Silver ore and scrap silver have to go through a refining process in order for the pure silver to be separated from the dross. Cupellation is when it is heated to 648 degrees C Celsius in a special furnace. First though, the silver scrap or ore is placed in a solution of 30 per cent to 35 per cent nitric acid. It takes an ounce and a half of nitric acid to dissolve one ounce of silver. The solution produces a white powder, silver chloride. When sodium carbonate is mixed with the silver chloride and placed in a cupellation furnace, the heat causes a chemical reaction and makes table salt and silver. The process works without the addition of sodium carbonate as well but then the heat releases poisonous chlorine gas as it produces the pure silver.
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Another method of refining silver is called the Patio Process and was used in Latin America by the Spaniards during the 16th century. Silver ore was ground to a powder and mixed with salt, powdered roast copper and liquid mercury. Then tethered mules walked around and around a small circle of earth on which the powdered mixture had been poured. The pressure of their feet crushed the powder into even smaller granules. Eventually the mixture dissolved in the liquid mercury. Like making liquor, the mixture was distilled and then placed in a cupellation furnace. The refined silver that emerged from the furnace was pure.
Passing an electric current through sulfated copper slime also produces a pure silver without the need for heating it to 1,200 degrees. Instead, silver nitrate dissolves without heat because the hydrometallurgical process regulates the concentration of sulphate ions in which the silver is placed. While the other processes of refining silver did not require sophisticated laboratory equipment, electro-refining silver has only been made possible by modern technology.
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