Birch tar reveals age of Bronze Age find

18 November 2019

The LARA radiocarbon dating laboratory at OCCR is a leading tool when it comes to analysing small material samples. This know-how was also in demand for dating the bronze hand of Prêles, a find that has made international headlines.

© Archäologischer Dienst des Kantons Bern, Philippe Joner

The discovery was an archaeological sensation. At the beginning of October 2017, two private individuals in the Bernese Jura came across a cast bronze hand wearing a gold bracelet. The bronze hand of Prêles is around 3,500 years old and is by far the oldest known bronze sculpture in the form of a human body part.

This exceptional find was dated at the OCCR, more precisely at the Laboratory for the Analysis of Radiocarbon with AMS at the University of Bern’s Department of Chemistry and Biochemistry. The laboratory is equipped with highly specialised technology that makes it possible to carry out 14C age determinations using even tiny material samples. For example, when OCCR researchers need age data about the individual layers of lake sediments used as climate and environmental archives, samples of just a few milligrams – or sometimes even smaller – must suffice.

© Archäologischer Dienst des Kantons Bern, Philippe Joner

However, 14C age determinations can only be performed on organic material, not on metal. Therefore, in the case of the Prêles find, it was not the hand itself that was dated, but rather the adhesive that craftsmen used 3,500 years ago to attach a gold sheet to the bronze. “For the analysis, we had five milligrams of plant material at our disposal. It was probably birch tar, a common Bronze Age adhesive,” explains Sönke Szidat, head of the LARA 14C laboratory. “For us, such tiny amounts of material are part of everyday life, but for archaeological dating it is unusual.” The result of the analysis: the bronze hand was made between 1507 and 1431 BC.

What the nearly life-sized hand was used for and what its significance was is still largely unclear. The University of Bern recently hosted an international colloquium to try to shed some light on the find from the Bernese Jura. It seems clear that the gold bracelet underlined the special status of the person buried with the bronze hand.

© Annette Boutelier

New method for dating bone material

The fact that the hand had been buried with someone became clear after canton Bern’s archaeological service carried out a post-excavation in Prêles in 2018. The archaeologists discovered a grave which contained the bones of an adult male as well as a finger that had broken off of the bronze hand. A bronze dagger and a human rib had already been found with hand.

© Archäologischer Dienst des Kantons Bern, Philippe Joner

The 14C laboratory also dated these bone finds – a big challenge from a methodological and technical point of view, as Sönke Szidat explains. “We generally have to remove any materials that could falsify the age of the samples. In the case of bones, the task is more complicated because contaminants penetrate them through their prolonged contact with the ground.” The humic substances released from the humus can also be found on collagen, the bone component that is isolated and finally analysed.

When first dated, the age of the bone material was only roughly the same as that of the bronze hand. In the LARA, meanwhile, researchers have refined a method that uses ultrafiltration to prepare bone samples. This method can also remove small contaminating molecules. “Now we are able to generate much more reliable data, even for poorly preserved samples,” notes Sönke Szidat. And this is why it has recently become clear that the age of the Bronze Age dignitary buried in Prêles and that of the sensational object found in his grave are exactly the same.