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Liquation is a metallurgical process in common use starting around 600 years ago. It is a way to separate valuable metals from ores that are a mixture of two or more valuable metals, by a process of heating the ore until the metal with a lower melting point drains away. The purification of metals goes back to ancient times, with methods for gold purification traced back to 6,000 BC. The process of liquation does not stretch back that far, as it only works well with certain types of natural alloys, and is a specialized process.
Up until the 13th century, only seven metals were known to exist in nature: gold, silver, copper, mercury, tin, iron, and lead. Until the start of the 17th century, the metallurgical practice for separating metals from ore most commonly involved introducing either carbon or hydrogen compounds into the furnace. Germany began widespread use of the liquation practice in the 16th century to separate silver from copper, when Georg Agricola described the process of liquation in his 1556 book, On the Nature of Metals.
The two earliest uses for liquation were the separation of silver from copper with lead as a solvent, and for removing tin from several types of minerals. In order for liquation to work, it must be done in the absence of air when lead is used, as the lead would not serve as a suitable solvent and would separate into litharge, otherwise, a form of earthy, poisonous lead solid with the chemical formula PbO. For this reason, liquation cannot be performed in an ordinary smelting furnace.
Metallurgical processes for liquation result initially in only partial separation of the alloys' metals. A typical copper-silver alloy may yield liquated lead that still contains 1-3% of copper, 10-30% lead, and the remainder as silver. The process is continued until sufficient silver is present in the liquated lead that is drained off, and this mixture is then cupelled, or refined further at the bottom of the furnace. An additional metallurgical method then takes place known as drying, essentially a prolongation of the liquation process to remove more lead from the remaining silver.
Though it seems simple and straightforward, the process of liquation is lengthy, requires special furnace conditions, and can yield ambiguous results as to the final composition percentages of metal. Metal is also lost in the process as slag, and, due to this and the length of processing, liquation has now been replaced by more efficient metallurgical methods. Nevertheless, liquation was so useful during the Renaissance period of history for its massive production of silver that it has been said to rival the invention of the printing press in importance, and is credited with reviving large parts of the European economy of the 15th-16th centuries.
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