Ancient metals corrosion

Spoto, G., Ciliberto, E., Allen, G. C., et al. (2000). Chemical and structural properties of ancient metallic artefacts multitechnique approach to study of early bronzes. British Corrosion Journal 35 43 17. 

Electrochemistry has been used for more than a century in the treatment of ancient metal artifacts [281], Ideally, this technique should be able to reverse the corrosion processes that have progressively transformed the metal into an ionic compound. Depending on the conservation state of the artifact, priorities have to be attributed and the treatment will be different if consolidation, stabilization, or cleaning is privileged. 

Another ak pollutant that can have very serious effects is hydrogen sulfide, which is largely responsible for the tarnishing of silver, but also has played a destmctive role in the discoloration of the natural patinas on ancient bronzes through the formation of copper sulfide. Moreover, a special vulnerabihty is created when two metals are in contact. The electromotive force can result in an accelerated corrosion, eg, in bronzes having kon mounting pins. 

Gold [7440-57-5] Au, is presumably the first metal known and used by humans. It occurs ia nature as a highly pure metal and is treasured because of its color, its extraordinary ductility, and its resistance to corrosion. Early uses ia medicine and dentistry date to the ancient Chinese and Egyptians. In the Middle Ages the demand for gold led to the iatense, unsuccesshil efforts of alchemists to convert base metals iato gold. These pursuits became the basis for chemical science. The search for gold has been an important factor ia world exploration and the development of world trade. 

Ancient iron structures sometimes show no sign of corrosion or at most, very little. The clean atmosphere of past centuries may be responsible in that it allowed a very thin adherent layer of oxide to develop on the surface [22], This layer very often protects against even today s increasingly aggressive industrial pollutants Very often the conditions of the initial corrosion are the ones that determine the lifespan of metals [23], A well-known example is the sacred pillar of Kutub in Delhi, which was hand forged from large iron blooms in 410 a.d. In the pure dry air, the pillar remains free of rust traces but shows pitting corrosion of the iron... 

The attraction of rubbed amber and some other effects of electricity were known in ancient times. We know from finding nails in an old wreck that the Romans knew about contact corrosion combined with electric current flow. A skin of lead as a protection against boring worms covered the wooden planks of the ship and was nailed down with copper nails. Galvanic coupIe.s formed between the lead and the copper nails and the less noble lead sheets around the nails corroded in the seawater and fell off. The shipbuilders discovered a simple solution and covered the heads of the copper nails with lead as well. Galvanic current flow between the two metals was eliminated and corrosion was prevented (26). 

FIGURE 38 Lead coffin. Lead coffin (first—third centuries c.E.) from Jerusalem, Israel. Lead, widely used in many ancient civilizations, was one of the first metals to be recovered from its ores. Lead objects date back from as early as the seventh century b.c.e. In Mesopotamia molten lead was used to fasten bolts and shafts into masonry. In Syria it was made into rods used as currency, and in Greece it was cast into coins. During the Roman Empire the use of lead become so widespread that the health hazards caused by lead exposure are suspected to have been one of the factors affecting the fall of the Roman Empire. Since it is very resistant to corrosion, lead was also used by the Romans, for making coffins as the one illustrated. 

Lead. Ancient lead objects excavated from the ground are often affected by ions in the groundwater and become covered by a white layer made up of a wide range of lead corrosion products (see Table 45). Sometimes, however, the early formation of a corrosion layer of cerussite (composed of lead carbonate), provides the metal with a protective coating that prevents the progress of the corrosion process. 

Tin is an essential trace element for animals. It is soft, pliable and colorless and belongs to group IV of the periodic table, and is corrosion-resistant to many media. Tin occurs in nature mostly as the oxide mineral cassiterite and is ubiquitous in the earth crust in an abundance of 2.5 x 10-4% (Clarke s number, 4 x 10 3). It is one of the earliest metals known to mankind, and evidence of its use dates back over 4000 years. The ancients... 

The importance of tin, as Dr. F. J. North of the National Museum of Wales pointed out, cannot be correctly judged from the quantities used. Since the days of ancient Rome, it has been applied as an extremely thin protective layer, or tin plate, to other metals to make them more resistant to corrosion and safer as receptacles for foods (224). In 1941 the National Museum of Wales held a special exhibition entitled Tin through the Ages in Arts, Crafts, and Industry. ... 

Today, metals unknown in ancient times, such as aluminum and titanium, play a leading role in modern technology. These metals are widely used, for instance, in the aircraft industry because of their low densities and high resistance to corrosion. 

The purified metal is hard but malleable and ductile. Its silvery white appearance can be polished to a lustrous finish, and provides about l/4th of the American 5-cent piece. Its corrosion resistance makes it good for coinage. The ancient Chinese produced an alloy called Paktong by smelting ores containing copper, nickel, and zinc thatis now known as nickel silver and is the base for high-grade silver-plated ware. 

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