Creating and exposing forgeries with chemistry

Art and culture was one of the twelve monthly themes of the 2011 International Year of Chemistry, selected to awaken the imagination of chemists and those interested in chemistry and create a desire to share their knowledge with non-chemists of all kinds.

When the allied troops finally found Göring’s art collection in an Austrian salt mine on 17 May 1945, they found a minor sensation. Among the 6 750 works of art was a hitherto unknown work by the master of the Dutch Golden Age, Johannes Vermeer. Vermeer never received recognition during his lifetime, 1632–1675, but during the latter part of the 19th century, his works were rediscovered and sold for sky-high prices.

Vermeer’s style is characterised by different light effects as well as frequent use of the sky blue colour ultramarine. Ultramarine means “beyond the seas” and the raw material for the pigment, the mineral lazurite or lapis lazuli, is a blue semi-precious stone which in the 17th century was mainly mined in Afghanistan. In 1822, the German chemist Gmelin discovered a method for producing synthetic ultramarine and soon even poor artists could paint in bright shades of blue. In Vermeer’s day, however, ultramarine was very expensive and his generous use of the colour became a signature that is found in paintings including “Girl with a Pearl Earring”, the subject of Tracy Chevalier’s novel of the same name.

So, in 1945 a new, unknown Vermeer had been found. The allied forces managed to trace the person who had sold the work to Göring, a banker called Alois Miedl. The tracks led from there to the immensely rich Dutch artist and art dealer Hans van Meegeren. On 29 May 1945, van Meegeren was accused of having plundered and sold Dutch cultural property to the enemy – a charge that could lead to the death penalty. The Vermeer painting had been sold for the tidy sum of 1 650 000 guilders, an astounding amount, today equivalent to somewhere between SEK 50 and 100 million. After three days in jail, van Meegeren called the investigators and explained that it was not a question of treason. The painting was a forgery and he had painted it himself. In addition, van Meegeren admitted that he had painted and sold a further five Vermeers, all of which had been affirmed as authentic by a number of experts. The experts protested and refused to accept that van Meegeren had fooled them; to prove his innocence – or rather his guilt – van Meegeren offered to show them how he had done it by painting another fake Vermeer – this time in front of the investigators. It must have been a fascinating demonstration.

In order to forge a painting, considerable chemical knowledge is required, and to expose it, even more sophisticated methods are needed.

“It was a Vermeer canvas, Viktor observed, or rather a skilful Vermeer forgery. According to the marshal of the realm, who talked unceasingly, it was entitled ‘Christ and the adulteress’. He explained that he had bought it from a Dutch art dealer.” - Extract from Kunzelmann & Kunzelmann, Carl-Johan Vallgren, 2009.

van Meegeren was, however, a master of forgery. He began by purchasing a canvas from the same period, in this case the 17th century, and carefully removed the old painting. It is after this that the chemistry comes in.

Chemical methods make it quite simple to expose when a forger has used a modern pigment. For example, Vermeer only had access to white lead, whereas titanium white and zinc white were not discovered until the 19th century (see fact box). van Meegeren, however, was extremely thorough and only used those pigments to which Vermeer would have had access – as well as ultramarine, these included white lead, indigo and the red mercury mineral cinnabar. In order to avoid being exposed, he even used paintbrushes made of badger hairs, as Vermeer did. Now only the really difficult part was left – getting the painting to age. When oil paint dries, a large number of chemical processes occur. Unlike watercolours, the components of oils do not evaporate when the paint dries; on the contrary, the oil used in oil paint contains many double compounds that slowly react with the oxygen in the air and initially the oil film becomes heavier as oxygen is bonded in. Oxidation also polymerises the chains of fat and once the process is complete, the canvas has been transformed into one complete crosslinked mass.

An old method used to expose a forgery was to rub the painting with alcohol. In a 17th century painting, all the chains of fat were cross-linked, while colour came off newer works. In order to achieve the right surface on the forgeries, van Meegeren had invented a method where he treated the surface with resol, a phenol formaldehyde resin, and then baked the painting in an oven at 120°C to crosslink the polymers.

When a painting ages, a thin network of cracks develops, known as craquelure, and to imitate this he bent and pried the painting over a steel groove until a fine web of cracks had formed. Finally, in order to recreate a proper 300 year layer of grime, he brushed the surface with India ink, which is a mixture of soot and water, and then carefully washed off the excess.

The result was perfect! The public rejoiced that a Dutchman had succeeded in fooling Göring and in October 1947 van Meegeren was voted the second most popular person in Holland. A few months later, however, he died following a series of heart attacks and the story could have ended there. The six forged works had been studied by a Belgian chemist, Paul Coremans, who confirmed that they were all most likely forgeries. A few years later, however, the owner of some of the fake paintings, Daniel George van Beuningen, sued Coremans, demanding £500 000 in compensation for the fall in value of the paintings. It was not until a decade later that another chemist, Bernard Keisch, was finally able to solve the problem.

Natural lead ore contains small quantities of radium-226, which has a half-life of 1 600 years and decomposes to chemicals including lead-210, which has a half-life of 22 years. Both Vermeer and van Meegeren used the pigment white lead, lead (II) hydroxide carbonate. When the pigment is produced, most of the radium is cleaned away, while both lead-210 and the stable lead-206 remain. There is therefore significantly more lead-210 than radium-226 in the pigment white lead to start with. If 99 % of the radium has been cleaned away, it takes around 150 years for the balance between radium-226 and lead-210 to become even again. In the case of the Vermeer forgeries, the levels of lead-210 were far higher than radium-226, which proved that the paintings were modern.

Today a range of chemical methods are used to expose forgeries and chemistry has made it extremely difficult to get away with art forgery. van Meegeren’s forged paintings continue to fascinate, however, and ironically they have become so valuable that forgeries appear every year!

- Ulf Ellervik, Professor of Bioorganic Chemistry at Lund University and enthusiastic disseminator of chemical knowledge and insights. 

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