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Authenticity and dating

Diffractogram of the mineral phases on the Sky Disk: malachite and cassiterite
Diffractogram of the mineral phases on the Sky Disk: the layers of corrosion consist entirely of malachite and cassiterite...
unbroken layer of crystallites which has fused firmly together
...which, as observed in a scanning electron microscope, consist of an unbroken layer of crystallites which has fused firmly together...
artificial corrosion structure
...whereas artificial corrosion would have a structure like scales lying loosely alongside each other.
corrosion layer built up over a long period and an artificial patina in contrast
That is why a corrosion layer which built up over a long period is a rich, dark green (left); in contrast, an artificial patina is light in colour, because the pores on the surface diffuse the light (right).
Lead 210 test results
The Lead 210 test: Sky Disc of Nebra compared to modern soldering tin. The Sky Disc no longer contains any Pb-201, so the material must have been smelted from ore more than 100 years ago.
Radiocarbon dating of the birch bark from the swords
Results of the radiocarbon dating of the birch bark from the swords.

As soon as the finds arrived at the Landeskriminalamt (State Criminal Investigation Office) in Magdeburg, they were tested to check their authenticity. There is rarely a single method of testing which on its own would tell whether an archaeological find of unknown provenance is 'genuine' or 'fake'. Most scientific methods of examining and dating objects can in fact be fooled by determined forgers, despite the widespread belief to the contrary. However, the effort required is extremely high, and this is multiplied to the point of impossibility if the forgers have to reckon with not one but a large number of different methods of testing.

The set of finds from Nebra can judged to be certainly genuine on the basis of the following criteria:

Indicator: radioactivity
Pb-210, a radioactive isotope of lead with a half-life of 22.3 years, is the product of a uranium decay series. The decay series proceeds through the element radium, which is present in small traces in ores and their subsidiary rocks. When the ore is smelted, the radium is separated out into the slag, and so the existing amount of Pb-210 passes into the smelted metal but the continuing supply of it stops. Thereafter, the radioactivity of the Pb-210 halves every 22.3 years.

The level of radioactivity in newly produced metal varies widely, but is in general very small. As a result, by at latest 300 years after smelting, it can no longer be traced in the metal. The level of radioactivity at the time of smelting is not known, so this method cannot be used to date an object. But when no radioactivity can be identified at all, it is a certain conclusion that the metal under examination is older than about 100 years. The measurement of Pb-210 in a small sample from the edge of the disc, carried out by the Bergakademie Freiberg, produced the result within a few days of testing: no measurable radioactivity from Pb-210 is present; all components of the alloy must therefore derive from metal produced at least 100 years ago.

Indicator: Corrosion
'Good' fakes for the art market are frequently accompanied by a note claiming that the corrosion had to be removed 'to preserve the object'. The reason: artificial corrosion can generally be spotted with the naked eye. It is usually light in colour, powdery or has simply been painted on with green paint. This will not stand up to any chemical and mineralogical examination.

The solid, intense green corrosion on the bronze disc was analysed using x-ray diffraction, and was identified as malachite (basic copper carbonate) and cassiterite (a tin oxide).  Products of corrosion that occur when an artificial patina is attempted (e.g. chloride compounds of copper such as atacamite and paratacamite) are not present. It is in fact possible to produce a malachite patina artificially. But the process is fairly laborious and still does not produce a patina with the compact crystalline character and intense green colour found on the disc. On the disc, the large size of the crystallites are appropriate to the long growth-period of 3600 years, and the secondary formations that have created a bubble-like structure are also typical for this. Synthetically produced malachite patina is composed essentially of fine crystals and has a much lighter colour.
Another significant characteristic of the corrosion is the presence of copper compounds which have formed on the gold plate. The gold contains so little copper that this copper corrosion cannot derive from the gold itself. It is in fact the result of an electrochemical process in the ground: gold and bronze have here formed a local galvanic cell. The surface of the gold is negatively charged (the cathodic zone) and copper ions diffused through the soil from the near-by bronze sections have precipitated onto it.

Indicator: Composition of the alloy
A forger would need to have recreated the typical composition of prehistoric bronze and gold alloys. The gold usually found on the market today is very highly refined and quite unlike gold that has been washed from gravel deposits. A forger would need to have got hold of gold nuggets from the right region, or to have 'simulated' their composition, including all major and minor component parts. The same difficulties are raised by the bronze, especially as it has a considerable arsenic content. These days it is exceptionally difficult to acquire arsenic on the open market, and working with this highly poisonous semi-metal is particularly dangerous.

Indicator: Soil adhesions and the correspondence of finds with findspot.
That the Sky Disc and the swords belong together is revealed by the examination of the remains of soil adhering to them. The soil on the disc matches that on the other finds, and is identical to the soil at the findspot on Mittelberg hill. The earth adhesions have completely fused with the corrosion during the long period in the ground. The soil had become so firmly attached to the corrosion on the bronze surface that it could only be detached with great difficulty during restoration work. On the gold plate, the earth could only be removed by chemical means, taking the malachite corrosion with it. This process of fusion must have happened simultaneously with the process of corrosion itself, so the soil cannot have been added to the objects later, e.g. as part of a forgery attempt. Corrosion products had also been diffused into the surrounding soil while the hoard was in the ground; this process also takes several hundred years. When the ground on Mittelberg hill was examined, far higher concentrations of copper and gold were found in the immediate area of the findspot, higher than the natural amount in the soil on the peak of the hill by several orders of magnitude.

 

Indicator: Scientific dating by the radiocarbon (C-14) method

The Sky Disc itself cannot be directly dated scientifically or typologically (i.e. by characteristic design forms). But, because it was deposited together with the other finds in the same pit - as the soil adhesions demonstrate -, it can be dated indirectly though these other finds. These can be dated typologically, but also by scientific methods. In one of the sword-hilts there were remains of birch bark which was evidently used to hold the grip in place. The bark was dated by the radiocarbon method to the period between 1650 and 1500 BC. The Sky Disc is at least as old as the swords and axeheads and probably somewhat older, as the numerous ancient alterations suggest.

 

The effort required for a forgery

Some of the criteria of authenticity set out above could perhaps be evaded with a certain amount of effort by an intelligent forger, but others (e.g. the corrosion) could not be falsified by any known method. On any scenario, a forger would have to go to enormous lengths to make a work that could meet all the test criteria applied here. Even the tiniest slip would result in failure. Consider the hypothetical scenario:

An 'old alloy' could still be created by melting down ancient objects, but these objects would all need to have a single, identifiable origin, for example they could all originally come from the Mitterberg region. Smelting metal out of new ore with a single origin would not work - and the Mitterberg deposit is anyway worked out - because then the metal would fail the Pb-210 test. The forger would need to acquire natural gold nuggets from the right region, and then work the disc and inlay the gold with prehistoric smithing techniques. To create a complex astronomical programme for the object, it would be wise to consult a number of experienced archaeoastronomers. Then the other objects of the hoard could be produced in the same manner. Early bronze age birch bark, its radiocarbon date correctly identified in advance, would then be placed in the sword-hilts. A radiocarbon date can in fact be faked: you set up a greenhouse, plant birch saplings and then pump in an isotopically correct mix of C-14 and carbon dioxide. After 20 years of growth in 'bronze age' C-14 air, the bronze age birch bark can be harvested. Then the finds must be artificially corroded with a solid crystalline malachite patina. But there is no known way to do that, and there is also no way that the soil adhesions - from the right findspot - could be made to fuse with the corrosion faster than would occur naturally. The findspot would need to have been carefully chosen for its astronomical peculiarities. Once the 'find' has been buried there, the surrounding earth can be evenly dosed with traces of gold and copper, and all that remains is to wait patiently for its 'discovery'.