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The impure product is then cupelled for further purification. This method is the same as described in Nagarjuna's Rasratnakara of 700 AD. This is also equivalent to the mixed amalgamation and cupellation processes of today.
It is also mentioned that lead is desilverised by liquation and crystallization. The lead thus obtained will contain about 2% Ag which, we know, cannot be removed due to the formation of eutectic mixture. However, silver so obtained has been termed as pure (Bis Biswa) useful for coin making. The impure silver is also purified with the help of lead used in various proportions. On heating. this in crucibles, or cupels, pure silver is obtained and impurities are either absorbed by the material of the crucible or volatalised. The pure silver could be converted into rods, ingots or foils. Different qualities of silver could be prepared and named by mixing it with a mixture of copper and brass in various proportions. The material absorbed by the crucible could be desilverised, if any, by mixing it with borax, sajji and fusing it strongly.
(6) Extraction of gold :-Gold is normally found in sands of the rivers and mountains or mines. The ore is mixed with a mixture of white chalk (calcium carbonate), salt (sodium chloride) and kallar (sajji mitti, mixture of sodium carbonate and sulfate) and heated strongly 3-21 times to get pure gold. There is always loss of weight in the original ore by this treatment. The gold ore may also be treated with the above mixture and kommans powder (containing perhaps lead, copper and tin) to obtain gold containing a small amount of copper in it. Like silver, the quality of gold could be determined by the amount of gold contained in it, the nongold material being a mixture of silver, copper and brass in various proportions. In a farmula, he gives that a fused mixture of 23 parts of copper with 77 parts of gold serves a good material to prepare various qualities of gold, The method of calculating the cost of a particular quality of gold has also been presented in the book. The gold extracted today is also based on the same basic principles but with a better quantitative accuracy.
(c) Extraction of copper :-The copper ore obtained from mines is ground and mixed with cowdung and dried. It is then heated strongly in a furnace with strong blasts until the slag forms. After the removal of slag, the copper so obtained is again heated by blasting to get it purified. The pure copper is then converted into either sheets or ingots,
According to the current practices, the ore is mixed with coke rather then. cowdung which serves to produce carbon particles or carbon monoxide while burning to supply necessary reducing agents. However, no flux seems to have been added in the olden times.
(d) Extraction of lead-The lead ore is ground and mixed with iron int the ratio of 2:3 and heated strongly in crucibles and furnaces. The iron might have served the purpose of removing sulfur from the sulfurous ores and reducing
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