Landesamt für Denkmalpflege und Archäologie Sachsen-Anhalt
Landesmuseum für Vorgeschichte
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Restoration

Removing the soil adhesions and corrosion

Image of the Sky Disc taken after confiscation
One of the first images of the Sky Disc after its confiscation: the surface is still very dirty and corrosion layers are firmly fused onto the gold plate.
Restorer Heiko Breuer removes soil adhesions from the back of the Sky Disc.
Restorer Heiko Breuer removes soil adhesions from the back of the Sky Disc.
‘Archäoplex', a complexing paste, is applied.
‘Archäoplex' is a complexing paste made up of chemical reagents that can be thickened to a paste with a silica gel. With this material, the layers of corrosion on the gold plate could be dissolved separately, without affecting the malachite patina of the bronze.

The restoration work on the Nebra Sky Disc began a few weeks after it was impounded in 2002. After it was thoroughly photographed and studied under the microscope, the soil adhesions were removed first, so that the original surface could be examined more closely.

The back of the disc, especially, still had large amounts of soil remains from the findspot. These had fused intensively ('cemented') with the corrosion of the metal. Over many weeks, the sandy-clay soil was removed, in part mechanically, but also with a mixture of ethylene glycol, ethanol and water. The soil remains were carefully preserved so that they could later be examined geologically for indications of their origin.

The surface of the gold created serious difficulties. It was covered by greenish malachite corrosion layers as hard as glass. These layers did not come directly from the gold plate itself, but had been diffused from the bronze parts of the disc's surface and had precipitated onto the more 'noble' metal. If they were removed mechanically, slight damage to the soft gold underneath would have been inevitable. That was out of the question, so a chemical method was chosen. After a long period of experimentation on a test surface, the choice was made to use as agent a paste developed at the museum itself. It contained the complexing agent EDTA, a formic acid/formate buffer, ethanol and water, and it was thickened to a paste with pyrogenic silica. The medium was applied with a paintbrush to the precise points which were to be cleaned, and then, after it had had time to work, it was removed again, taking with it the blue reaction products from the corrosion. This ensured that only the malachite on the gold surfaces was affected, but not that on the bronze. On the bronze, the corrosion was left intact: during the long period in the ground, the disc has in places corroded completely, though the corrosion has maintained the original shape of the metal. Removing this corrosion would therefore have been a massive alteration in the actual substance of the object.

Repairing damage

The missing section prior to restoration.
The missing section prior to restoration. The grooved mark left by the pick can be seen clearly.
The new piece of gold plate is set into place.
The new piece of gold plate is set into place.
The Sky Disc immediately after the final restoration work was completed.
The Sky Disc immediately after the final restoration work was completed.

The inappropriate methods used in the illegal excavation caused serious damage to parts of the Sky Disc. The blows of the digging tool, a customised fireman's pick, bent the upper edge of the disc and knocked out one star. The pick also ripped out a strip of the gold plate in the large image of sun or full moon. Both the star and the fragment of gold plate were successfully retrieved later. The star was almost undamaged, but the piece of gold plate from the larger image was very badly creased and twisted and there would have been little point in putting it back onto the disc. It was therefore preserved for further study. So that the Sky Disc could be displayed in a complete and aesthetically satisfying state, a new piece of gold plate of the same composition was made. It was cut to the exact size and set precisely in the gap using a special glue made of silicone rubber, so that it can be removed again at any time necessary.