Ancient Metals and Alloys

The corrosion of metals and alloys generally starts at the surface with the formation of an outer layer, which may develop into a crust of corrosion products. If a crust is formed, it generally has a layered structure comprising two or more compoxmds (1) an outer, rather stable, mineralized layer that often covers entirely the surface of the objects, and xmdemeath, (2) a less mineralized, unstable, and chemically active layer. Some corrosion layers may also bind ugly and disfiguring earthy accretions. 

Copper and the Copper Alloys. Copper and its alloys are relatively resistant to corrosion dry, xmpolluted air rarely affects them at normal temperatures surfaces of the metal or its alloys exposed to polluted air, even under ordinary atmospheric conditions, however, are tarnished by pollutants such as hydrogen sulfide and/or carbon dioxide. Given sufficient time, the activity of the pollutants result in the formation of a usually green layer, known as patina, which coats and surroxmds the bulk of the metal or alloy (see Fig. 40). If the patina is chemically stable, that is, if it is hard, is non-porous, and covers the entire surface of an object, it protects the xmderlying metal core from further corrosion. Such a patina consists mostly of basic 

Most corrosion processes in copper and copper alloys generally start at the surface layer of the metal or alloy. When exposed to the atmosphere at ambient temperature, the surface reacts with oxygen, water, carbon dioxide, and air pollutants in buried objects the surface layer reacts with the components of the soil and with soil pollutants. In either case it gradually acquires a more or less thick patina under which the metallic core of an object may remain substantially rmchanged. At particular sites, however, the corrosion processes may penetrate beyond the surface, and buried objects in particular may become severely corroded. At times, only extremely small remains of the original metal or alloy may be left rmdemeath the corrosion layers. Very small amounts of active ions in the soil, such as chloride and nitrate under moist conditions, for example, may result, first in the corrosion of the surface layer and eventually, of the entire object. The process usually starts when surface atoms of the metal react with, say, chloride ions in the groundwater and form compounds of copper and chlorine, mainly cuprous chloride, cupric chloride, and/or hydrated cupric chloride. 

Under favorable environmental conditions, a chemical equilibrium is established between a corroded surface layer and its surroundings, which may lead to the preservation of the bulk of copper thus ancient objects made 

Bronze disease is the name given to a form of corrosion of bronze and some other copper aiioys, in which iight biue-green outgrowths form on the surface (see Fig. 41). it is an especiaiiy obnoxious form of corrosion that particuiariy attacks ancient excavated bronze objects. Unless terminated by speciaiized treatment soon after excavation, bronze disease usually results in the complete corrosion and total destruction of the object. 

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Of all the ancient metallic artifacts that have been left from antiquity, coins are among the most numerous. Since ancient times coins have generally been made from coinage metals or, mostly, from coining alloys, whose chemical and physical properties and economic qualities make them suitable to be used for this purpose. Until the twentieth century, gold, silver, copper, and their alloys were practically the only metals from which coinage was made. All these metals and alloys have the following properties ... 

Cf. Paul Diergart, Journal fur PraTctische Chemie, Ncuc Folge, Vole. 61, 66, 67 Zeitschift fur Angewandte Chemie, 1903. Cf. also J. A. Phillips, Metals and Alloys Known to the Ancients, Journal of the Chemical Society, Vol. 4, p. 252 f. 

Traditions of ancient writers attribute some discoveries in these lines to India or Persia, or other Asiatic countries, but as to whether any of these countries contributed in any important way to the development of Egyptian chemical knowledge, or whether at some time these countries learned their arts from Egypt, we cannot safely determine from such tradition. It is quite certain that both in China and in India the chemistry of the metals and alloys, methods of dyeing and the use of certain chemicals in medicine were practiced at ancient periods, but their chronology is diffi-... 

Atomic absorption is suitable for the analysis of several types of ancient inorganic materials, e.g. metals and alloys, silicates and minerals. Only a few milligrams of sample are required typically 10 mg may be dissolved in 25 ml for analysis. Electrothermal methods may require even less sample and are thus very attractive in this field. Often papers describing results obtained by atomic absorption give little or no analytical details. Table 4 lists some of these publications to illustrate the potential scope of atomic absorption spectrometry in archaeology, but the review of Hughes et al. [210] remains the best source of experimental detail. 

Metallurgy is like glass-making chemistry at high temperatures and it has been of great importance in the discovery of elements, especially metals. Ancient metallurgy was of course not carried out to discover new elements but to make metals and alloys for tools, weapons and objets d art. In the history of element discoveries, however, we often meet metallurgical furnaces and processes. 

Tin [7440-31 -5] is one of the world s most ancient metals. When and where it was discovered is uncertain, but evidence points to tin being used in 3200—3500 BC. Ancient bron2e weapons and tools found in Ur contained 10—15 wt % tin. In 79 ad, Pliny described an alloy of tin and lead now commonly called solder (see Solders and brazing alloys). The Romans used tinned copper vessels, but tinned iron vessels did not appear until the fourteenth century in Bohemia. Tinned sheet for metal containers and tole (painted) ware made its appearance in England and Saxony about the middle of the seventeenth century. Although tinplate was not manufactured in the United States until the early nineteenth century, production increased rapidly and soon outstripped that in all other countries (1). 

The fundamental goal in the production and appHcation of composite materials is to achieve a performance from the composite that is not available from the separate constituents or from other materials. The concept of improved performance is broad and includes increased strength or reinforcement of one material by the addition of another material. This is the well-known purpose in the alloying of metals and in the incorporation of chopped straw into clay for bricks by the ancient Egyptians and plant fibers into pottery by the Incas and Mayans. These ancient productions of composite materials consisted of reinforcing britde materials with fibrous substances. In both cases the mechanics of the reinforcement was such as to reduce and control the production of cracks in the brittle material during fabrication or drying (2). 

Relatively soon after ancient humans recognized the metals and their special properties, they also discovered ways to make alloys. Some alloys were produced in antiquity directly, by the smelting of ores that include two metals in their composition or mixtures of ores of different metals. Arsenical copper, bronze, and brass, for example, three alloys of copper... 

Calcium oxide was used in ancient times to make mortar for building with stone. Both the metal and calcium compounds have many industrial as well as biological uses. Metallic calcium is used as an alloy agent for copper and aluminum. It is also used to purify lead and is a reducing agent for beryllium. 

Tin is anther ancient metal that continues to have a variety of uses. The inorganic form is used in food packaging, solder, brass, and as an alloy with other metals. The organic forms of tin, triethyltin and trimethyltin, are used as fungicides, bactericides, and generally as antifouling agents for boats. 

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