Agricola, Georgius, De re metallica, 1912/1950
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              <s>A wall which will be called the “second wall” is constructed of brick
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              or stone, two feet and as many palms thick, in order that it may be strong
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              enough to bear the weight. </s>
              <s>It is built fifteen feet high, and its length depends
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              on the number of furnaces which are put in the works; there are usually
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              six furnaces, rarely more, and often less. </s>
              <s>There are three furnace walls, a
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              back one which is against the “second” wall, and two side ones, of which I
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              will speak later. </s>
              <s>These should be made of natural stone, as this is more
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              serviceable than burnt bricks, because bricks soon become defective and
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              crumble away, when the smelter or his deputy chips off the accretions which
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              adhere to the walls when the ore is smelted. </s>
              <s>Natural stone resists injury
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              by the fire and lasts a long time, especially that which is soft and devoid
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              of cracks; but, on the contrary, that which is hard and has many cracks
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              is burst asunder by the fire and destroyed. </s>
              <s>For this reason, furnaces which
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              are made of the latter are easily weakened by the fire, and when the accretions
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              are chipped off they crumble to pieces. </s>
              <s>The front furnace wall should be
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              made of brick, and there should be in the lower part a mouth three palms
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              wide and one and a half feet high, when the hearth is completed. </s>
              <s>A hole
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              slanting upward, three palms long, is made through the back furnace wall, at
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              the height of a cubit, before the hearth has been prepared; through this
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              hole and a hole one foot long in the “second” wall—as the back of this wall
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              has an arch—is inserted a pipe of iron or bronze, in which are fixed the nozzles </s>
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