Silicon: Silica Heated with Magnesium Powder under Magnesia, Acid-Washed (Vigouroux, Thorpe 1912)

Magnesium burns the oxygen out of silica; extra magnesia keeps the reaction from running wild. A run of acids then dissolves everything but the silicon. Result: amorphous silicon, a maroon powder with only 0.4–1 % impurity. Source: Thorpe, Dictionary of Applied Chemistry vol. 4 (1912), p. 669, after Vigouroux (Compt. rend. 1895, 120, 94, 1161). Not given: amount of magnesia, temperature and vessel. Picture drawn by Youblob.