Metals and Metallurgy

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METALS AND METALLURGY. Metals comprise a large group of chemical elements which are distinguished from non-metallic substances by their high conductivity for electricity and heat, properties resulting from the presence of “conduction” electrons that are free to move about within a metal, not being bound by specific atoms. Metals are also characterized by their high reflectivity for light. A polished sheet of metal, called a speculum, was used in ancient times as a mirror (cf. 1 Cor 13:12). However, the widespread use of metals in ancient times, and to a considerable extent today, is dependent upon other properties which permit them to be shaped by hammering, melted and cast into molds and alloyed with other metals. Alloying is carried out to increase strength and improve other properties.

The metals used in pre-Biblical and Biblical times were almost entirely copper, gold, iron, lead, tin and silver (q.v.), although mercury and zinc also were used. Some of the properties of these metals are:

Of these metals copper and gold commonly occur in the native state with gold almost certainly the first metal known to and used by man. It is too soft to be used for weapons or tools, but much used for jewelry and decorative purposes. Native copper is also soft, but it was found that it hardened appreciably when hammered and so was used for making weapons such as daggers and tools such as sickles. Terrestrial native iron is rare, but the majority of meteorites, which are extraterrestrial bodies, are mainly iron with some nickel. This metallic material was used before 4000 b.c. as was gold and copper, with the common use of copper c. 4500 b.c. heralding the Chalcolithic age (copper-stone age).

Silver also is found in the native state and its use by man for jewelry and decorative purposes began c. 4000 b.c. The use of lead, tin, mercury and zinc was dependent upon metallurgical discoveries relating to their smelting, refining, alloying and working as was the extension of the use of copper as the copper-tin alloy, bronze (q.v.). This was also the case, much later for the copper-zinc alloy, brass (q.v.), and for the common use of iron.

Metallurgy is the science and technology of metals. It covers the processes of producing metals by extracting them from their ores, of the refining and purification of these ores and the working of them mechanically or alloying them to adapt them for various uses.

The development of metallurgy during the pre-Christian era can be summarized as follows:

Before 4000 b.c. Native gold, copper and meteoric iron hammered into shape, with the copper and iron hardened; melting, casting and annealing of copper.

4000-3000 b.c. Native silver hammered into shape; reduction of oxidized ores of copper (e.g. malachite, q.v.) and lead; smelting of natural mixed ores to produce copper alloys, including bronze; melting and casting of copper alloys; accidental reduction of oxidized ores of iron.

3000-2000 b.c. Smelting of copper sulphides and tin oxides with metallic tin becoming an important item of trade; production of sponge iron; extraction of silver by cupellation with lead; making of gold leaf and metal wire.

2000-1000 b.c. Bellows used in furnaces; iron reduced from ore and forged without melting to produce wrought iron—important by 1600 b.c.; steel made by carburization in a hearth and by 1200 b.c. hardened by quenching; brass made from copper and zinc ores c. 1500 b.c. (not important until 200 b.c.); high-tin bronze (speculum) for mirrors.

1000-0 b.c. Vast expansion in production of metals, particularly iron; iron and steel welded into composite tools and weapons; mercury distilled from ores; separation of gold by amalgamation with mercury; stamping of coins (c. 700 b.c.); more general use of bronze.


Metallurgy of gold. The gold of the ancient Near E occurs in the native state with that used by early man recovered from stream sands and gravels where the gold is present as small flakes, or sometimes as nuggets. This gold was recovered by washing away the other mineral grains of the sand which have a density about one sixth that of gold. Washing also was used to separate gold mined from veins, after the ore had been ground to a small size (mines and mining, q.v.).

The washing of gold ores is depicted on Egyp. monuments of the 1st dynasty (2900 b.c.). The simplest and earliest means of washing was by hand, in pans. Other means used included washing the stream sand or crushed ore over a sloping table or by sending the material down an inclined sluice with transverse ripple bars behind which the gold collected. The legend of the Golden Fleece was based on an expedition (c. 1200 b.c.) to Armenia to obtain alluvial gold by washing gold-bearing sands over sheepskins.

By Rom. times native gold (and silver) were extracted from ore by means of mercury, the process being called amalgamation. The ores are crushed in water and mixed with mercury while being agitated. The metallic gold (and silver) adheres to the mercury (quicksilver) and particles of the amalgam adhere to one another. These aggregates become large enough and heavy enough to sink in running water which washes away the other mineral particles. This is much more efficient than just washing gold-bearing material with water. The amalgam is separated by heating in retorts. The mercury is driven off as a vapor, condensed and reused, while the gold is melted and cast.

Whatever means of separation used, the gold generally contains other metallic elements. An early method of refining was heating the gold with lead, salt and barley bran which act respectively as scorifier, flux and reducing agent. This was done in an airtight clay crucible which remained in a hot fire for five days. By this time only the gold remained, with the other components of the charge absorbed by the clay of the crucible. Sometimes tin was added to the charge to harden the gold. The separation of base metals, such as copper and tin, also was carried out from a very early time by the method of cupellation. The gold to be purified is melted with lead, which is oxidized by the oxygen of the air. The molten lead oxide forms a slag into which the base metals go and with which they are separated off from the refined gold (cf. 1 Chron 28:18; Mal 3:3). However, any silver remained. From c. 600 b.c. onward this silver was separated from the gold by heating in a crucible with salt. The silver is converted to silver chloride, which passes into the molten slag, leaving the gold.

Methods of working gold were developed in ancient times. Soldering with gold-copper alloys was known before 3000 b.c. Before 2500 b.c. most jewelry techniques, such as inlay, stamping, repoussé and granulation, were known. Gold was hammered into thin gold leaf and wire made by cutting sheet. These various techniques were used more than a thousand years later by the children of Israel (e.g. Exod 25:31; 39:3).

Metallurgy of copper. Native copper often occurs as large lumps. Though soft, it is hardened appreciably when hammered and the first fabricated metallic articles used by man for other than adornment were made of copper. This was prob. 8000 b.c. More than two thousand years later it was found that copper could be melted (at 1083oC) and cast into desired shapes.

The reduction of copper ores to metallic copper in a red hot charcoal fire was, almost certainly, a repeated campfire accident, possibly where brightly colored oxidized minerals of copper (turquoise, malachite, q.v.) were being mined for ornamental and decorative purposes (mines and mining, q.v.). The next step was to make a hole in the hearth to collect the molten metal and to line this with clay, a material which pottery manufacture had demonstrated to be fire resistant. Subsequently rudimentary furnaces, enclosed by stones, evolved.

The copper ore initially smelted was the weathered, oxidized portion of the lode that cropped out at the surface and could be mined using wooden shovels, antler picks and flint hammers. The copper produced from such weathered surface outcrops in the ancient Near E before 2500 b.c. contained only 0.5 percent impurities. Subsequently the realization that there was copper in the deeper, unweathered parts of the rock mass led to the mining of less pure ore and the production of copper with 2-3 percent impurities. However, this metal was both harder and much easier to cast than the purer metal. Its production was the first step toward the deliberate mixing of ores and the production of various alloys of copper, of which bronze was the most important.

Bronze was made by smelting copper and tin ores together with charcoal, using a forced draught. This was created, before 1800 b.c., with the lungs; later bellows were used. The draught was through a non-inflammable clay nozzle, with the molten metal collected in a clay crucible and then cast into ingots, or directly into molds. Later bronze was made from copper and tin previously reduced from their ores. The copper-zinc alloy, brass, was initially produced by heating copper with charcoal and smithsonite, the naturally occurring zinc carbonate. Later it was made from copper and zinc, both previously reduced from their ores.

Metallurgy of lead. Lead is reduced easily from its ores, particularly the oxidized ores such as lead carbonate (cerussite). The earliest method of smelting, which may have been the first metallurgical process practiced by man, was to place the ore with wood in a hole in the ground, and fire it. The lead that was produced then ran along a gutter to a second hole, where it was collected. In the case of the chief lead ore, galena (lead sulphide), roasting is carried out in an oxidizing atmosphere. At a moderate temperature lead oxide and lead sulphate are formed from the lead sulphide. With increased temperature, and assisted by the addition of a small amount of flux (e.g. quicklime), the remaining lead sulphide reacts with the two oxidized products to produce lead and the gas sulphur dioxide.

Any copper, antimony or bismuth are oxidized and form a scum on the surface, mixed with a little lead oxide (litharge). This is taken off. The metal is desilvered by cupellation.

Metallurgy of silver. The earliest source of silver was native silver which occurs mainly in an upper, secondarily enriched zone of silver lodes, as at Laurion, Greece. Subsequently silver was extracted from its ores by smelting with lead in a simple furnace, often following a preparatory roasting in the open air. The resultant lead-silver alloy is melted on a flat dish (cupel) of bone ash or marl. The lead, together with any other base metal impurities are oxidized by an air blast directed at the surface of the molten metal. The impurities are skimmed off (dross—Ezek 22:18) and the last portions of the oxidized impurities are absorbed by the porous cupel. Only the silver, free from base metals, but containing any gold or platinum that may have been present, remains. This process of cupellation is thought to have been used by the Babylonians.

Metallurgy of tin. Almost the sole ore of tin is cassiterite (tin oxide). This is an uncommon mineral in the ancient Near E, but a metal in which the Phoenecians traded (cf. Ezek 27:12), particularly with Cornwall, England. Cassiterite was smelted in a hole in the ground by means of a charcoal fire and a forced draught. The tin oxide reacts with the carbon of the charcoal with the production of tin and carbon monoxide gas. To assist in obtaining the temperature needed, the furnace prob. had alternate small amounts of ore and burning charcoal added while the forced draught was in operation.

Metallurgy of iron. The earliest metallurgical working of iron was cold hammering with flint tools, of meteoric iron. Ornaments were fabricated and weapons and tools made. Native iron prob. was first reduced from its ores in large camp fires adjacent to rocks containing the oxides of iron-magnetite, haematite and limonite. This accidentally smelted product would have been a dark spongy mass, not at first recognized as a metal, while remnants of unreduced ore would have rendered the mass non-malleable, and so useless. Only when air was excluded during cooling, following a sufficiently high fire temperature (800-900oC), would a coherent lump of metal have been produced. Hammering, aided by heat, welded such small pieces of sponge iron into larger pieces and hardening took place if heating was followed by sudden quenching in water.

The slowness of the ancients to make this discovery, which cleared the way for the massive use of iron and opened the door to the Iron Age, prob. resulted from their experience with copper, a metal which softened when heated and was unaffected by quenching. However, once the secret of producing hard wrought iron was discovered, it was jealously guarded, in turn by the Hitties of Asia Minor, and then by their conquerors, the Philistines (cf. 1 Sam 13:19, 20). The method involved using forced draught in pits or primitive furnaces in which the iron ore was reduced to metallic iron by charcoal. Then the glowing ball was pulled out of the furnace (cf. Deut 4:20; 1 Kings 8:51; Jer 11:4) and while still white hot hammered vigorously (forged), both to expel slag and to weld the hot metal into a coherent mass. The iron was not melted and the product was wrought iron. Accidentally, and later by design, ordinary iron was subjected to carburization when it was reheated in a charcoal forge. In this way additional carbon was absorbed with the resultant product being steel. These methods used by ancient man, with modifications and improvements of equipment and technique, produced all the iron up to the 14th cent. Only then was liquid pig iron, requiring temperatures in excess of 1500oC, and cast iron produced.

Metallurgy of zinc. The preparation of the metal zinc referred to as mock silver, by heating the oxide with coal was described about 7 b.c. However, the zinc of brass almost certainly came from smelting smithsonite (zinc carbonate) with charcoal and with copper. Smithsonite was known from the silver mines of Laurion, Greece.

Metallurgy of mercury. The mercury used for separation of gold from its gangue was made, as at present, by roasting cinnabar, the naturally occurring mercury sulphide, in a current of air. The mercury vapor is carried on with the air and the liberated sulphur dioxide and is condensed by cooling. The cinnabar would have been obtained from Spain or Italy.

Bibliography

T. A. Richard, Man and Metals. A History of Mining in Relation to the Development of Civilization, Vols. I and II (1932); J. R. Partington, A Textbook of Inorganic Chemistry, 6th ed. (1950), 776-780, 786-789; F. C. Thompson, “Metallurgy,” EBr, 15 (1970), 232-235; also references under Copper, Gold, Iron, Lead, Silver, Tin.