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Production technique

There is hardly any relevant scientific method which has not been used to examine the Nebra hoard.
The metallography of a section of the Sky Disk exhibits a feature called 'twinning'. This shows that the disc was repeatedly re-heated in the forge to relax the metal again after it had been cold-worked by the smith.
This is what the rough cast for a sword from the Nebra hoard will have looked like. In this form, the high quality of the finished object could hardly be guessed. But even in the originals there were flaws in the casting, as revealed by x-ray tomography.
Driven by countless gentle taps from a hammer, the bronze chasing-punch slides across the object. This will have been the final stage of the long process of metalworking that transformed a rough piece of cast metal into the finished early bronze age work.

The Sky Disc is a work of the smith's craft. It was created from a simple, disc-shaped ingot, probably with a diameter of 15-29 cm. The ingot's metal was an alloy of copper that was unusually soft for the bronze age, containing only around 2.5 % tin and 0.2 % arsenic.

This lightly alloyed bronze was chosen intentionally, so that the disc could be beaten out to a diameter of 32 cm, working it when the metal was cold. Smithing bronze demands a high level of skill. Through the process of cold-working the metal, the material gets steadily harder and more brittle. So that cracks do not form, the smith has to re-heat the piece in the forge a few times in the course of his work, to make the metal re-crystallise. This 'relaxes' the metal again, so that, when the piece has cooled, the smith once again has a relatively soft, malleable material. The piece will never have been worked while hot.

Metallographic studies have given a clear picture of the smith's work-process. To see the traces of ancient production methods, researchers detached a tiny block of metal from the middle of the material, using electrical discharge machining ('spark erosion') to cut it out, and then they examined the block's surfaces. The metal block was later set back into the disc and the miniscule gaps were filled, so that this intervention is no longer visible.

The cut surfaces of the little block clearly show the traces left by the smith in the material. It exhibits 'twinning', a sign that the disc was re-heated in the forge repeatedly (around 2-3 times) and then once more after the object had been given its final form. The bands of dark brown discoloration come from material inclusions in the cast metal, which got flattened out during smithing.

The swords were produced by a quite different method from the Sky Disk. They received their basic form at the casting stage of production. Their metal is much harder than that of the disc. It is a typical tin bronze with a 10% tin content. Material like this can hardly be smithed at all as it is too hard and brittle. X-ray tomography images show that the centres of the swords contain a lot of air-bubbles. These regularly occur in the conventional casting method of casting ('gravity casting') and there is hardly any way to avoid them. The swords were probably cast in clay moulds using the lost-wax technique. This produces rough casts which must be filed and then strengthened by careful hammering. The final step is the chased decoration. To create the exceptionally fine lines on the blades and in the grips, fine chasing-punches of hard bronze were used.