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April 2018

Old and new carbon

In the early 1950s, only a few years after Willard Libby’s invention of the technique of radiocarbon dating, the Museum of Applied Science in Melbourne decided to establish a dating laboratory. The basics of the technique are probably well known to chemists but – just to recapitulate – cosmic ray neutrons interact with nitrogen-14 nuclei to produce carbon-14 and a hydrogen nucleus. The carbon-14 is radioactive, losing an electron (β-particle) to revert to nitrogen-14, with a half-life of about 6500 years. In living organisms, an equilibrium concentration of carbon-14 is maintained by constant exchange with the atmosphere, but when an organism dies this equilibrium is no longer maintained and the concentration of carbon-14 falls steadily as the atoms decay. Ages up to about 40 000 years can be determined by careful counting. Libby checked the validity of his method by determining the age of wood from archaeological sites of known longevity, and he also showed that really old materials such as natural gas and coal had no remaining radioactivity.

Libby burned the wood or charcoal, and treated shells with acid, to get carbon dioxide. This gaseous product was reduced with magnesium metal to give solid carbon, which was coated on the inside of a Geiger counter. The game had moved on a bit by the time chemist Anne Bermingham was setting up the Melbourne laboratory: she converted the carbon back to carbon dioxide and filled a proportional counter with it. The Museum’s main client was E.D. Gill, curator of fossils at the National Museum of Victoria, who had already begun to send samples of charcoal from Aboriginal hearths to Libby and to other overseas dating facilities, before the Melbourne one opened.

At first, a vacuum line was constructed and sample preparation was carried out in the basement of the Chemistry Department of the University of Melbourne, with assistance from staff there. In the mid-1950s, the facility was transferred to the Museum and its official opening took place in 1961, but it was always under-resourced and this had consequences. The laboratory struggled through the 1960s and it was eventually closed down in 1970. My article about it has just been published in Historical Records of Australian Science.

Other combustion-based radiocarbon dating laboratories opened at the University of New South Wales, the University of Sydney, and – the only one still operating – the Australian National University. Dates are also determined at ANSTO where the measurement is done by accelerator mass spectrometry.

In my wider reading, I came across a lot of information that did not fit into the main narrative but was nonetheless of considerable interest. For example, carbon-14 dating has been used to date historic iron samples. Since the Industrial Revolution, iron and steel production has been based on coke that is in turn produced from coal that is millions of years old and therefore bereft of radioactive carbon-14. Before then, iron was produced with wood charcoal that, at the time of combustion, did contain measurable concentrations of carbon-14. When first produced, the iron (cast iron) can contain up to 5% carbon, and that’s enough to enable dating if a large sample is taken. Burning at about 800°C serves to release carbon dioxide, which can be purified in traditional fashion via calcium carbonate and used in a counter.

South Africa’s N.T. van der Merwe did his PhD at Yale in the late 1950s, applying the carbon-14 dating technique to samples of American colonial cast iron. Archaeological evidence suggested the iron was from the 17th and 18th centuries, and this was confirmed by carbon dating. For example, iron from a Massachusetts site where production had occurred in the period 1648–78 was dated as 1600 ± 60. Older iron was available from excavations in China; for example, one from Szechwan thought to be from 221–220 BCE was dated at 180 ± 100 BCE. Back home, van der Merwe turned his attention to local sites. Charcoal excavated from ancient Transvaal dwellings had carbon-14 dates in the range 960–1130 CE, and 19th century dates were assigned to slag from some iron and copper smelters.

Some carbon-14 determinations are made not to see how old a sample is, but to see whether its carbon is ancient or modern. Since most industrial chemicals are manufactured from coal or petroleum sources, they will not contain any carbon-14. An organic natural product, by contrast, will be more or less in equilibrium with the global pool of carbon-14 and therefore easily distinguished from its industrial cousins. It has been demonstrated that certain marine natural products that contain bromine attached to aromatic rings, such as the polybromodiphenyl ethers, are natural products and not synthetic flame retardants. And it has been shown that certain biofuels really were ‘bio’ rather than just look-alike products of the petrochemical chemical industry.

Ian D. Rae FRACI CChem ( is a veteran columnist, having begun his Letters in 1984. When he is not compiling columns, he writes on the history of chemistry and provides advice on chemical hazards and pollution.

Some carbon-14 determinations are made not to see how old a sample is, but to see whether its carbon is ancient or modern.

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