Bronze is a mixture, or alloy, of copper and usually tin that has been
in use for thousands of years. Compared to pure copper, bronze is much harder, and has a brighter, more golden, color that may have been just as
appealing to ancient craftsmen as its hardness.
Bronze is still favored for casting because it expands slightly as it cools. This makes sure that the metal fills the mold entirely and
reproduces all of the details in the original.
Bronze made with 12% tin is harder than pure iron, and, unlike iron, will not continue to oxidize once the immediate surface has developed the
characteristic patina. Bronze is also not as brittle as iron. In modern times, bronze has been made with aluminum or silicon instead of tin, but these
formulas are not usually used for casting sculptures.