Scientists who invented the technique show how the technology that detects fake bank notes by using their inks' unique "magnetic signature" can now authenticate oil paintings, too.

A technology that can frustrate art counterfeiters has grown out of the efforts to fight bank note forgery.  Paolo Costa Ribeiro and Anna Vila of the Pontifical Catholic University of Rio de Janeiro in Brazil, together with their colleagues, had made magnetic maps of Brazilian bank notes. And they found that each note has a unique signature that remains stable over time. They suggested that these signatures could be used as an identification tag to help distinguish a real note from a fake one.

The special inks used to print bank notes around the world are made of ferrofluids, which generate weak magnetic fields that can be measured using instruments such as superconducting quantum interference devices (SQUIDs). A SQUID can be simply scanned across the note to record a "map" of its magnetic flux.

Now, these scientists have extended the technique of “magnetic signatures” used in the ink of bank notes to authenticate artwork.

We propose a new technique for authentication of oil paintings, using a scanning SQUID technique to measure its magnetic field. The paintings are pre-magnetized in an homogeneous field of 100 G. It was observed that the response depends on the ferromagnetic properties of each paint independent of its colour. This shows that a magnetic image could be used as a magnetic signature for authentication purposes.

- From The Magnetic Imaging of Oil  Paintings by P Costa Ribeiro, H Lins de Barros, J C Portinari, C S Wolff, S M Kafensztok, H R Carvalho, D Acosta-Avalos, O G Symko,  A C Bruno

In the case of oil paintings, the magnetism comes from minerals, like magnetite, in the oil-paint pigments. As with bank notes, the signature could be used to identify the artwork and might even serve as a tag for cataloguing purposes by museums and insurance companies.

In a report published in The Journal of Applied Physics, Ribeiro and his team concluded that “The magnetic imagings of four similar oil paintings done with the same visual color, [but] obtained using different mixtures of oil paints for each painting, were different. This shows that the magnetic imaging can be used as a fingerprint for the oil paintings.”

According to Jose Garcia of the University of Barcelona, a researcher in authenticating paintings, "Although there are other non-destructive techniques with a spatial resolution image better than this one, it could be an interesting research line for the future." He describes the technique as an interesting new approach to the global analysis of artworks.

Can the same technique vouch for the authenticity of sculptures, ceramic figures, reliefs, carvings and other artworks? If magnetic maps can be obtained from them, then such technologies can make great strides toward fighting what is now the world’s fourth largest illegal market.