Locked in time

I have started this post many times and put it to one side, as I struggled to find the best explanation of carbon -14 dating!   So here goes!

Rob collecting charcoal on site at Golden Cap

Radio carbon 14 is a radioactive isotope and is  a method  to date organic materials like  wood and bone by  measuring the amount of carbon-14 they contain. All living things take in carbon 14 directly or indirectly from the air they breath and the food they eat, it’s all to do with carbon dioxide. The amount of carbon-14 in the air is a small amount but has been more or less at a constant level for thousands of years, adjustments can be made to compensate for the fluctuations in levels. Once an organism dies, it stops taking in carbon-14, and the carbon-14 it contained at the time of death decays at a set rate and the radioactivity of the material decreases. Calculations are made and results are produced at different levels of probability, and depending on the sample can produce a narrow range of dates. (thanks to the help of the  BBC education web site) 

Charcoal under the Bronze age barrow on Golden Cap

Charcoal is the most popular sample for dating but burnt grain is even better, due to type of tree the charcoal is from. If it is from an oak tree then the tree may have been hundreds of years old already when the wood was burnt and can produce a wide date range. Grain, even if stored, will only have had a short time to take in the carbon so it will produce a better result.

When collecting a sample you need to make sure you have enough and also to bag it so it doesn;t get contaminated by other samples or other carbon sources. These days techniques are so refined that we do not usually wrap samples in tinfoil as we did in the 1980s, but we still ban smoking in the trenches  for the health of our samples and diggers!

All non smokers on sites were always jealous of smokers as they had more breaks from digging than us!

Charred grain and seeds collected from soil samples

Hopefully this has made som sense, I am not very scientific minded as I lean more to the arts side of archaeology!  We are at the moment waiting for some dates from our excavations at Chedworth, as the six to eight weeks deadline approaches. I will post the results when we have them.

One of the charts showing results from samples from Badbury Rings

Burnt grain you can see the details of the grain hull