Читать книгу The History of Mining - Michael Coulson - Страница 10

2. The Bronze Age

As with the Stone Age, the Bronze Age is difficult to pin down in terms of dates, particularly as there is strong evidence that stone weapons and tools were still being used when man had begun to smelt and fashion copper into weapons, creating an extended passage of time from the Stone to the Bronze Age. It is also the case that gold was well known in the Bronze Age, so whilst we tend to define the period as one where there was a major technological breakthrough enabling copper to be alloyed with tin to create bronze for weapons and metallic items, the mining and usage of other metals, including lead and zinc, went on in tandem.

It is also the case that copper objects were found long before the onset of the Bronze Age; the oldest found, a copper pendant from Shanidar in Iraq, is believed to date from around 6500 BC. Such copper objects were unlikely to have been made with mined metal but would have been worked using native copper picked up off the ground.

The Bronze Age is usually split into periods which depend on the area of the world being discussed. The early Bronze Age in the Near East, then far more advanced culturally and technologically than Europe, began around 3500 BC and the final stage, the late Bronze Age, ended around 1200 BC when the Iron Age began to emerge. In Europe the Bronze Age continued in some places up until 700 BC. The early Bronze Age also saw the invention of writing in Mesopotamia (now Iraq), which helped provide the means for chronicling the development of society and commercial life.

Gold is mentioned, along with silver, in the Code of Menes which was promulgated by King Menes of Memphis – effectively the first Egyptian Pharaoh – in around 3100 BC. It is probable that there was knowledge of gold for millennia before because the sight of gold nuggets gleaming in alluvial settings must have caught many an ancient eye. However, the ability to work the material, other than in its native form using crude beating methods, would not come until the technological advances of the Bronze Age.

Mining methods

The crucible of the technology of metallurgy, providing the ability for man to work metal, was Anatolia (now Turkey) where tin deposits allowed the alloying of this key metal with copper in order to make bronze. This technology spread to the area which today encompasses the eastern Mediterranean/Middle East, with copper being mined in Cyprus, the Negev desert of Israel, Egypt, Persia (Iran) and the surrounding Gulf, all providing material for the metalworkers.

Interestingly the appearance of bronze did not lead to a material breakthrough in mining methods; that moment had to await the Iron Age. Bronze was considered too precious and too soft a metal to be used for mining tools, and mining continued to be done using basic stone tools. The depths mined remained modest, usually no more than 30 feet, although as the Bronze Age proceeded and demand for all metals increased, concomitantly the volumes of bronze needed increased, requiring a more organised structure with larger, probably slave, workforces.

The mining methods used were related to the tools available so stone and animal bones such as antlers were predominant. Stone hammers, made from as hard a material as possible and usually round in shape, were used to break up softer rocks in which mineralisation had been identified. Bones were used to lever loose rocks and to wedge into cracked rock in order to break it into smaller, more easily worked pieces.

As might be expected, these stone hammers were vulnerable to breakage or chipping and probably didn’t have a long life; they were also difficult to make as the stone heads had to be attached firmly to the handles. Even so, such hammers were used extensively, and in Greenland over 10,000 stone hammers were found in the 19th century close to fragments of a meteorite which hit the earth in the Stone Age. These hammers had been used to try and break pieces off the meteorite, which was essentially made of iron.

Extracting metal

If the rock that Bronze Age miners were attempting to break up was particularly hard a technique called fire setting was used. We will come across this technique – which consisted of lighting large fires close to the rock face where the metal could be seen – frequently later in the book. The fires were tended for many hours and as the rock got hotter and hotter it began to crack. The process was then completed by throwing cold water onto the hot rock face, intensifying the cracking and allowing miners to access and lever out fragments containing metal from the rock face.

A copper mine at Rudna Glava in the mountains of north-east Serbia used fire setting to mine malachite (copper carbonate ore) in very hard rock at what were then considerable depths of around 70 feet. Earlier, and at greater depths of perhaps 300 feet, fire setting was used in Egyptian gold mines to break up the hard, gold-bearing rock face. It is unlikely that there was much in the way of ventilation in such early underground mines so working conditions must have been particularly tough.

We have seen that in mining for flint the ancients had accumulated a body of geological knowledge that helped them identify promising environments for finding flint. They were also likely to use visible markers such as water colour to spot where certain metals might lie close to the surface, the water having become stained by the ore as it passed over it. Copper was known in the Stone Age but then would have been found in small accumulations in the form of nuggets and raw (native) copper on the ground, or washed out of rivers on to the bank. In due course larger quantities in the form of copper veins were found. During this extended period man would have experimented with the copper, finding that it was soft enough to shape by beating it with stone hammers.

The onset of the Bronze Age came about as a result of man developing smelting techniques to work the copper into objects of use. The key to copper smelting was to create fires with sufficient heat to turn the copper ore into molten metal and molten slag. When this had been achieved the metal and slag could be separated. The earliest smelters consisted of an open hearth made out of fire-resistant bricks. Charcoal was then loaded on to the hearth and lit, and after that copper ore was placed on top of the charcoal. The use of charcoal required a large quantity of wood – it has been calculated that 5 cubic metres of wood would produce 300 kg of charcoal, which in its turn would produce just 1 kg (2.3lbs) of copper from the ore.

The next step in the development of smelting technology was the use of crucibles. The charcoal and copper ore were put in a crucible which was made out of ceramic material. The crucible pot was then placed in a crude furnace and the temperature raised by the use of bellows. The ore would then reduce to metal and slag as in the open-hearth process. When the copper had been freed it could then be alloyed with tin, usually within a range of 3% to 8% added tin, to produce bronze. Another property of bronze was that it was much easier to cast than copper on its own – copper tends to contract as it cools becoming fractionally porous and adding tin counters this tendency allowing easy separation of the bronze item from its mould.

As the casting of bronze developed other metals were tried as additives, such as lead and zinc, but the resultant metal was not really bronze, and it had faults in terms of reduced strength and a tendency to brittleness if the added metal content rose above 3%.

Over the years ancient bronze pieces have been unearthed in a wide variety of places from the Middle and Near East, to Britain and South America. Analysis of these objects shows that different percentages of tin were used in the bronze-making process depending on what kind of object was being made. For example, a sword requiring a hard, sharp edge would use no more than 3% tin but other items such as plate might contain more. It is worth remembering that although the start of bronze making constituted a major technological step forward in the use of metals, metallurgical precision in terms of blending took centuries to perfect.

Even as the Bronze Age progressed other metals were catching the eye of blacksmiths and forgers. One of the key new metals was iron, which we come to next. Of course the coming of the Iron Age did not see the use of bronze disappear but iron ultimately increased the range of products that man could develop and so opened up another avenue for social and economic advance.