GEORGE BERNARD / SCIENCE PHOTO LIBRARY GEORGE BERNARD / SCIENCE PHOTO LIBRARY
Saltpetre extraction. Historical engraving showing equipment used to extract saltpetre, or potassium nitrate, from soil. The soil, placed in tubs, has running water washed over it to dissolve and remove the saltpeter (right). The water is collected and placed into a vessel on top of a stove. The water evaporates leaving only solid material, which will include saltpetre. The saltpetre is then extracted by filtration. Saltpetre was used to make gunpowder. Taken from The Laws of Art and Nature, in Knowing, Judging, Assaying, Fining, Refining and Inlarging Bodies of Confin'd Metals (1683), John Pettus, a translation of the work by Lazarus Erckern.
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