Metalwork

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Early Techniques

The earliest metalworking was of copper, perhaps as early as the 11th millennium, using small nuggets of native copper picked up in streams or from the ground. These nuggets were presumably at first considered a special kind of attractively colored stone, and by grinding and beating—methods already used for working stones, flints, and obsidian—they could be shaped into ornaments.

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Annealing

The next step was the discovery, about 5000 bc, that these special stones could be worked on with repeated hammering if the mass was heated to a full red color and cooled from time to time, and that this kept the metal soft and workable. Ordinary wood fires produced sufficient heat for this process, called annealing. Repeated hammering without annealing will cause the metal to become too hard and brittle, with resultant jagged cracks.

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Smelting

The next discovery came after the development of the closed two-chamber pottery kiln, which produced a far greater heat than the open fires adequate for the earlier low-fired pottery. This took place probably before 4000 bc and led, after some 500 years or so, to the smelting first of small pieces of native copper, malachite (which under certain conditions will render into copper), and finally large amounts of copper ores, in furnaces that initially resembled the two-chamber pottery kilns. It was not until copper ores were smelted that any significant increase in the supply of copper and copper products could take place.

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Alloys

Knowledge of smelting ultimately led to knowledge of mixing different ores together in the smelting process to produce simple alloys. This followed an intermediate period, about 3000 bc, when compound ores—rocks bearing one or two visibly different metallic particles—were observed to produce a superior metal. Copper produced by smelting continued to be shaped at first into small tools and ornaments by the grinding and beating methods long in use for working native copper. Weapons and tools dating to the late Predynastic period in Egypt (around 3000 bc) have been found, however, that were indubitably cast from smelted copper; at Ur of the Chaldees in Mesopotamia, in the royal graves of the 1st Dynasty (c. 3100-2907 bc) a profusion of beautifully worked objects in gold, silver, electrum (a natural alloy of gold and silver), copper, and even primitive bronze has been excavated, many made both by open-mold and lost-wax methods of casting.

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Application of Techniques

By 2500 bc, at the least, all the main techniques for working metals had been very slowly pioneered in the treatment of copper over the preceding 3000 years. By that time these techniques were already being applied to other metals, such as silver, gold, and natural alloys of electrum and bronze. Techniques used for shaping were hot and cold forging or beating, which developed into hammering and raising techniques, using smooth hematite hammers; annealing; grinding, which led to the polishing and fine abrading used in the production of mirrors; piecing flat sheets of metal together with lapped seams or rivets and subsequently with solders; and casting. After the discovery of smelting, battering was used to flatten the cakes of metal into sheets; some form of battery continued to be necessary until the invention, in the late 17th century, of the rolling mill in which sheet metal was produced by mechanical means.

Joining, beating, annealing, raising, and casting were and remain the artistic methods used for shaping metals, although other methods, such as spinning, have been introduced for industrial shaping. The shaping methods were presumably first worked out by the late Neolithic farmers, who were also part-time miners, prospectors, and smelters in the hilly region of northeastern Iran.