The impact was so pervasive that scholars traditionally divide ancient history into Stone Age, Bronze Age, and Iron Age.
In the Americas, pre-Inca civilizations of the central Andes in Peru had mastered the smelting of copper and silver at least six centuries before the first Europeans arrived in the 16th century, while never mastering the smelting of metals such as iron for use with weapon-craft.
To extract the metal, workers must make these compounds undergo a chemical reaction.
Smelting therefore consists of using suitable reducing substances that combine with those oxidizing elements to free the metal.
The others – copper, lead, silver, tin, iron and mercury – occur primarily as minerals, though copper is occasionally found in its native state in commercially significant quantities.
These minerals are primarily carbonates, sulfides, or oxides of the metal, mixed with other components such as silica and alumina.
A few practical examples: Reduction is the final, high-temperature step in smelting, in which the oxide becomes the elemental metal.
A reducing environment (often provided by carbon monoxide, made by incomplete combustion in an air-starved furnace) pulls the final oxygen atoms from the raw metal.
In the case of carbonates and sulfides, a process called "roasting" drives out the unwanted carbon or sulfur, leaving an oxide, which can be directly reduced.
Roasting is usually carried out in an oxidizing environment.
However, since it was easy to cast and shape, workers in the classical world of Ancient Greece and Ancient Rome used it extensively to pipe and store water. Tin was much less common than lead and is only marginally harder, and had even less impact by itself.