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Polychrome metalwork techniques
Overview of polychrome metalwork techniques in the early bronze age.
scoring of grooves
To attach the pieces of gold plate to the Sky Disc, deep grooves were scored into the bronze of the disk...
beating down the edges
... the edges of the gold were laid into the grooves, which were then beaten down with blows from the punch, wedging the gold plate in place.
copper strip inlay
A copper strip is inlaid by the same technique as in the swords from the Nebra hoard.

- a very rare technique of the early bronze age

The heavenly bodies depicted on the disc are made of gold plate of a thickness of around 0.2-0.4 mm. They are fixed to the bronze firmament with a specialised technique called damascening. Neither glue nor solder is used to attach them to the bronze underneath. The edges of each piece of plate reach deep into the substance of the disc and are wedged under it. As a technology, this is a combination of two new decorative techniques that originate in the eastern Mediterranean: plating, i.e. covering a surface with gold plate, and inlay, i.e. setting pieces of metal into an object of a different metal.

Inlaid works are extremely rare in early bronze age central Europe. It is only recently that some inlaid sword blades have been found at the Vreta Kloster in Sweden (Östergötland). Another at Marais de Nantes (France), of uncertain origin, and an axe-blade from Thun-Renzenbühl (Switzerland)  have been known for longer. In the eastern Mediterranean inlaid metalwork was more widespread and this area should be seen as the origin of this elegant technique. The beautifully executed Mycenaean works are well kown, as are the even earlier pieces from the Near East.

However, the manner in which the 'plate inlay' on the Sky Disc was executed differs greatly from the works from the eastern Mediterranean, so it seems that here only the idea of multicoloured metalwork was transported, but not the technology itself. To attach the pieces of gold plate, the bronze was first softened by re-heating it and letting it cool, and then the outlines of each graphic motif were scored into the disc as grooves that cut deep under the surface of the metal. The tools used were chisels of hard bronze with a high tin content. The pieces of gold plate were then cut to the right size and laid in these grooves, and finally the ridges of bronze that had been raised up when the grooves were cut were beaten down over the edges of the gold. In this way the gold plate was fixed in place permanently.

Early bronze age metal inlay is so rare that it is surprising that the swords of the Nebra hoard are also inlaid. In this case, strips of pure copper were inlaid in the blades. Even though this technique is very work-intensive, the optical effect is not very satisfactory if the metal is bright and polished. The pale reddish copper does not form much of a contrast to the pale gold colour of the swords' primary metal, bronze.

However, to increase the contrast in colour, the material can be given an artificial patina, that is, a coloured layer of oxidation. This can be done by heating the object, or, more simply, by using urine to ferment it; the result is that the copper inlay becomes dark brown while the bronze is nearly unchanged in its gleaming golden colour. It is likely that the Sky Disc, too, originally had a dark patina, to increase the contrast with the golden heavenly bodies. Creating a patina is also necessary to prevent the object from oxidising in an uncontrolled, blotchy way.

(Kopie 1)

Experimental inlay work
Experimental inlay work
dark patina

production of artificial patina
In contrast to its present appearance, the surface of the Sky Disc may have had a dark patina, from which the golden heavenly bodies shone out impressively.
The last stage of production after the swords were cast (left) and after the inlay, chasing and polishing (centre), was the production of an artificial patina (right) to give the inlay a stronger visual impact.