Of the
seven metals known in antiquity, only
gold regularly occurs in nature as a
native metal. The others –
copper,
lead,
silver,
tin,
iron, and
mercury – occur primarily as minerals, although
native copper is occasionally found in commercially significant quantities. These minerals are primarily
carbonates,
sulfides, or
oxides of the metal, mixed with other components such as
silica and
alumina.
Roasting the carbonate and sulfide minerals in the air converts them to oxides. The oxides, in turn, are smelted into the metal. Carbon monoxide was (and is) the reducing agent of choice for smelting. It is easily produced during the heating process, and as a gas comes into intimate contact with the ore. In the
Old World, humans learned to smelt metals in
prehistoric times, more than 8000 years ago. The discovery and use of the "useful" metals – copper and bronze at first, then iron a few millennia later – had an enormous impact on human society. 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.
Copper and bronze -tripods, from the Chinese
Tiangong Kaiwu encyclopedia of
Song Yingxing, published in 1637 Copper was the first metal to be smelted. How the discovery came about is debated. Campfires are about 200 °C short of the temperature needed, so some propose that the first smelting of copper may have occurred in pottery
kilns. (The development of copper smelting in the Andes, which is believed to have occurred independently of the
Old World, may have occurred in the same way. A mace head found in Turkey and dated to 5000 BC, once thought to be the oldest evidence, now appears to be hammered, native copper. Copper–tin bronzes, harder, and more durable, were developed around 3500 BC, also in Asia Minor. At present-day the direct product of copper smelters is
anode copper which has a purity ranging from 98.5 to 99.8%.
Tin and lead The earliest known
cast lead beads were thought to be in the
Çatalhöyük site in
Anatolia (
Turkey), and dated from about 6500 BC. However, recent research has discovered that this was not lead, but rather cerussite and galena, minerals rich in, but distinct from, lead. Since the discovery happened several millennia before the invention of writing, there is no written record of how it was made. However, tin and lead can be smelted by placing the ores in a wood fire, leaving the possibility that the discovery may have occurred by accident. Recent scholarship however has called this find into question. Lead is a common metal, but its discovery had relatively little impact in the ancient world. It is too soft to use for structural elements or weapons, though its high density relative to other metals makes it ideal for
sling projectiles. 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. They also used it as a
mortar in stone buildings. Tin was much less common than lead, is only marginally harder, and had even less impact by itself.
Early iron smelting The earliest evidence for iron-making is a small number of iron fragments with the appropriate amounts of carbon admixture found in the Proto-Hittite layers at
Kaman-Kalehöyük and dated to 2200–2000 BC. Souckova-Siegolová (2001) shows that iron implements were made in Central Anatolia in very limited quantities around 1800 BC and were in general use by elites, though not by commoners, during the
New Hittite Empire (~1400–1200 BC). Archaeologists have found indications of iron working in
Ancient Egypt, somewhere between the
Third Intermediate Period and
23rd Dynasty (ca. 1100–750 BC). Significantly though, they have found no evidence of iron ore smelting in any (pre-modern) period. In addition, very early instances of
carbon steel were in production around 2000 years ago (around
the first-century.) in northwest
Tanzania, based on complex preheating principles. These discoveries are significant for the history of metallurgy. Most early processes in Europe and Africa involved smelting iron ore in a
bloomery, where the temperature is kept low enough so that the iron does not melt. This produces a spongy mass of iron called a bloom, which then must be consolidated with a hammer to produce
wrought iron. Some of the earliest evidence to date for the bloomery smelting of iron is found at
Tell Hammeh, Jordan,
radiocarbon-dated to . Lejja, a village in Nsukka, Enugu State, Nigeria, is recorded to have engaged in iron smelting as far back as pre-2000 BC.
Later iron smelting From the medieval period, an indirect process began to replace the direct reduction in bloomeries. This used a
blast furnace to make
pig iron, which then had to undergo a further process to make forgeable bar iron. Processes for the second stage include fining in a
finery forge. In the
13th century during the
High Middle Ages the blast furnace was introduced by China, which had been using it since as early as 200 b.c during the
Qin dynasty.
Puddling was also introduced in the
Industrial Revolution. Both processes are now obsolete, and wrought iron is now rarely made. Instead, mild steel is produced from a
Bessemer converter or by other means including smelting reduction processes such as the
Corex Process. ==Environmental and occupational health impacts==