Agricola, Georgius, De re metallica, 1912/1950
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              with the mouth and then a blue flame will be emitted. </s>
              <s>In the end the tubes
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              are weighed, and if their weights prove equal, he who has undertaken this work
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              has not laboured in vain. </s>
              <s>Lastly, both are placed in another balance-pan and
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              weighed; of each tube four grains must not be counted, on account of the
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              silver which remains in the gold and cannot be separated from it. </s>
              <s>From the
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              weight of the tubes we learn the weight both of the gold and of the silver
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              which is in the button. </s>
              <s>If some assayer has omitted to add so much silver to
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              the gold as to make it three times the quantity, but only double, or two and a
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              half times as much, he will require the stronger quality of
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              aqua
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              which
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              separates gold from silver, such as the fourth quality. </s>
              <s>Whether the
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              aqua
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              which he employs for gold and silver is suitable for the purpose, or whether
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              it is more or less strong than is right, is recognised by its effect. </s>
              <s>That of
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              medium strength raises the little bubbles on the tubes and is found to colour
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              the ampulla and the operculum a strong red; the weaker one is found to
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              colour them a light red, and the stronger one to break the tubes. </s>
              <s>To pure
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              silver in which there is some portion of gold, nothing should be added when
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              they are being heated in the cupel prior to their being parted, except a
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              bes
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              of lead and one-fourth or one-third its amount of copper of the lesser weights.
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              <s>If the silver contains in itself a certain amount of copper, let it be weighed,
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              both after it has been melted with the lead, and after the gold has been parted
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              from it; by the former we learn how much copper is in it, by the latter how
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              much gold. </s>
              <s>Base metals are burnt up even to-day for the purpose of assay,
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              because to lose so little of the metal is small loss, but from a large mass of
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              base metal, the precious metal is always extracted, as I will explain in
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              Books X. and XI.</s>
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              <s>We assay an alloy of copper and silver in the following way. </s>
              <s>From a
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              few cakes of copper the assayer cuts out portions, small samples from small
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              cakes, medium samples from medium cakes, and large samples from large
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              cakes; the small ones are equal in size to half a hazel nut, the large
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              ones do not exceed the size of half a chestnut, and those of medium size come
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              between the two. </s>
              <s>He cuts out the samples from the middle of the
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              bottom of each cake. </s>
              <s>He places the samples in a new, clean, triangular
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              crucible and fixes to them pieces of paper upon which are written the weight
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              of the cakes of copper, of whatever size they may be; for example, he writes,
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              “These samples have been cut from copper which weighs twenty
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              centum­
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              pondía.”
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              When he wishes to know how much silver one
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              centumpondíum
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              of
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              copper of this kind has in it, first of all he throws glowing coals into the
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              iron hoop, then adds charcoal to it. </s>
              <s>When the fire has become hot, the paper
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              is taken out of the crucible and put aside, he then sets that crucible on the
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              fire and gradually heats it for a quarter of an hour until it becomes red hot.
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              </s>
              <s>Then he stimulates the fire by blowing with a blast from the double bellows
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              for half an hour, because copper which is devoid of lead requires this time to
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              become hot and to melt; copper not devoid of lead melts quicker. </s>
              <s>When
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              he has blown the bellows for about the space of time stated, he removes the
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              glowing charcoal with the tongs, and stirs the copper with a splinter of wood,
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              which he grasps with the tongs. </s>
              <s>If it does not stir easily, it is a sign that the </s>
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