https://www.iitutor.com To a large extent, cultural achievements have been defined in terms of the materials used during particular periods of human history such as the Stone Age, Copper Age (5000-3000 BC), Bronze Age (3000-1000 BC) and Iron Age (from 1000 BC). Until approximately 14 000 years ago the main materials used by humans were wood, stone, bone, shells and plant products. Archaeological evidence suggests that humans first used metals around 14 000 years ago. Gold, silver and copper can be found as almost pure metals in various parts of the world and were initially used by humans to make jewellery, ornaments and tools. The ability of these metals to be beaten and bent into various shapes and their relative scarcity made them prized possessions. Most metals however, are chemically combined with other elements forming naturally occurring compounds called minerals. At the beginning of the nineteenth century, Humphry Davy used electrolytic procedures to extract active metals such as potassium, magnesium and calcium from their salts. In turn, these active metals can be used to extract other metals from their compounds. For example, titanium can be extracted from titanium chloride by heating it with magnesium or sodium metal. Pure titanium was not isolated till 1910 although impure forms had been discovered a century before. As increasingly sophisticated methods of extracting metals from their mineral ores were discovered, new metals such as iron, and lead became available. Another important discovery was that the properties of metals could be significantly altered by combining them with other elements to form alloys, such as bronze, brass and steel. The Age of Copper began about 8000 years ago in the Middle East. Native copper, which was harder than gold, was mined and used for simple tools such as pins and hooks as well as for jewellery. The blue and green copper ores found in the mountains were used for decorative purposes. By 6000 years ago humans had discovered ways of converting these copper minerals into metallic copper by a process called smelting. This process involved strongly heating the crushed ore in a reducing environment rich in carbon (charcoal) and low in oxygen. Bellows were used to blow air into the furnace to cause the charcoal to burn and raise the temperature and produce carbon monoxide, which created a reducing atmosphere. Under these conditions the heat energy supplied was sufficient to break the ionic bonds in the lattice to release the metal. The copper particles that formed in the reaction were mixed with a glassy slag formed from the quartz present in the ore body. Further improvements in furnace design over the next 1000 years produced sufficiently high temperatures so that the copper remained as a liquid, which could be run off and cast into tools such as spears, arrowheads and axe heads. Copper was also used for kitchen utensils such as plates and cups. Its high thermal conductivity made it useful for cooking pots. Its high lustre when polished made copper sheets useful as mirrors. Bronze is an alloy of copper and other metals such as arsenic or tin. Bronze was first discovered about 5000 years ago when impure copper ores containing arsenic and lead minerals were smelted. The copper that formed had different physical properties due to the presence of these other metals. Arsenic bronze was eventually replaced by tin bronze as tin bronze was not as brittle as arsenic bronze. Tin bronze was used to make cutting tools such as knives and axes and these were superior to copper as they maintained a sharper edge that could be readily resharpened. By 4000 years ago an extensive trade in tin bronze had developed between Europe and the Middle East. Its rarity made it even more prized than gold. The high nickel content of the meteoric iron resulted in an alloy that was corrosion resistant. High temperature smelters in which iron ore was mixed with charcoal were required to produce the metal from iron ore. The charcoal burns in the blast of air to form carbon monoxide, which reduces the iron ore to iron. The earliest iron samples were spongy masses containing impurities that were often difficult to remove. Controlling the amount of carbon in the iron was a major problem. Blacksmiths developed a method of hammering the iron at red heat to squeeze out the solid impurities as well as burn off the carbon. In all these examples, pure iron was never obtained. Instead, various types of iron alloys such as carbon-steels were formed. Unlike meteoric iron, these carbon steels tended to readily corrode (rust) unless other metals such as nickel or chromium were added to the mixture. Iron and steel replaced bronze and other copper alloys in many applications because of the superior strength and hardness of iron alloys. CB3111 http://youtu.be/nfmsynWPyrk

• published: 22 Oct 2013
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