Early Bronze and Copper Technology From the Dawn of History Until Early Historic Times (2000 B. C. - 400 B. C.)
Anthropologists speak of the Copper Age, the Bronze Age, and the Iron Age as steps or stages through which societies and cultures pass in the course of their advancement toward becoming true civilizations. What is not often explained in these discussions is that there is a vast difference between a societyís use of metals and their ability to systematically extract them from their ores. Many early societies utilized naturally occurring copper, silver, and gold by taking the metal as it was found in nature and hammering it into a useful form such as a tool or ornament. These peoples were able to create beautiful and intricate objects such as jewelry, votive figurines, pins, and weapons by employing the techniques of hammering and cold working alone. In fact, some societies even made tools of the nickel - iron alloys found in fallen meteorites.
Mining and smelting ores for their metal was an entirely different story. Two major technological problems faced any ancient people who sought to reduce naturally occurring metal ores to the pure metal. The first was simply attaining an adequately high temperature, and the second was a matter of basic chemistry which was not fully understood until the early Nineteenth Century A. D. Not only did the copper bearing ores have to be heated to temperatures above about 1600 degrees Fahrenheit, but they had to be heated in the presence of a substance that would remove the oxygen which clung tenaciously to the copper in the form of oxides.
Most cooking fires never got any hotter than 1300 degrees Fahrenheit or 705 degrees Celsius. At about 1475 degrees F., (800 degrees C.), an impure lump of spongy copper will form amongst the embers of a fire, if it has been allowed to form good, hot coals which are essentially the carbon after all other portions of the wood have burned off. It takes about 2000 degrees F. (1090 degrees C.) to bring copper to a fully liquid state in which it can be cast into ingots or other useful shapes by pouring it into molds. No open fire burning naturally can attain such temperatures. In order to reach a heat of 2000 degrees, the fire must be forced. Early metalworkers accomplished this by blowing through a leather tube right into the base of the flame. In order for the tip of the tube to withstand the heat, it was often fitted with a clay nozzle. Later, the process was made a little more efficient by using a leather bellows with which an assistant could pump large volumes of air into the fire while the metalsmith tended to the crucible of hot metal or the heated metal object he was forming, welding, or hammering.
Removal of sulfur and oxygen from the ores required a different technique. Today, ores are roasted before they are smelted. The roasting removes the sulfur and other elements combining with the metal in ores. The important thing to remember is that the key to the whole process was control of the amount (or lack of) oxygen in the presence of the heated ores. In the roasting process, you want to have an oxygen rich flame so the sulfur and other elements can be easily burned off. What you have left after roasting is copper oxides. In ancient times, the roasting process most probably was not a separate step, but occurred by derault while the smelting furnace was coming up to temperature.
It is in the handling of oxygen and dealing with the metal oxides that all the ancient black magic, ritual, trial - and - error technology, closely guarded secrets, and other mysterious stuff associated with ancient metalworkers and blacksmiths comes into play. For now, the oxygen rich fire that was so helpful during the roasting phase had to now be transformed into an oxygen starved flame in order to rip those greedy oxygen atoms away from the metal to which they are so happily married in the copper oxide molecules. Now, this all had to be accomplished by a craftsman who had never a clue as to what chemistry was involved in the process and had never heard of oxygen and probably thought his powerful breath or the air from his bellows contained some special spiritual qualities that came only from some godís special blessing on metal craftsmen. In the smelting process, nature aided him in one crucial way while a whole mathematical universe of probability worked against him since he had no idea of what was actually happening. When one considers that the secrets of smelting copper ores (and later iron) were learned and lost possibly hundreds or thousands of time throughout human history or considers the number of failures, painful accidents with hot metal, or unfortunate victims sacrificed to gods, it is amazing that the processes were discovered at all. But this is the old chicken and the egg situation. Modern civilization with its knowledge of chemistry would not have come about if we hadnít learned to extract and use metals thousands of years previously. Conversely, we had to learn how to use a complicated (again, it became much more so when we started using iron) chemical process without understanding it in order to obtain the metals in the first place.
Remember that I stated in the previous paragraph that nature did work in one way to favor the early metalsmiths. The nature of a fire is that all the volatile oils and resins in wood will burn up very quickly in the early stages of a fire. Though these components burn quickly, they have neither the energy content nor the ability to produce high heat that carbon in the form of charcoal has. Then, the wood cellulose which actually contains some oxygen will first char, then burn. As this progresses, especially in the case of hardwoods like oak and maple, what is left is a glowing coal of carbon which becomes purer and purer as volatile components burn off. Charcoal is produced as part of the burning process. This natural tendency of a fire to produce essentially pure carbon is the one way in which nature came to the rescue of the early metalworkers, for pure carbon is the only fuel the ancients had available to them that could even come close to producing the temperatures needed to smelt metals. Pure carbon will burn quite hotly in the presence of a forced air draft even if air is only about 20 percent oxygen and the resulting fire will generate three products. While a fire burning naturally will produce mostly carbon dioxide and ash, with a little carbon monoxide, a forced fire will produce greatly increased quantities of super hot carbon monoxide.
Now if there is anything an oxygen atom loves more dearly than a copper atomís sweet embrace (or the even more cloying one of an iron atom in danger of being slighted and forsaken) it is the siren song of a hot carbon monoxide molecule. By very deftly controlling the oxygen flow in a semi enclosed and brick or clay lined furnace containing copper ores and charcoal formed from burning wood, the more advanced copper age metalworkers were able to produce the 2000 degree temperatures and oxygen starved (reducing, as the chemist would say) fires needed to separate copper from its various oxides and convert it into freely flowing liquid metal. In order to reach this stage. A lot of trial and error was needed by his predecessors in the art of metal refining. Fires in furnaces with less control of oxygen and fuels that were largely wood in which the charcoal making was accomplished in the same event as the metal refining were employed throughout the course of humankindís learning of the metal refining process. These gave varying degrees of success but provided enough encouragement for the craftsmen to try and modify their process time after time until they could efficiently and quickly produce the pure metal. Often the most fruitful technique would be to force the fire until the coals were good and hot and the ores and the clay or brick lining of the furnace was soaked with heat. If done correctly, the forced fire would produce enough carbon monoxide which would burn with the oxygen ripped off from the copper oxide molecules and the fire actually use oxygen tied up in the copper oxide molecules until most of the copper oxides were converted to pure copper and the oxygen from this source was gone.
One of the earliest sites discovered so far where copper was refined on a large scale is the ancient town of Tal - I Iblis in Iran. It appears that a large proportion of the townís inhabitants were employed in the mining and refining of copper. Archaeologists have discovered ancient pottery stained with residues from the copper refining process, as well as several different types of pottery kilns and copper refining furnaces.
Many archaeologists believe that these techniques for smelting copper were discovered in the process of firing ceramics in early kilns. Pottery making technology was reasonably well developed before 4000 B. C., and the potters had by that time learned much practical chemistry through their use of glazes. Though they understood it but little, they were using the properties of silicates to control whether their pottery was of the high fired type (like fine bone china today) or a medium fired variety (like a terra - cotta plant pot or roof tile) or even a low fired type (fired just enough so that it doesnít turn into mud when it gets wet). High and low here refer to the temperature and length of time the piece was kept in the kiln and determine to what degree the particles of kaolin or other clay fuse together to form glass. The chemistry of glazes was another entire field altogether and it was through controlling the temperature and the amount of oxygen present (these were closely guarded secrets) that the color, texture, and pattern of the finish were controlled. The glazes containing copper compounds were used to create the brilliant blue, aqua, and green finishes which were quite as popular with the ancients as they are today. If fired correctly, they formed a hard glass coating on the piece.
The reason for the foregoing discussion of pottery is that many researchers today believe that the discoveries of smelting techniques came about when ancient potters found bright bits of pure copper in their kilns when removing the pieces of fired pottery after the kilns cooled off. They were accomplished practical if not theoretical chemists in that they knew how to control fire and oxygen in order to produce a desired look to their finished pieces. The authors of the Time - Life Metalsmiths book used as the principal source of information in this article express wonder in print how long it took the potters to catch on that their glazes were the source of metallic copper found in their kilns. The author of this article would take a different path down the road of conjecture and what if. He contends that these were the keepers of the most advanced and complex technology of their day, and were quite experienced in the use of their powers of observation when it came to some new phenomenon associated with their firing techniques. Furthermore, their work would be in high demand by the aristocratic and ruling classes, the consumers or art and beautiful objects. It was probably part of good professional practice for these craftsmen to be on the lookout for some potentially useful or desirable byproduct of their operations. On the contrary, many were probably deliberate experimenters. What would be surprising would be if they did not make a connection between the most brilliant blue and green minerals used in their glazes and the work of their contemporary metalsmiths who were taking similarly colored malleable rocks, hammering off the green malachite scale covering them, and cold forming copper weapons and jewelry. It is simply human nature to wonder about the possible implications of a curiosity or a discovery, and to make connections with other little bits of knowledge that come their way.
Bronze and brass are alloys of copper. Ancient people found that the addition of tin to their copper would aid the molten metal in filling the mold better and would result in a harder, stronger, and tougher metal. Alloys of copper and primarily tin are called bronze. Some bronzes were refined from naturally occurring ores containing tin as well as copper. The rich metal deposits of Anatolia and Asia Minor were an ancient source of ores. Cornwall in the western part of the island of Britain was a source of tin dating back to prehistory. The Mycenian Civilization was known for its advanced bronze technology and they produced many excellent weapons and objects of art in this alloy. A beautiful example of Mycenian plate armour is featured Dr. Johanna Keirns' Tour of Greece's ancient places.
Zinc was added to copper to form brass in much later times. Brass had a much lighter, yellower colour than copper, which is a reddish colour in its pure state. While the deliberate production of bronze was carried out from about 2000 B. C. on, brass was not used extensively until Roman times. An alloy of brass consisting of 80 percent copper and 20 percent zinc, called orichalcum by the Romans, was the metal used to strike the Roman coin denominations known as the Sestertius and the Dupondius.
Time - Life The Metalsmiths story of copper, pp. 30 - 53. bronze, 54 - 81. smelting temperature of copper, p. 36. Association of discovery of copper refining with early potters, p. 37. early copper refining centre of Tal-I-Iblis, pp. 41, 44.
Wonderful images of ornate Roman bronze parade and gladiatorís helmets can be seen in the following works:
The Colosseum by Peter Quennell, front of dust jacket (Wow!) and p.47
Treasures of Ancient Rome by Peter Clayton, p.150. images of bronze heads wearing ornate helmets, pp. 148 - 149
The Great Invasion by Leonard Cottrell,. images of various types of helmets in use by the Roman army in Britain, center section of plates between pp. 64 and 65
Sinnigen and Boak Page 12
Grant Page 14