Iron

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Iron

Materials and technology

Materials

Iron can be used alone and in alloys in a variety of ways. However, iron requires some processing before it can be used to shape the objects used by people. Iron is often found on archaeological sites in the form of wrought or cast iron.

Wrought iron is a fairly pure form of iron containing >0.1% carbon and 1-2.5% glassy slag inclusions. The inclusions give wrought iron its characteristic fibrous structure - the slag is mostly composed of iron silicate fayalite (Selwyn 2004). Wrought iron is tough, malleable, and remains strong under tension. Wrought iron is used to produce hardware, hinges, chains, fences, wire, etc.

Steel is an iron/carbon/other alloy with a carbon content of about 0.2-2%. The additional carbon makes steel harder and stronger than wrought iron. High carbon steels are generally >0.8% carbon. Stainless steels have low carbon contents (<0.15%) and contain nickel along with at least 10.5% chromium (Scott and Eggert 2009). Depending on the alloy, steel may or may not be magnetic. Rapid cooling, or quenching, hardens steel; tempering, or reheating, lowers hardness and restores ductility. Carbon content and the temperatures used for hardening/tempering steel can be adjusted to achieve a wide range of hardness, strength, and ductility. Steels are used for tools, weapons, construction, etc.

Cast iron alloys contain 2-4% carbon with varying amounts of impurities. Cast iron is hard but also more brittle than wrought iron or steel. Cast iron has been used to make cooking vessels, cannons and cannonballs, stoves, architectural features, etc.

Sections of iron can be joined through welding. Through this process, oxidation on the surface is removed by mechanical or chemical means, which allows iron with differing carbon contents to be joined. Wrought iron sections can also be joined by overlapping edges and hammering them together until a cohesive surface is formed (Cronyn 1990).

Deterioration

Despite some differences in composition, both kinds of iron corrode in similar ways. If iron is exposed to a moist, oxygen rich environment, it will begin corroding from the surface towards the object’s core. The metal in the object may disperse into the surrounding environment, be deposited within another object, or form a crust around the object itself (Cronyn 1990). These corrosion products may have differing hydration states, depending on the environment that the iron was recovered from (Hamilton 2000). In dry environments, a thin patina or crust forms on the surface of an iron object. As the humidity increases, the thickness of the crust increases (Cronyn 1990). These crusts can either protect the remaining metal within the object, or spall off, leaving a new surface to corrode (Hamilton 2000). In high humidity terrestrial environments, iron can be reduced to a shapeless red-brown lump of iron oxides that preserve little to no metallic iron. Wrought iron will corrode, leaving sections of slag surrounded by corrosion products. After interacting with sulphates or phosphates, iron will exhibit a black or blue-black surface layer. Iron can also form green films, due to the presence of hydrated oxides (Cronyn 1990).

Iron found on marine sites exhibit interactions with their environment. Concretions of calcium carbonate, shell, and other debris will form around iron (Cronyn 1990). This layer of concretion can actually slow or stop galvanic corrosion, which occurs when two metals with different electrochemical properties are in contact with each other or very close to one another in an electrolytic solution (Hamilton 2000). Inside these concretions, some or all of the iron can be reduced to oxides due to the reduction of iron. And as in terrestrial sites, wrought iron will corrode along the slag stringers within the object. Cast iron may become graphitized under concretions, reducing the iron to corrosion products and graphite. In this case, the cast iron object retains its original size and shape, but loses density and mechanical strength (Cronyn 1990, Hamilton 2000).

Depending on the composition of iron and the environmental factors it is exposed to, iron can form a variety of corrosion products. These include ferrous hydroxide, ferro-hydroxide, anhydrous and hydrous forms of ferrous chloride, ferrous sulfide, ferro-ferrous oxide (magnetite), hydrated magnetite, ferric hydroxide, ferric oxide, and anhydrous and hydrous forms of ferric chloride. Of these products, the ferrous chlorides are the most dangerous to iron. Ferrous chlorides will form ferric oxide or ferric hydroxide and hydrochloric acid when exposed to oxygen and moisture (Hamilton 2000). While iron may appear stable during and after excavation, corrosion can easily begin, or accelerate, after the artifact is removed from its original location. This is due to changes in humidity and oxygen levels. Therefore, the same elements that pose a threat to the survival of iron artifacts in archaeological contexts can continue to be an issue long after excavation unless humidity and oxygen levels are controlled (Cronyn 1990, Hamilton 2000).

References

Cast iron. Encyclopædia Britannica Online, s. v. http://www.britannica.com/EBchecked/topic/98324/cast-iron (accessed 03/18/13)

Cronyn, J. M. 1990. Elements of Archaeological Conservation. New York, New York: Routledge.

Hamilton, D. L. 2000. Metal Conservation: Preliminary Steps. http://nautarch.tamu.edu/CRL/conservationmanual/File9.htm (accessed 03/18/13)

Scott, David A. and Gerhard Eggert. 2009. Iron and Steel in Art: Corrosion, Colorants, Conservation. London: Archetype Books.

Selwyn, Lyndsie. 2004. Metals and Corrosion: A Handbook for the Conservation Professional. Ottawa, Ontario: Canadian Conservation Institute.

Further reading

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