Engraving. Copper ore was subjected to prolonged heating in order to remove sulphur and other impurities. The resulting calx was heated in a furnace, taken out and calcinated again. This proces was repeated up to 26 times and could last up to 30 weeks as other 'impurities' such as silver and gold were removed. Illustration from 'Regnum Subterraneum sive Minerale de Cupro et Orichalco' (The Subterranean or Mineral Kingdom in respect to Copper and Bras) by Emanuel Swedenborg (1688-1772) published in Dresden in 1734. This book formed part of his 'Opera philosophica et mineralia' (Philosophical and Logical Works), which dealt with the creation of the world, and drew on Swedenborg's interpretations of metallurgy and metaphysics.
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