The Nebra Sky Disk, which is more than 3,600 years old, is considered the oldest concrete representation of the sky in the world. But only now have archaeologists discovered how the bronze disc, which is the size of a plate but only a few millimeters thick, was made. Microstructural analyzes and tests with replicas reveal that the Bronze Age blacksmiths processed the casting blank in a complex hot forging process. The bronze disk was heated ten times and then hammered flatter.
The Nebra Sky Disk was made around 1600 BC, perhaps even 100 to 200 years earlier, in what is now Central Germany. The Bronze Age work of art consists of a 31 centimeter large, thin bronze disk on which stars, the sun and the crescent moon are applied made of sheet gold – clear indications of an astronomical reference to the sky disk, even if the exact function of the disk is still disputed. The find is considered a unique evidence of the early Bronze Age in Central Europe and is part of the UNESCO “Memory of the World” document heritage. At the same time, the Nebra Sky Disk is one of the best-researched archaeological objects ever.
Juraj Lipták/ State Office for Monument Preservation and Archeology Saxony-Anhalt
With structural analyzes and experimental archaeology
However, it was still unclear how the bronze disc, consisting of around 97 percent copper, 2.6 percent tin and only tiny traces of nickel, zinc and arsenic, was once made. The high copper content and low tin and arsenic content of bronze made the metal difficult to cast, even when molten. The disc, which is only a few millimeters thick but large, cannot simply have been cast, as Harald Meller from the State Office for Monument Preservation and Archeology (LDA) Saxony-Anhalt and his team explain. The creator of the sky disk must therefore have reworked the cast, much thicker and smaller blank by forging.
Meller and his team have now examined in more detail how the cast blank could become the finished bronze disc. To do this, they re-analyzed a sample taken from the edge of the disk in 2002 using state-of-the-art metallurgical methods, including X-ray spectroscopy and electron scattering. These make the microstructure of the metal visible and thus provide information about the thermal and mechanical processing of the bronze. At the same time, the researchers had copies of the bronze disk made by an experienced coppersmith. He processed the cast blanks using various forging techniques, and the resulting replicas were also subjected to microstructural analysis.
Elaborate forging process in ten cycles
The tests and analysis revealed that the sky disk was not formed into its final shape through simple cold forging. Instead, it must have been manufactured using a complex hot forging process. The cast blank was heated several times to 700 degrees and then flattened by hammering until the metal became too cold and hard. By comparing the disk shape and microstructure with different stages of the replicas, Meller and his team conclude that the Bronze Age blacksmith needed approximately ten cycles of heating and hammering to give the sky disk its shape. Finally, it was heated one last time to make the metal more resistant to cracking.
“The fact that the investigations produced such fundamental new findings more than 20 years after the sky disk was secured not only once again demonstrates the extraordinary character of this century find, but also how highly developed the art of metal processing was already in the Early Bronze Age,” says Meller. The elaborate production of the bronze disc demonstrates that the early Bronze Age craftsmen were not only outstanding casters, but also mastered the further processing of bronze artifacts, for example through hot forging, at the highest level. “The production of the sky disk was in no way trivial,” said Meller and his colleagues. Although axes and bronze jewelry were already being made by forging back then, the sky disk was unique in its shape, size and volume.
“In addition, the sky disk is impressive evidence of how important it is for the advancement of knowledge to subject well-known and supposedly researched finds to a new examination when new methods become available,” says Meller.
Source: State Office for Monument Preservation and Archeology Saxony-Anhalt – State Museum of Prehistory; Specialist articles: Scientific Reports, doi: 10.1038/s41598-024-80545-5