Note 205 — Chapter XXXVI · HOW TO MAKE ROYAL CEMENT.

Cellini appears not to have quite understood the process. Geber, who gives the oldest description of it, ‘Alchemiae Gebri Arabis Philosophi Solertissimi Libri, etc. Joa͠n: Petreius Nuremberge͠n denuo Bernae excudi faciebat,’ anno 1545, p. 51, gives the ingredients thus: ‘Vitriol (ferrous sulphate), sal ammoniac, flower of copper (scale of oxide of copper formed by heating the metal with access of air), ground old earthen pot, sulphur in the smallest quantity or none at all, man’s urine, together with similar sharp and penetrating substances,’ etc. See Percy’s ‘Metallurgy,’ Murray, 1880; Part I., p. 385. Prof. Roberts-Austen adds that ‘usually the “cement,” and the gold to be purified, were placed together into a porous earthen pot, and not between the joints of the brickwork.’