text: Heap leaching dates back to the second century BC in China, where iron was combined with copper sulfate. By the time of the Northern Song Dynasty, a copper alloy was able to be recovered by leaching.
  Leaching can also be traced back to alchemy. Early examples of leaching performed by alchemists resembled mixing iron with copper sulfate, yielding a layer of metallic copper. In the 8th century, Jabir Ibn Hayyan, an Arab alchemist, discovered a substance he coined "aqua regia". Aqua regia, a combination of hydrochloric acid and nitric acid, was found to be effective in dissolving gold, which was previously thought to be insoluble.