Boron: Boric Oxide Reduced with Magnesium Filings (Moissan, Thorpe 1912)

Magnesium tears the oxygen out of boric oxide at red heat. Acids then wash away everything that is not boron. Result: chestnut-brown boron powder, nearly pure; a trace of magnesium boride stays. Source: Thorpe, Dictionary of Applied Chemistry vol. 1 (1912), p. 498, after Moissan (Thorpe: 'the best method'). Not given: the furnace and acid strengths. Picture drawn by Youblob.