BRONZE is exceeded only by stone in the length of time it has served man as material for fashioning implements, utensils and works of art. Almost as soon as primitive humans discovered copper they seem to have learned that it was hardened in an alloy with other non-ferrous metals, especially tin. A number of bronzes, Western and Oriental as well as prehistoric and more modern, have been analyzed and the great majority of them contain from 80% to 90% of copper. Although a basis of copper is essential to the composition of bronze, the remainder of the alloy generally consists of greatly varying proportions of tin, lead and zinc, with traces of other metals. Tin is most often used with copper, but lead adds fluidity to the alloy, while zinc does not so readily oxidize as the tin. Modern French foundries use an alloy of 90% copper, 6.5% zinc and 3.5% lead for sculpture castings, with sometimes even higher percentages of zinc, to produce a softer, more easily worked alloy. In modern work, and some mediaeval work it would be difficult to make a sharp distinction between bronze and brass, the latter being an alloy of copper and zinc.
Obviously, then there is no single formula for bronze. One might suggest a formula of 90% copper and 10% tin as a norm for bronze, understanding that it is possible greatly to vary the proportion of copper to other metals and that the basic 10% for tin may be divided among a variety of other non-ferrous metals.
Bronze is superior to pure copper for casting both simple and more complicated works of art in that it has a much lower melting point than copper. Copper melts at about 1083° C., but with the addition of 10% of tin the melting point is lowered to 1005°C., besides which the tin increases the fluidity of the molten metal. Pure copper also tends to absorb gases, becoming porous, and, equally serious when accurate castings are required, it cools quickly and contracts excessively. An addition of even 5% of tin makes it possible to cast copper with greater fluidity and less heating. It is likely that most mediaeval cast objects, generally described as "copper gilt"--are bronzes with proportions of tin so small as to affect the characteristic copper color very little. 1
At present there is little information for a history of bronze casting and it has seemed best for this catalogue to describe only one of the methods of bronze casting in use today. 2 The "cire perdue" or "lost wax" process is most generally used for smaller sculpture. The sand mould process is most satisfactory for large works where fineness of detail in casting is not important or where there is little undercutting. The following description is of the "cire perdue" process. 3
THE SCULPTOR'S MODEL: This may be of any material. Usually the model received by the foundry is a plaster cast from the original clay model of the sculptor.