Durability of Metals from Archaeological Objects

Don’t ask me why I was curious about this subject. Ok…I will tell you. For entertainment, I sometimes watch Turkish TV shows (from YouTube). Conspiracy theorists are ubiquitous on Turkish TV. They make bold claims. Almost all their claims are questionable and some of them are flat wrong.  One of these claimed that all metals disintegrate in 1400 years. He is wrong, of course. Many objects made from gold, silver, copper, bronze, iron, lead, and tin have survived for several thousand years.

This old report from 1980 explains clearly.

Durability of metals from archaeological objects, metal meteorites, and native metals 

Note: the web site provides access to the PDF document but delivery is very slow.

Abstract:

“Metal durability is an important consideration in the multi-barrier nuclear waste storage concept. This study summarizes the ancient metals, the environments, and factors which appear to have contributed to metal longevity. Archaeological and radiochemical dating suggest that human use of metals began in the period 6000 to 7000 BC. Gold is clearly the most durable, but many objects fashioned from silver, copper, bronze, iron, lead, and tin have survived for several thousand years. Dry environments, such as tombs, appear to be optimum for metal preservation, but some metals have survived in shipwrecks for over a thousand years. The metal meteorites are Fe-base alloys with 5 to 60 wt% Ni and minor amounts of Co, I, and S. Some meteoritic masses with ages estimated to be 5,000 to 20,000 years have weathered very little, while other masses from the same meteorites are in advanced stages of weathering. Native metals are natural metallic ores. Approximately five million tonnes were mined from native copper deposits in Michigan. Copper masses from the Michigan deposits were transported by the Pleistocene glaciers. Areas on the copper surfaces which appear to represent glacial abrasion show minimal corrosion. Dry cooling tower technology has demonstrated that in pollution-free moist environments, metals fare better at temperatures above than below the dewpoint. Thus, in moderate temperature regimes, elevated temperatures may be useful rather than detrimental for exposures of metal to air. In liquid environments, relatively complex radiolysis reactions can occur, particularly where multiple species are present. A dry environment largely obviates radiolysis effects.”

The summary section of the report:

“6.2 SUMMARY OF METAL DURABILITY

Gold has been durable in essentially every environment. Silver, while durable in most environments, including seawater, has been subject to some corrosion, particularly by chlorides.

Copper has an impressive record of durability on land and in seawater. Float copper having evidence of glacial action has shown low rates of postglacial corrosion. Bronze, with better mechanical properties also has survived for up to five millenia in favorable environments. Tylecote [1] indicates that bronze canisters could be engineered for use in seawater for at least 1,000 years.

Over the centuries, archaeological iron appears to be less durable than the other materials mentioned above. Even so, ferrous materials have shown impressive durability in several environments, exemplified by the iron articles in the Tutankhamen tomb (1300 BC), the iron pillar of Delhi (400 AD), and cast iron water pipes (Versailles, France, since 1665 AD). Metal (Fe-Ni) meteorites show a wide variety of corrosion responses, but some with estimated terrestrial ages of 5,000 to 20,000 years have survived impressively. However, Tylecote [1] was not impressed with iron durability in seawater.

Lead sheathing on ancient ships and in Roman water systems held up sufficiently well to merit recommendation for 1,000 year service in seawater. Tin has not held up well in seawater, but a tin bracelet dated to 2500 BC appears to be well-preserved. Pewter has held up quite well in seawater over several centuries.”

[1] R. F. Tylecote. Durable Materials for Seawater: The Archaeological Evidence. British Nuclear Fuels Limited (BNFL), Report 314 (R), 1977.