Subscribe now for full access and no adverts
The Nebra Sky Disc is a remarkable object, considered by many to be one of the most important archaeological finds of the last century. The prehistoric bronze disc decorated with gold inlays is believed to represent the earliest known depiction of the cosmos, and the advanced astronomical knowledge that its creators seemingly held has sparked global fascination. However, at 32cm in diameter and just a few millimetres thick, the Nebra Sky Disc is also a testament to the impressive craftsmanship of the people who made it.
The disc was discovered in 1999 by treasure hunters carrying out illegal metal-detecting at the site of a prehistoric enclosure near Nebra in eastern Germany. It was unearthed as part of a hoard that contained two swords, two axes, two spiral bracelets, and a chisel as well. Through stylistic analysis of these objects, the deposition has been dated to c.3,600 years ago. By then, the disc had been through several phases of use, and its creation is associated with the Early Bronze Age Únětice culture (see here). The disc initially ended up on the black market, but it was recovered by the police in 2002 and has undergone extensive scientific research in the intervening years. These studies have revealed information about the ways in which its golden designs were reimagined over time (see CWA 113), as well as its original colour (black like the night sky), and the possible origins of its raw materials (which include copper from the Austrian Alps and gold from Cornwall). The exact details of the disc’s manufacture, though, remained unknown until now.
The recent study used several metallographic techniques, including optical microscopy, energy dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction, to examine the microstructure of the metal in a small sample originally taken from the outer area of the disc in 2002. The researchers investigated the mechanical properties of the material, too, by measuring its microhardness. Heating and forging can change the microstructure of metal as well as affecting its hardness, so looking at these things can provide information about the processes that the object has undergone. The other element of the project involved experimental archaeology. Experienced coppersmith Herbert Bauer created several replicas of the bronze disc, each representing different stages of the manufacturing process. The microstructure of these replicas was then compared to the sample from the original.
Through these tests, researchers were able to confirm that the Nebra Sky Disc could not have been cast in its final state; instead, the process must have begun by casting a small, compact bronze preform, which then had to be worked by hand to turn it into a large flat disc. In order to achieve this without creating cracks in the material, it was concluded that multiple cycles of heating and hot forging would have been required. First, the preform would have been heated to nearly 700°C to make the metal easier to work. It would then have been forged using a helical hammering technique, working from the centre of the object out towards the rim in a spiral. However, as the metal was forged, it would begin to cool and would require heating again – a process known as annealing. It appears that the Nebra Sky Disc probably underwent ten cycles of this heating and forging in order to achieve the final product. It is also likely that it was heat-treated again at the end of the process to prevent cracking during the finishing stages of the disc’s creation, which included patination and the addition of the gold inlays through damascening.
Interestingly, the cast preform used to make the replicas required 55 cycles of heating and forging to reach the correct final size. However, the microstructure of the metal was much more similar to the Nebra Sky Disc in Replica Stage 2, which reflected the process after just ten cycles. It therefore seems that the preform used to make the Nebra Sky Disc must have been larger and flatter to begin with than the one used for the replicas, and consequently required considerably less working.
These findings reveal the complex hot-forging process that created the Nebra Sky Disc, demonstrating that Early Bronze Age metalworkers were even more skilled than previously understood. They reinforce how special the Nebra Sky Disc was: other sheet-metal ornaments are known from this period, but these are much smaller and would not have involved the same sophisticated sequence of heating and forging. The study, which has been published in Scientific Reports (https://doi.org/10.1038/s41598-024-80545-5), highlights the importance of re-examining even well-known objects as new scientific methods and technologies become available.
Text: Amy Brunskill / Images: Dieck et al., Scientific Reports 2024; reproduced with permission by State Office for Heritage Management and Archaeology, Saxony-Anhalt State Museum of Prehistory, photo: J Lipták, Munich
