Gold from the richest grave in the cemetery at the 5th millennium site of Varna, Bulgaria. This grave contains c.3kg of gold items decorating the body of the deceased. Varna is considered one of the key archaeological sites in world prehistory. Photo: E. Pernicka

While studies of ancient gold metallurgy and the color characteristics of gold alloys are well supported by modern research, the color properties of prehistoric copper alloys, such as tin bronzes or arsenical copper, the most abundant type of metal artifacts in prehistory, have largely been understudied. Until now.

In a study published in the Journal of Archaeological Science, an international team of Serbian and UK researchers have developed a Cu-As-Sn (Copper-Arsenic-Tin) color ternary diagram to uncover the original colors of archaeological artifacts now patinated through age and exposure.

The study was prompted by the discovery of the world's earliest tin bronze artifacts four years ago in Serbia and the ongoing debate into what significance color played in the advancement of metal-making technologies.

"Given the acknowledged importance of aesthetics in ancient metallurgy, we decided to experimentally replicate the most common prehistoric alloys, made of binary and ternary combinations of copper, arsenic and tin and produce a color chart that comes the closest to showing the true 'bling' of such artifacts in the past. We were inspired by modern jewelery making where similar color charts are used to explore properties of gold-copper-silver alloys," said Miljana Radivojevic, lead author and researcher at the McDonald Institute for Archaeological Research, University of Cambridge.

"Our laboratory is one of the few in Europe to hold a license to experiment with arsenic, which is why we were approached to develop the study and produce 64 metal samples of variable copper-tin-arsenic compositions," explained Zeljko Kamberovic, leader of the Serbian team, from the University of Belgrade Faculty of Technology and Metallurgy.

"The copper-tin-arsenic ternary color charts enabled us to re-evaluate the claim that early tin bronzes in the Balkans had a distinctive golden hue," said Radivojevic. "It is now highly likely that the production of this new alloy in the Balkans at the same time as gold could have been dictated by the demand for the 'exotic' golden hue, or its closest imitation."

"This research, although driven by the case study in the Balkans, yielded a valuable representation of color of the most commonly produced prehistoric alloys worldwide. We now have the means to bring the original shine to the items that have lost their original aesthetic appeal during several millennia of deposition below ground," stated MartinĂ³n-Torres, from the UCL Institute of Archaeology, where chemical and colorimetric analyses for this study were conducted.

Radivojevic added that she anticipates these color charts being widely used in teaching or museum exhibits, "helping students and museum visitors to imagine how the majority of ancient metal objects looked a couple of thousands years ago."