown into the furnace, and then the melted material would be
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              chilled, and would form sows around the nozzle, and thus obstruct the opening
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              into the furnace, which would cause great damage to the proprietors'
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              property. </s>
              <s>If the ores agglomerate and do not fuse, the smelter, mounting
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              on the ladder placed against the side of the furnace, divides the charge with
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              a pointed or hooked bar, which he also pushes down into the pipe in </s>
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