Forensic provenance: the science of art authentication

The value of an artwork can vary by orders of magnitude if its provenance can be established; increasingly, science is coming to the aid of curators to prove provenance.

Scientists from the Australian Synchrotron recently assisted conservators from the State Library of New South Wales (SLNSW) to determine provenance of a series of exquisitely detailed, unsigned, scientific drawings of birds and flowers that date back to the years immediately following the arrival of the First Fleet.

The drawings from SLNSW’s TAL & Dai-ichi Life Derby Collection depict local plants, birds and fish, and feature iridescent effects through the use of gold, silver and brass leaf with watercolours.

 

Drawings from SLNSW’s TAL & Dai-ichi Life Derby Collection.

The drawings are believed to be originals done in Australia, but the intricate gold leaf work was rarely practised in European watercolours and so there was some thought that the drawings were in fact copies done in another country such as India.

By determining the composition of the gold leaf and the other finely ground mineral pigments that had been used to create the drawings, it was hoped that the identity or nationality of the artists could be established.

Cultural materials conservators Kate Hughes and David Thurrowgood from SLNSW took five of the drawings to the synchrotron in June 2014. Due to their delicate nature the drawings cannot be touched, so the synchrotron presented an opportunity to examine the very fine detail of the drawings, in which many of the brushstrokes are finer than 0.1 mm.

The analysis was carried out on the synchrotron’s X-ray fluorescence microscopy (XFM) beamline with Daryl Howard. This non-destructive synchrotron technique can map which elements (such as gold, copper, zinc and iron) are present in a sample, along with their locations and chemical form. XFM can resolve details as small as 0.1 mm across and detect much lower concentrations of elements than laboratory-based techniques. It also collects data on timescales that make it possible to scan entire works of art.

Kate Hughes (SLNSW) examines the results of her X-ray examination of the bird drawings.

XFM is more commonly used for research and development work to help the mining industry improve its productivity and environmental performance, and by biomedical researchers. Recent uses include examining the role of metals and metal-containing enzymes in diseases such as Alzheimer’s and investigating how the body deals with zinc oxide nanoparticles in sunscreens. It is increasingly important for agricultural researchers, with applications that include assisting the development of rice and other grains with improved levels of essential minerals, and the development of more-effective phosphorus fertilisers.

Preliminary results from the XFM scans revealed that the gold leaf was of very high quality, indicating the artworks were produced in Australia using European materials. Highly refined, the gold leaf contained only a small percentage of copper and, Hughes said, very few impurities.

“This is a really strong indication that the gold was refined in the UK or in Europe,” she said. “You would expect a greater degree of impurities in gold refined in India due to [it] not having the technique as refined as Europe did at the time.”

The synchrotron data also showed that the pigment in the watercolours was highly refined - suggesting the artist had not ground the materials themselves but had purchased the paints for the trip to the new colony.

The researchers are now developing a pigment composition database and will use their synchrotron findings to validate a method for assessing other works in the SLNSW collection using laboratory-based technologies. The information will be shared with colleagues at the Natural History Museum in London.

Private collector scores a Raphael

A work belonging to a private collector in Cordoba, Spain, has recently been attributed to the great Raffaello Sanzio da Urbino (Raphael) - the famous Renaissance painter - following scientific investigation at the University of Granada. The painting, entitled the Small Madonna of Foligno, depicts a scene identical to that of the Madonna of Foligno and was probably a preliminary version of Raphael’s painting, which is exhibited in the Vatican Pinacoteca.

Small Madonna of Foligno - Raffaello Sanzio da Urbino.

Luis Rodrigo Rodríguez-Simón, lecturer in the Faculty of Fine Arts at the University of Granada, has identified and reliably attributed the work, hitherto by an unknown artist, following a minutely detailed study lasting several years.

He conducted a technical, scientific study applying a series of advanced instrumental techniques and analytical methods: X-ray, infrared photography, infrared reflectography, fluorescence under ultraviolet light, analysis of paint layers, scanning electronic microscope linked to an energy-dispersive X-ray microanalysis system, gas chromatography-mass spectrometry and micro Raman spectroscopy.

The painting arrived in Cordoba from France in the late 19th century. The paint layer study has revealed that the work was transferred from wood to canvas in the second half of the 19th century. A preparation of several layers of lead white over a set of three canvases has been found. This corresponds to the way in which paintings were transferred from one support to another at that time in France. Other paintings by Raphael - The Ecstasy of Saint Cecilia (Pinacoteca Comunale, Bologna, Italy) and The Madonna of Foligno itself (Vatican Pinacoteca) - underwent the same change.

The University of Granada researcher discovered two hidden fragments of paper, stuck to the frame, which confirm that the change of support happened in France. The first is written in French, in iron-gall ink, and gives the date as ‘16 Avril’ and the year, 1888. The other is part of a page from a printed catalogue of works of art to be sold through the Hotel Drouot auction house in Paris and dated in 1872.

Using infrared photography, Rodríguez-Simón identified Raphael’s preliminary sketches for the painting, as well as a combination of different graphic techniques in the underdrawing. “The practice of working with different drawing instruments, ranging from chalk to brush, has been found in many of Raphael’s works,” he says.

Moreover, the study found a direct correspondence between the underdrawing of the Virgin’s head in this painting and a drawing on paper in the British Museum, London, known as Study for the head of the Virgin, proving that both were created by the hand of Raphael himself.

In the Small Madonna of Foligno, two letters decorate the cuff of the Virgin’s tunic: the capital letters ‘R’ and ‘U’, the initials of Raffaello de Urbino. “Raphael stamped a similar rubric in the decoration that is part of the brocade adorning the same cuff in the major work, held in the Vatican Pinacoteca, with the same theme,” explains Rodríguez-Simón.

Similarly, he has also discovered the first letters of the name Raffaello or Raphael and the year 1507, which have been incised, when the paint was fresh, in the flesh colour of the Virgin’s right hand.

Infrared photography has also led to another discover of major importance: the existence of numbering on both the upper and right sides and short hairsbreadth lines all around the edge of the painting, some 2.9 cm apart. “These graphics can be explained by the use of the method of squaring to transfer this composition to a larger scale, as shown by the number of squares and the fact that they are so small,” says Rodríguez-Simón.

Forged facts

When Peter Silverman bought a ‘German, early 19th century’ coloured chalk-and-ink portrait in 2007, few would have imagined that scientific investigation could have turned his $21,850 purchase into a $160 million investment in a drawing by Leonardo da Vinci.

With so much money at stake, the portrait known as La Bella Principessa has been subject to ongoing scientific investigation.

La Bella Principessa.

Firstly, many of the drawing’s pigments were analysed and it was determined that none of them had been invented after da Vinci’s time period. A sample of the parchment was sent to the Swiss Federal Institute of Technology, in Zurich, for radiocarbon dating. The parchment was dated between 1440 and 1650, which fits in well with da Vinci’s lifetime (1452-1519) but does not establish when the drawing was done as it could still be a much more recent drawing on old vellum.

Martin Kemp, Emeritus professor of art history at Oxford University, thought he could see da Vinci’s hand in the drawing - how the hair was bunched, the modulation of colours, the precise lines which were definitely drawn by a left-hander … But such expert opinion does not prove provenance.

Next the drawing was sent to Pascal Cotte for high-resolution multispectral scanning at the Research Laboratory Lumiere-Technology in Paris.

These multispectral images were used by Peter Paul Biro, who has pioneered a different approach to authenticating pictures. Biro looks for the artist’s fingerprints, impressed in the paint or on the canvas. Using advanced image-processing software, Biro claims to have subtracted the background noise in Cotte’s images, until only the clearest parts of a fingerprint remained. This fingerprint is said to be “highly comparable” to a fingerprint on da Vinci’s St Jerome that hangs in the Vatican.

Sadly, Biro’s credibility and honesty have been seriously questioned and his assertions no longer assure provenance.

Kemp and Cotte still assert that the portrait is a da Vinci and have done a lot of work on establishing that the drawing had been torn from a bound volume. Against all odds, Kemp tracked the volume down to Poland’s national library in Warsaw.

The portrait is claimed to have been cut from one of the Sforziadas, the copies printed in the 1490s, dedicated to the Duke Ludovico Sforza’s father and family. Each copy was illuminated with a differently illustrated frontispiece depending on the dedicatee. The copy, from which the vellum is extracted, was given at the wedding, in 1496, of Ludovico Sforza’s natural daughter, Bianca, to his faithful Commander, and subsequent son-in-law, Galeazzo Sanseverino.

Stitch-holes along the portrait’s left-hand margin are a perfect match for the spacing of the holes bound in the Sforziadas.

Technical analysis confirmed that “the vellum of the portrait closely matches, in all respects, the physical characteristics of the remaining sheets”, Kemp says. “Vellum sheets are made by an elaborate process of shaving a calf, kid or lambskin to a desired thickness. Even in a batch of sheets the thickness will vary. The thickness of the portrait parchment is entirely consistent with the Warsaw book’s folios.” The volume even bears an incision where the blade that removed the sheet slipped.

However, the very murky pseudo-science of the fingerprint has tarnished the portrait’s reputation and no one is offering the $160 million the portrait would be worth if its provenance could be proven.