Horse manure an unlikely ally in unmasking art forgeries

Medieval sites with the paintings selected for radiocarbon dating of lead white. (A) Marguerite of Bavaria’s dressing room in the Château de Germolles, Burgundy, France (Photo: E. de Lavergne), (B) The restitution of the destroyed rood screen in the Church of the Cordeliers, Fribourg, Switzerland (marked in red) and some fragments of the wall painting discovered during excavation. Images:  Beck et al.

Horse dung residues have emerged as a potential new weapon in efforts to unmask art forgeries.

Art forgers have been around for centuries. While experts find some easy to identify, others are much harder, especially those that date closer to the creation of the original work.

Investigations to authenticate paintings rely on an advanced knowledge of art history and a collection of scientific techniques, including paint analysis, imaging and radiocarbon dating.

As Lucile Beck and her colleagues at the University of Paris-Saclay in France point out, the absolute dating of paintings is crucial for tackling the problem of fake art.

Radiocarbon dating is the only technique that gives access to an absolute time scale, but its application is limited to organic materials such as wood, canvas or natural binder.

Being able to extend absolute dating to the range of inorganic pigments used for colours in art would make it possible to overcome the lack of available materials for dating easel and mural paintings.

Paintings consist of several superimposed layers containing two main ingredients, pigments and binder, that may or may not be covered with varnish.

Some of these components are good candidates for radiocarbon dating since they originated from natural organic materials.

Until the 20th century, the binder was mainly made from plant or animal material, such as vegetable oils for oil painting, egg for tempera and more rarely beeswax.

However, preservation of the binder over the centuries is not always certain and it can be subject to modern contamination by synthetic resins used for restoration, retouching or varnishing.

As a result, the amount of original carbon still present can be very low, except for recent art works or forgeries.

So, could there be a way of dating inorganic pigments used in paints?

Beck, who works at the Laboratory for the Measurement of Carbon 14 at the university, turned her attention to lead carbonate, more commonly known as white lead.

White lead which is used in painting has to be synthesized through a process of lead corrosion which has been used since antiquity.

The corrosion process involves metallic lead, vinegar and organic substances such as horse manure or tan bark left to ferment for several weeks. It became known as the stack or Dutch process when mass production using this method started from the 16th century.

Back then, white lead was used in 16th century cosmetics to whiten the face of ladies at court, as well as in pigments in paintings.

The process piqued the interest of Beck, who wondered if Carbon 14 residues might be present in this inorganic pigment because of the use of horse manure and plant material in the process. If so, this could allow the accurate dating of paintings.

Beck and her colleagues developed a thermal process to neatly separate the carbon originating from lead white without contamination from other sources such as the likes of calcium carbonate pigments, which can contain geological or fossil carbon.

The resulting carbon derived from the technique proved reliable for the dating of the paint layers.

The technique has been successfully used for the absolute dating of well-documented medieval paintings from the Château de Germolles in Burgundy, France, and on fragments of paintings recently discovered in the church of the Cordeliers in Fribourg, Switzerland.

Beck said the study showed that it was possible to extract all the carbon from lead carbonates by thermal decomposition in order to date lead white pigments and paintings by the radiocarbon method.

From the 19th century on, new synthesis processes were developed to improve yields and lower production costs.

“In summary, the methodology developed here can be applied to any archaeological objects or works of art made of carbonates synthesized by the corrosion process.

“Cosmetics and paintings produced from antiquity to the 19th century and that contain lead white can now be dated by the radiocarbon method.

“However, there are some limitations.

“In the case of more complex mixtures of paints, and in particular in presence of other carbonates or recent acrylic resins, a prior analysis of the constituents is recommended so as to adapt the extraction protocol in order to prevent any contamination from geological or fossil carbon.

“Absolute dating of paintings that contain inorganic pigments has been achieved,” the study team concluded.

“The successful dating of lead white in medieval paintings confirms that the radiocarbon dating method can be extended to inorganic paint materials such as synthesized carbonate pigments.

“The radiocarbon dating of lead white is innovative and opens new perspectives.”

Carbon 14, they say, is an efficient marker to distinguish an ancient lead white manufactured by corrosion from lead whites manufactured by modern processes.

“In the field of art history and forensic sciences, dating lead white pigment provides a new tool for dating paintings.

“Since lead white was extensively used by the greatest artists, Da Vinci, Vermeer, Van Gogh and many others, we anticipate that this study will be a starting point for developing new approaches to authenticate paintings and detect forgeries. The impact for the art market and for museums is considerable.”

The full study team comprised Beck, Cyrielle Messager, Ingrid Caffy, Emmanuelle Delqué-Količ, Marion Perron, Jean-Pascal Dumoulin, Christophe Moreau, Christian Degrigny and Vincent Serneels.

Beck, L., Messager, C., Caffy, I. et al. Unexpected presence of 14C in inorganic pigment for an absolute dating of paintings. Sci Rep 10, 9582 (2020).

The study, published under a Creative Commons License, can be read here.

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