Smelting

Perhaps 1,000 years after humans learned about melting virgin copper, they found that still another stone, a brittle one directly useless for tools, would produce liquid copper if sufficiently heated while in contact with charcoal. This step was epoch making, for it was the discovery of smelting, or the separation of a metal from a chemical compound called ore. Smelting, as differentiated from melting, was the first metallurgical operation and is still the principal method of gaining metals from their ores. Copper was the first metal to be smelted; it was another 1,000 years before iron was reduced from its ores.

Mycenaean dagger, bronze with gold, silver, and niello, 16th century BC. In the National Archaeological Museum, Athens. Length 16.3 cm.

As mined, raw ore is a nonchemical mixture of ore proper (heavy) and earthy matter, or gangue (light); the two may be largely separated by crushing the raw ore and washing away the lighter gangue. The ore proper is a chemical compound of oxides, sulfides, carbonates, hydrates, silicates, and small amounts of impurities such as arsenic and other elements. Smelting frees the metal from the various combinations with which it is bound into the compound form. A preparatory step is to heat the washed ore (roasting, or dressing) not only to dry it but also to burn off sulfides and organic matter. Early practice involved heating the ore in intimate contact with charcoal to provide the essential reducing atmosphere, producing a metallic sponge made up of metal and slag. For chemical as well as practical reasons, the iron of tools, wrought iron, continued to be worked out of the spongy mass until the Middle Ages.

Originally copper smelting was terminated at the spongy stage. Early smelters soon discovered that better results were obtained when the metallic sponge was left in the furnace and subjected to draft-induced high temperatures. The metal became liquid and seeped down to the hearth, as did the slag, which, being lighter than the metal, floated over it, permitting recovery of the copper.

At some time during the copper period, a new kind of “copper” happened to be made by smelting together two separate ores, one bearing copper, the other tin. The resulting metal was recognized as being far more useful than copper alone, and the short period of copper tools came to an end. The new metal, a copper–tin alloy of mostly copper, was bronze. It was produced in the fluid state at a temperature less than that needed for copper, could be formed economically by casting, and could be hammer-hardened more than copper. The tin noticeably increased the liquidity of the melt, checked the absorption of oxygen and other gases, and suppressed the formation of cuprous oxide, all features that facilitated the casting operation. A two-piece, or split, mold, impracticable for copper, worked very well with bronze. Furthermore, it was found that bronze expanded just a bit before solidifying and thus picked up the detail of a mold before it contracted in cooling.

The earliest bronzes were of uneven composition. Later, the tin content was controlled at about 10 percent, a little less for hammered goods, a little more for ornamental castings. The edges of hammered bronze tools of this composition were more than twice as hard as those obtained from copper.

The Bronze Age of tools and implements began about 3000 BCE. In the course of the following 2,000 years the much more abundant iron supplanted bronze for tools, but bronze continued to be used in the arts.

All of the early metals were expensive commodities in antiquity and were monopolized by kings, priests, and officials. Most metal was diverted to the manufacture of weapons for professional soldiers. Industrial use was severely limited. The metal chisel was used on rock for buildings of state or for fashioning furniture for the wealthy; the common people living in a mud or reed hut had no reason to own such a tool.

Generally speaking, molds for copper and bronze were of baked clay, although soft rock was sometimes carved; metal molds are known from about 1000 BCE. Sectional molds of three and four pieces, permitting more complex castings, are known from about 2600 BCE. The earliest metal tools and implements were simply copies of existing rock models. It was only slowly that the plasticity of the new medium and especially the possibilities inherent in casting were appreciated. The rock dagger, for example, was necessarily short because of its extreme brittleness. With copper and then bronze, it became longer and was adapted to slashing as well as to stabbing. Casting allowed forms that were impossible to execute in flaked stone, such as deeply concave surfaces. Holes could be cast in, rather than worked out of, the solid.

Sometimes the process was reversed. There were, for example, pottery imitations of bronze vessels for the poorer classes, with such necessary adjustments as a heavier lip for the pottery jug. The lines of bronze daggers have been noted in rock daggers of a later date, despite the difficulty of imitating a metal object in stone. Bronze axheads were copied in stone, even to the shaft hole, which was difficult to produce and impractical for a rock tool; it is possible that some of the rock replicas of bronze daggers and axes were used for ceremonial rather than utilitarian purposes.

Malleable metal had several advantages over a brittle material, such as rock or bone or antler. It could be severely deformed without breaking and, if badly bent, could probably be returned to service after straightening. It was shock resistant and chip-proof, good qualities for use in the ax, adz, and chisel, and the edges could be kept keen by hammering or abrasion; its sharpness was, however, inferior to that of good stone. In particular, metal allowed the fashioning of many small items, articles of a size awkward to make of bone or horn, such as pins, fishhooks, and awls. Copper pins were stronger, tidier, and more attractive than the fish bones and thorns they replaced for securing clothing; even in the 3rd century BC there were shapes resembling the modern safety pin. Tweezers were invented, but whether for depilatory or surgical purpose is unknown; there are artifacts presumed to be scalpels. Plates, nails, and rivets also developed early.

The most common tools were awls and pointed instruments suitable only for wood and leather. Woodworking was facilitated by the invention of the toothed copper saw, made of smelted metal and cast to shape. Edged tools—the ax, adz, and chisel, at first similar to rock models—became predominant, and, although not nearly as sharp as the tools they replaced, they had the advantage of toughness and could easily be resharpened. In particular, the chisel made it possible to use cut rock for construction purposes, principally in temples and monuments. Abrasive sand under metal “saw blades” allowed rock to be cut neatly, just as the sand under tubes (made from rolled-up strips) that were turned provided a boring device for larger holes.