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Technique may reveal world of hidden paintings

July 30, 2008
Courtesy Delft University of Technology
and World Science staff

Be­neath the top lay­ers of many paint­ings are to­tally dif­fer­ent pic­tures, which were at some point paint­ed over.

A new tech­nique can re­veal some of these hid­den paint­ings in un­prec­e­dent­ed de­tail, say sci­en­tists who used it to newly re­veal a work by Vin­cent van Gogh. It is a por­trait of a wom­an, lurk­ing un­derneath his pic­ture Patch of Grass.

A woman's head re­vealed be­neath Patch of Grass by van Gogh. (Cour­tesy DE­SY Ham­burg)
Ex­pe­rts es­ti­mate that about one third of the famed artist’s early paint­ings con­ceal oth­er com­po­si­tions, ac­cord­ing to the re­search­ers, from Delft Uni­ver­s­ity of Tech­nol­o­gy in The Neth­er­lands, the Uni­ver­s­ity of Ant­werp in Bel­gium and oth­er in­sti­tu­tions.

Tech­niques usu­ally used to re­veal con­cealed lay­ers of paint­ings, such as con­ven­tion­al X-rays, have lim­ita­t­ions. The new meth­od, billed as non­de­s­truc­tive, em­ploys X-rays gen­er­at­ed by a so-called syn­chro­tron radia­t­ion source. This is a de­vice that ex­ploits the fact that ac­cel­er­at­ing elec­trons, the par­t­i­cles that car­ry elec­tric charge, gen­er­ate radia­t­ion as they move.

In the new tech­nique, a paint­ing is sub­jected to an in­tense but very small bun­dle of X-rays from this type of source. Atoms with­in the paint­ing give off ti­ny flashes of light when struck by the rays. 

In the new meth­od, re­search­ers said, these flashes, called flu­o­res­cence, dif­fer de­pend­ing on the chem­i­cal el­e­ment be­ing struck. This means dif­fer­ent co­lors of paint can be dis­cerned.

Com­pared to con­ven­tion­al meth­ods, syn­chro­tron radia­t­ion al­so al­lows the mea­sure­ments to be less dis­tort­ed by the up­pe­r lay­ers of paint, the sci­en­tists said. Their study is pub­lished in the July 29 on­line is­sue of the re­search jour­nal An­a­lyt­i­cal Chem­is­try.

Patch of Grass, paint­ed in Par­is in 1887 and owned by the Kröller-Müller Mu­se­um in Ot­terlo, The Neth­er­lands, was al­ready known based on pre­vi­ous re­search to con­ceal the vague out­line of a head. The ex­pe­ri­ment­ers scanned the paint­ing with a syn­chro­tron radia­t­ion source called DO­RIS at Deutsches Elek­tron­en-Synch­ro­tron re­search cen­ter in Ham­burg. Over two days the ar­ea co­vering the im­age of a wom­an’s head was scanned, a square meas­ur­ing 17.5 cm (7 inches) per side. 

The com­bina­t­ion of the dis­tri­bu­tion of the el­e­ments mer­cu­ry and an­ti­mony from spe­cif­ic pig­ments pro­vid­ed a “colour pho­to” of the paint­ed-o­ver por­trait. The work, re­search­ers  said, will en­a­ble art his­to­ri­ans to bet­ter un­der­stand the ev­o­lu­tion of Van Gogh’s work and pave the way for re­search in­to many oth­er con­cealed paint­ings.

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Many paintings conceal—beneath their top paint layers—totally different pictures, which were later painted over. A new technique reveals these hidden paintings in unprecedented detail, say scientists who used it to newly reveal a work by Vincent van Gogh. It is a portrait of a woman, lurking underneath his picture Patch of Grass. Experts estimate that about one third of the famed artist’s early paintings conceal other compositions, according to the researchers, from Delft University of Technology in The Netherlands, the University of Antwerp in Belgium and other institutions. Techniques usually used to reveal concealed layers of paintings, such as conventional X-rays, have limitations. The new method employs X-rays generated by a so-called synchrotron radiation source. This is a device that exploits the fact that accelerating electrons, the particles that carry electric charge, generate radiation as they move. In the new technique, a painting is subjected to an intense but very small bundle of X-rays from this type of source. Atoms within the painting give off tiny flashes of light when struck by the rays. In the new method, researchers said, these flashes, called fluorescence, differ depending on the chemical element being struck. Compared to conventional methods, synchrotron radiation also allows the measurements to be less distorted by the upper layers of paint, the scientists said. Their study is published in the July 29 online issue of the research journal Analytical Chemistry. Van Gogh’s Patch of Grass, painted in Paris in 1887 and owned by the Kröller-Müller Museum in Otterlo, The Netherlands, was already known based on previous research to conceal the vague outline of a head. The experimenters scanned the painting with a synchrotron radiation source called DORIS at Deutsches Elektronen-Synchrotron research center in Hamburg. Over two days the area covering the image of a woman’s head was scanned, a square measuring 17.5 cm (7 inches) per side. The combination of the distribution of the elements mercury and antimony from specific pigments provided a “colour photo” of the painted-over portrait that had been painted over, the researchers found.The work, they said, will enable art historians to better understand the evolution of Van Gogh’s work and pave the way for research into many other concealed paintings.