Analyses of antique bronzes give us some idea of their art of mixing and coloring. We presume that they soon abandoned the use of copper and pure bronze; the former yields porous casts and of a poor color; the latter material was, in later times, too costly. Lead was probably used at first for its fusibility only, but afterward it was certainly introduced for economical reasons. This cheap material was often added in very considerable quantity until they learned that leaden bronzes did not have a fine color either while fresh and clean, or when old and covered with patina. We have also seen that zinc, as well as lead, was often added. As the color of zinc alloys was red to light golden yellow (red metal, brass), they tried to dispense with tin entirely, as its price was higher than that of zinc (cadmia, as it was called). But they soon became convinced that for fine statues, at least, a small quantity of _tin was a necessity_. Generally a zinc-brass was used for statues. To prevent the metallic constituents from separating during fusion, the mass was kept thick and pasty by putting in old scrap bronze that had been often melted and contained oxides. The smelters also knew that the metals, particularly the tin, grew smaller every time it was melted, in consequence of oxidation, slagging, and evaporation.[19] The Romans therefore added, besides the scrap bronze, an eighth part of "silver lead," _i. e._, a mixture of tin and lead. Finally, in regard to the color of the castings, the ancients collected valuable experiences. Cadmia (zinc ore) was used to impart a golden color to the bronze.[20] Alloys rich in tin were used for mirrors, and arsenic was employed to make them white.[21] The moulds originally employed were very primitive. For simple objects a corresponding hole was dug in the sand or clay soil. Complicated figures had to be formed in clay, and the metal was cast in the clay mould. If the mould was to serve for several castings, it had to be made of baked clay, stone, brick, or other durable material. Organic substances were mixed with the clay to prevent uneven shrinkage and cracking. Hollow casting is more difficult; first a core is formed corresponding to the hollow in the figure; over this the figure is formed, and over that the mantle, _i. e._, the negative, or mould. The latter is taken off, the figure taken away from the core, the mantle replaced, and the metal poured into the space between the core and the mantle. In this case it is difficult to take off the mantle so clean and put it back so accurately that the parts will not be disturbed. To avoid this difficulty a wax model may be built on the core, and the mantle formed over this, and then when the mould is dry it can be heated and the wax melted out. The Phoenicians and Egyptians must have used one or the other of these devices for their hollow castings.