Book IX artificially coloring of metals such as gold, silver, copper Page 210



	198	DE NATURA FOSSILIUM

	that has been scraped off is placed to one side in a vessel that has been coated with lead oxide. When enough material is collected the latter vessel is placed over a charcoal fire and the material burned a little. After cooling the material is washed in pure cold water. When it has settled the water is decanted and it is dried in the sun. This is the caeruleum that has been so highly esteemed for a long time.¹¹ Some people, in order to avoid the cost of using sheets of silver, make caeruleum from a mixture of three parts of quicksilver, one part of artificial sal ammoniac and two parts of sulphur. The sulphur is pulverized and then melted in a vessel coated with lead oxide. Then the sal ammoniac, also pulverized, is added together with the quicksilver and the mixture stirred with a wooden stick until thoroughly mixed. The mass is then cooled and pulverized. The powder is then placed in a glass vessel that is coated with two inches of chemists mud. An opening is left in the mud and the vessel is set aside to dry. After it is thoroughly dried it is placed on an iron tripod over a charcoal fire with a sheet of iron covering the opening. It is heated slowly and the iron sheet removed from time to time in order to watch the mixture. When it has melted down the opening is closed with mud and the vessel is heated for an hour over a hotter fire. It is then heated over a still hotter fire until it gives off a blue smoke. The caeruleum is found in the bottom of the glass.¹² Just as copper smelters produce diphryges, cadmia, pompholyx, spodos, flowers of copper, and copper scale, lead smelters produce sandyx, cerussa, cerussa plumbaria, and ochra plumbaria. I shall now take up these latter substances. Cerussa, that the Greeks call ψψμίθων, is made in the following manner from lead. First twigs are placed in a large earthenware jar and then strong vinegar is poured into it until only about three inches of the twigs project above the acid. A sheet of lead is placed on the twigs. Some do not use twigs but fasten the lead on a piece of wood that suspends it just above the acid. Each sheet of lead weighs a pound. To prevent the escape of the vinegar the jar is covered with an earthen lid and sealed with mud. In summer it is set out in the sun, in winter it is placed over a furnace, oven or bath. The lead sheets are scraped every ten days until entirely eaten away. Sometimes they are onty scraped every thirty days. After the sheets are entirely eaten away the pure liquid that covers the cerussa is decanted and the glue-like mass that remains in the bottom of the jar is transferred to another vessel and dried in the sun. When dry it is crushed, either in a hand mill or in some other way and sieved. The coarse material that is usually hard and solid is crushed again and again until it passes through the sieve. At one time, according to Dioscorides, the finest cerussa came from Rhodes, Corinth, and Lace-daemon; the next best from Puteoli. In our time it is made in all parts of ¹¹ A silver-lead ammonium acetate. ¹⁸ One would not expect this to produce a blue compound.