Radiocarbon is a destructive process, but with the advent of AMS dating, milligram sized carbon samples are now required. Bone is sometimes challenging to date because of the often low preservation state of the collagen that is found in ancient bone, such as those from the Palaeolithic. Usually we need 400-700 mg of bone for a good attempt at an AMS date, which can make a large hole in a bone or an ugly cut if a saw is used to sample it. To overcome this, in Oxford we have developed a keyhole sampling technique that we apply to precious samples. This involves drilling a 2 mm wide hole into an artefact and then widening the inside of the hole, causing little significant external damage and making it possible to preserve rare and important objects to a much greater extent than previously has been the case.
Recently, we visited the Hungarian National Museum in Budapest to sample a series of bone projectile points from a series of key sites along the Danube corridor for Rachel Hopkins' doctoral research project. Dr Andras Marko (pictured above left) is the curator of the material. He was understandably a bit nervous about our drilling programme. These samples are 30-40,000 year old bone points from the early Upper Palaeolithic, and are quite rare across Europe. They are often displayed in museums and therefore are difficult to access for dating. Split-based bone points are delicate and thin objects manufactured from bone and antler. Sometimes we see ivory points or perforated deer teeth made into ornaments. They have great archaeological value.
In Budapest we looked carefully at the inventory of osseous objects in Andras' care and decided which to prioritise and sample for AMS dating. The first object was sampled, you can see a picture of it left, and its small size. We decided to sample it through the base where the point had been broken. The damage to the exterior of the object is very small and constitutes a hole around 3 mm across. This type of sampling hole does not compromise the integrity of the object and when done with the curator, gives he or she confidence that the material is not substantially changed by the sampling process (see below for the sample hole we left).
Sample destruction can also be reduced by measuring the amount of nitrogen in the bone or tooth prior to taking a larger samples for AMS dating. For %N, as we call it, we need 3-5 mg of bone powder, a sample size equivalent to a few grains of salt. High nitrogen concentration in bone means likely high collagen, since the nitrogen comes from the protein component not the inorganic phase. In the case of the Hungarian samples, we already had a good idea that there was collagen, so we were able to sample with confidence, but in many cases we have a strong suspicion that dating will be difficult, so we take a %N sample.
Hopefully, using techniques like this, the days of large scale sample destruction are over. The archaeological record is finite, and we have to look at ways of conserving important collections. We also share material with other scientists, in particular geneticists and isotope geochemists, to again help reduce the amount of destructive sampling we need to do.
(Left) Rachel sampling one of the bone points in the museum in Budapest.