How did they do it? New insights into the formation of the Nebra sky disk

The Nebra Sky Disk, which is over 3,600 years old, is a unique find of international standing and has been part of the UNESCO “Memory of the World” register since 2013. It can be considered one of the best-researched archaeological objects. However, the question of the manufacturing process of the object has not yet been fully resolved.

Due to the material composition and previous research, it was known that the disc could not easily be cast to its final size. New metallographic analyzes show that the sky disk was manufactured using a complex hot forging process. Around ten cycles were necessary until it reached its final dimensions, each of which included heating to around 700 °C, forging and subsequent annealing to relax the metal structure again.

Since it was confiscated by the police and returned to Saxony-Anhalt in 2002, it has been the subject of extensive scientific research, which in recent years has yielded important findings, for example about the raw materials or the original color of the sky disk.

Due to the material composition and previous research, it was known that the disc could not easily be cast to its final size.

Despite its simple appearance, forging a bronze disc with a diameter of around 31 centimeters and a thickness of a few millimeters is by no means a trivial task. The latest metallographic studies, now published in Scientific reportstherefore devoted themselves to the question of how the bronze disc was made from a single cast.

The study was carried out by the State Office for Monument Preservation and Archeology of Saxony-Anhalt in collaboration with the Otto von Guericke University of Magdeburg, Chair of Metallic Materials (Prof. Dr. Thorsten Halle), and the company DeltaSigma Analytics GmbH, Magdeburg. The most modern metallographic techniques were used.

A small sample from the exterior of the Sky Disk, first taken in 2002 for various archaeometallurgical studies and since reinstated, was again temporarily removed and re-examined. In addition to microstructural analyzes on color-etched surfaces with a light microscope, the most modern imaging techniques were used: energy-dispersive X-ray spectroscopy and electron backscatter diffraction.

Hardness measurements were also carried out. These were accompanied by parallel experimental tests in which the experienced coppersmith Herbert Bauer, Halle (Saale), made a replica from a cast blank.

The results of the metallographic analyzes show that the sky disk was manufactured using a complex hot forging process. Around ten cycles were necessary until it reached its final dimensions, each consisting of heating to around 700 °C, forging and then annealing to relax the metal structure again.

In his attempts to produce the copy, H. Bauer had to carry out significantly more forging cycles than could be proven for the original sky disk. The casting blank for the original must have been slightly larger and thinner than that for the replica.

“The fact that the investigations continue to lead to such fundamental new findings more than 20 years after the sky disk was secured not only shows once again the extraordinary character of this find of the century, but also how highly developed the art of metal processing was even back then in the Early Bronze Age,” says state archaeologist Prof. Dr. Harald Meller.

The latest research results make it clear that the early Bronze Age craftsmen were not only excellent casters, but also mastered complex bronze processing techniques, such as hot forging, at the highest level. With their extensive experience and knowledge, they not only managed to produce numerous axes in series, but also forged a workpiece that is unique from today’s perspective, such as the Nebra Sky Disk.

“In addition, the sky disk shows how important it is to re-examine apparently well-known finds when new methods become available,” says Meller.