Radiocarbon dating: the science that helps archaeologists ‘time travel’

One of the things I love most about archaeology is the variety of work. I’m currently working on my PhD, which means that a “typical” week could see me in the lab, on fieldwork, writing, teaching, at a conference or workshop, or meeting with people to chat about research. This week, I’m at the Australian Nuclear Science and Technology Organisation (ANSTO) in Sydney, learning to prepare charcoal samples for radiocarbon analysis. The ‘Day of Archaeology’ actually falls on my last day/travel home to Western Australia, so rather than focus on one day I thought I’d share what I’ve been doing for the past week.

Sadly for Superman, radiocarbon dating is only useful for samples that are less than ~50,000 years old. Photo credit: radio-carbon testing via photopin (license)

Sadly for Superman, radiocarbon dating is only useful for samples that are less than ~50,000 years old. Photo credit: radio-carbon testing via photopin (license)

Accurate dates are very important for archaeological research. Scientific dating of excavated material, using techniques like radiocarbon analysis and optically stimulated luminescence (OSL), provides a time frame with which to assess the archaeological record. But most archaeologists are not chemists or physicists, so we often collect and send our samples to specialist laboratories, and then wait weeks or months before we receive an email with results of the analysis with limited understanding of what goes on in the meantime. Sometimes these samples are sent off accompanied by a whispered prayer to the universe: ‘please let them arrive safely, be large enough, and be uncontaminated!’

I’ve collected and sent many samples for analysis before, but this week I was fortunate enough to travel to ANSTO to meet their researchers and learn how charcoal samples are prepared and pre-treated for analysis. This opportunity is courtesy of an AINSE research grant, and means that I’ve been able to work on samples I collected during my fieldwork last year.

ANSTO uses an isotope analysis technique called “Accelerator Mass Spectrometry”, or AMS, for radiocarbon analysis, which involves firing a sample of carbon through a particle accelerator to measure carbon isotope ratios. This method can be used to get accurate ages from very small samples (as small as 10 mg); very useful for archaeologists as we really don’t like destroying our samples!

Charcoal samples packaged and ready for submission to the radiocarbon laboratory.

Charcoal samples that were collected in the field, and then packaged ready for submission to the radiocarbon laboratory.

Before a sample can be analysed, it needs to go through a series of pre-treatments to remove contaminants, and combustion to prepare it for analysis in the mass spectrometer. The charcoal is converted into carbon dioxide, before going through a series of processes to extract contaminants (like nitrogen). The carbon dioxide gas is then heated (with hydrogen gas and an iron catalyst to remove the oxygen) to produce graphite – solid carbon – before it is finally ready to be sent through one of ANSTO’s particle accelerators (check out this video to see more about the particle accelerator, or this one for a more detailed explanation of what goes on inside it).

Sample being cleaned of excess sediment and cut into small pieces for chemical pre-treatment.

Sample being cleaned of excess sediment and cut into small pieces for chemical pre-treatment.

So this week, I’ve been dealing with the first half of this process: sample pre-treatment and combustion. It’s a fairly straightforward process – even for a non-chemistry-minded person such as myself – similar to following a recipe (albeit one that involves sterile facilities and acids!). Pre-treatment involves scraping the outside off the charcoal sample, then treating it with acid, then an alkali solution, and then acid again. The sample is then dried and repackaged into small glass vials before being combusted at around 900°C.

Samples sat in first acid wash

Samples sat in first alkali solution to remove humic acids.

So that’s been my work for this week: travel to a facility on the other side of the country, and a week in a small, sterile lab learning a new skill. Who says archaeology isn’t glamorous? As I post this, I’m packing up and getting ready to head back to the airport to fly home. It’s been a fun, challenging week as I’ve tried to wrap my head around the chemistry and physics involved in the process.

As for my samples? I won’t know how old they are for another few weeks. But having now seen them through the stages of collection, preparation and pre-treatment, I’ll be keeping a close eye on my email inbox for the results!


Many thanks to the staff at ANSTO’s Institute for Environmental Research for their efforts in teaching me about these processes. Any errors are mine.

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