A well guarded family heirloom, the “genuine” Rembrandt that great-grandmother once bought from an art dealer, can turn out to be a big disappointment. Etchings and copperplate engravings by the Dutch master were popular with copyists and counterfeiters alike. The latter copied the masterpieces with intent to deceive, the former to learn the craft. Today they are exposed as copies only by an expert like Michael Matile, Deputy Director of the ETH Collection of Prints and Drawings. (1)

Magnifying glass instead of chemistry

However, most of the methods to recognise copies have nothing to do with chemistry, as Matile explained at the CHAB Forum of the Department of Chemistry and Applied Biosciences in the week beginning 30 October 2006. The Curator of the Collection of Prints and Drawings reaches for his magnifying glass to prove for example whether an etching or copperplate engraving is genuine or forged.

“Rembrandt etchings were popular subjects to counterfeit,” said Matile. Like the famous Hundred Guilder Print which the Dutch artist produced in 1647. More than 100 years later the original copper plate came into the possession of William Baillie, a Captain in the British army. He was a big fan of Rembrandt and imitated his style. However the copper plate was obviously severely worn down in places. Baillie, who was not untalented, made good the missing areas and printed 100 new copies. To limit the edition he subsequently cut the plate up into four individual pieces from which he in turn prepared new prints. Today it is unclear whether the Briton acted with intent to deceive. “Obviously fraudsters were well able to spread such fragments around on the 18th Century art market,” said Matile. In today’s climate a restorer would certainly refrain from changing an original in that way.

Precious stones under the laser

Whereas a forged Rembrandt can be recognised simply by eye, a high-tech laboratory is needed to check the authenticity of precious stones. Detlef Günther, ETH Professor for Trace Element- and Micro-Analysis (2), demonstrated how much effort is required to track down counterfeit sapphires, diamonds or rubies. This is done by ablating microscopic amounts in the nanogram range from the stones with a laser beam, evaporating and ionising them and analysing them in a mass spectrometer. “The shorter the wavelength of the laser, the less material is ablated,” said Günther.

Finally the distribution of the elements and isotopes provides information as to whether a precious stone is genuine. The composition of the elements even allows conclusions to be drawn about the geographic origin of the stones. However, the amount of sample ablated by the laser is so tiny that no damage is apparent on the surface of the stone.

Archaeology uses non-destructive methods

This method also enables the origins of archaeological finds to be investigated. For example ETH chemists have collaborated with the Swiss National Museum to elucidate how humans manufactured copper artefacts in the Neolithic period in 4300 BC.

In the 18th Century Captain Baillie cut Rembrandt’s Hundred Guilder Print into four pieces and sold the fragments as originals. (Photo: ETH Collection of Prints and Drawings) large

Günther says that in the past this required copper to be abraded off the items being examined and dissolved. However that destroyed the finds, some of which were valuable. Nowadays the specimens are abraded with a laser. The chemist considers that “The damage is minimal and invisible without a microscope.”

The copper analyses by the ETH chemists have helped archaeologists to complete a blank area on the trail map of the early copper trade. The copper used in East Switzerland originated from Eastern Europe. However West Switzerland was supplied with the metal from what is now France.

The Sky Disk: ingeniously forged or uniquely authentic?

Even chemical methods cannot prove whether the Nebra Sky Disk (3), which some experts suspected was a clever forgery, really is genuine. This is the opinion of the historian Barbara Ihrig of the Basel Historical Museum, where this exhibit can be seen until the end of January. (4)

Among other reasons, the Sky Disk made headlines because it was found by grave robbers on the Mittelberg in Saxony-Anhalt, was smuggled through receivers, and was finally impounded thanks to a spectacular operation. Speculation and rumours have grown up around the Sky Disk.

Not 100 percent certainty

Here again chemical analyses of the composition of the copper have brought greater clarity. German researchers discovered that the copper of the Sky Disk contains, among other things, a high arsenic level of 0.7 percent, which is evidence that the object is genuine. They were also able to elucidate the source of the copper. The raw material originated from what is now Austria – typical for that period. Finally, the patina of the Sky Disk contains small blisters of malachite. The historian emphasises that “This indicates slow ageing.” It would also be possible to patinate copper artificially, but patinating substances of this kind contain salts that are not found in the Sky Disk’s patina.

Ihrig explained that “It is impossible to be 100 percent certain that this object is authentic.” She said this was also connected with the fact that the excavation was not sufficiently documented. “But why would anyone want to counterfeit an object of this kind at such great expense?"