Early metals and smelting
- Related Topics:
- cutlery
- saw
- wrench
- flake tool
- vise
The discovery that certain heavy “stones” did not respond to hammerblows by flaking or fracturing but were instead soft and remained intact as their shapes changed marked the end of the long Stone Age. Of the pure, or native, metals, gold and silver seem to have attracted attention at an early date, but both were too soft for tools. The first metals of value for toolmaking were natural copper and meteoric iron. Although they were scarce, they were tough and potentially versatile materials that were suited for new purposes, as well as many of the old. They also introduced a new problem, corrosion.
Metalworking
Copper occurs in native state in many parts of the world, sometimes in nuggets or lumps of convenient size. It is malleable; that is, it can be shaped by hammering while cold. This also hardens copper and allows it to carry a sharp edge, the hammered edge being capable of further improvement on an abrasive stone. After a certain amount of hammering (cold-working), copper becomes brittle, a condition that can be removed as often as necessary by heating the material and plunging it into cold water (quenching). The softening operation is known as annealing, and repeated annealings are necessary if much hammering is required for shaping.
Among early toolmakers, nuggets of copper were hammered into sheets, divided into strips, and then separated into pieces to be worked into arrowheads, knives, awls, choppers, and the like. Copper was also shaped by beating pieces of the soft metal into appropriately shaped rock cavities (molds).
Meteoric iron, widely distributed but not in heavy deposits, was a highly prized material more difficult to fabricate than the softer copper. Its celestial origin was recognized by the ancients: the ancient Egyptians called it black copper from heaven, and the Sumerians denoted it by two characters representing heaven and fire.
Like copper, iron hardens under the hammer and will then take a superior edge. Iron can be annealed, but the process is quite different from that of copper because, with iron, slow cooling from a high temperature is necessary. Meteoric iron is practically carbonless and, hence, cannot be hardened in the manner of steel; a high nickel content of about 8 percent makes it relatively corrosion resistant.
For early toolmakers, small meteorites were the most convenient sources of iron, but larger bodies were hacked at with copper and rock tools to yield tool-sized pieces for knives, spear points, arrowpoints, axheads, and other implements. Meteoric iron was beaten into tools in much the same way as copper, although it could not be forced into a mold in the manner of the softer metal. Much rarer than copper, meteoric iron also was often used for jewelry, attested to by burial finds of necklaces of iron and gold beads, iron rings along with gold rings, and ornaments in sheet form.
Casting
In casting, a liquid metal is poured into a cavity or a mold, where it takes the shape of the mold when it congeals; casting shapes the metal to essentially final form once a proper cavity has been prepared. Some touch-up work may be needed; for an edged copper tool, such as an ax or knife for example, hammering the cutting side gives a keen edge.
A great step forward was made with the discovery that gold, silver, and copper could be melted and cast with many advantages. Casting meant that the size of the tool was no longer dependent on the size of a chunk of available copper. Old tools could be added to a melt instead of being thrown out. This reuse of old metal accounts in part for the scarcity of virgin-copper implements.
To make the procedures of melting and casting possible, several innovations were required. Pottery making, already well established, provided the knowledge of heat-based processes. Clay vessels were essential to working with fluid metal, for, in all but the most primitive operations, it was necessary to convey the melt from furnace to mold. Aside from providing crucibles, pottery making taught how to restructure a fire with a deep bed of prepared charcoal to provide a heat superior to that of a simple campfire. Tongs of some sort had to be devised to carry the hot crucible; it is surmised that green branches were bent around the pot and replaced as needed.
A number of forms of molds were developed. The most primitive was simply an impression of a rock tool in clay or sand to give a cavity of the desired form. A more durable mold resulted when the cavity was worked into stone. Cavities of uniform depth allowed flat but profiled pieces to be cast. For example, some ax blade castings were roughly T-shaped, the arms of the T being afterward bent around to clasp a handle of some sort, with the bottom of the T becoming the cutting edge. A one-piece mold, prepared for a dagger, could have a groove for most of the length of the cavity to provide a stiffening rib on one side. With experience, closed but longitudinally split and, hence, two-piece molds were devised, each side having a groove down the middle to furnish a strengthening rib on both sides of the blade.
Split molds for copper were not desirable because pure copper is a poor metal for casting. It contracts a good deal on cooling and has a tendency to absorb gases and thereby become porous, blistered, and weak. Also, molten copper exposed to atmospheric oxygen contains embrittling cuprous oxide.