guia do novo coronavirus
Imprimir Republish

Chemistry

Secrets under the paint

X-ray fluorescence provides intimacy to nineteenth century paintings

Cristiane Calza/COPPE-UFRJPedro Américo: portable equipment investigates the painting Moses and JocabedCristiane Calza/COPPE-UFRJ

For more than a century, the young woman depicted by painter Eliseu Visconti in the picture Gioventú hid a study for another work of art by the same artist, Reward of Saint Sebastian. The revelation comes from the archaeometric work of chemist Cristiane Calza, a researcher in the postgraduate engineering program of the Federal University of Rio de Janeiro (Coppe/UFRJ). Instead of the stereotypical lab full of test tubes and foaming substances, her work environment is surrounded by works of art from many different eras, from the paintings that decorated the sarcophagi of Ancient Egypt to nineteenth century paintings, also encompassing ceramic thongs of the Marajoara people, who lived in the island of Marajó between the fifth and the fourteenth centuries. By examining paintings down to the detailed level of the atoms with the aid of X-ray fluorescence techniques and X-rays, she uncovers the secrets that lie beneath the paint, characterizes the pigments that were used in the palette of each painter and points out touch-ups and wear on the canvases, thus providing guidance for future restoration work.

Having been invited to analyze the works of art at the National Museum of Fine Arts (Museu Nacional de Belas Artes) in Rio de Janeiro, Cristiane, who has always had a penchant for archeology, history and the fine arts, fell in love with nineteenth century Brazilian painting and ended up analyzing 33 pictures by artists such as Rodolfo Amoedo, Eliseu Visconti and Félix Émile Taunay using X-ray fluorescence, which reveals the atoms of which the layers of paint are comprised, along with computerized X-rays. The main objective was to characterize the pigments used by each painter, incorporating this information into a database that will be available for restorers, conservers, art students and researchers.

This work was possible because during her doctoral studies Cristiane developed a portable X-ray fluorescence system made to measure to analyze works of art and archeological material. It is a box only slightly larger than a book, which she can take to the museum and which she has carried to Peru, to analyze pre-Columbian gold. Very large or very valuable works (the painting First Mass, by Victor Meireles, one of the paintings studied by the expert, is insured for US$3 million) cannot be transported to laboratories that have the standard X-ray fluorescence equipment.

“The technique is non-invasive and doesn’t harm the works of art”, stresses the researcher. The device releases a beam of X-ray focused on a half-centimeter circle, producing a process known as a photoelectric effect: as they move in order to reestablish equilibrium, the electrons also emit X-rays, which the equipment detects and reproduces on the computer screen in the form of energy emission curves. Each chemical element has its own characteristic energy emission signature. With this information in hand, Cristiane can then infer what pigment was used on that spot in the picture.

Cristiane Calza/COPPE-UFRJRodolfo Amoedo: Study of a woman, painted over white leadCristiane Calza/COPPE-UFRJ

The important thing is that some pigments reveal the time when the painting was done. They may look similar, but what distinguishes the paints used are the chemical elements in them – which the researcher from Coppe manages to see.  White zink, used to this day, was first produced in the eighteenth century, but only in 1835 did it reach a price that most painters could afford;  white titanium, on the other hand, only appeared in the twentieth century. A blue spot analyzed with X-ray fluorescence may reveal the presence of iron or of cobalt atoms, for example. In the first case, the painter used Prussian blue pigment, created in 1704; in the second, cobalt blue, used since 1807. The ochre pigments are not used much: back in prehistoric times, they were used to color rupestrian paintings and they continue to be used to this day. “They are cheap pigments, derived from clay”, explains Cristiane. Some other pigments, on the other hand, are now prohibited because they are carcinogenic, such as those that are mercury, arsenic or lead based.

In her analysis, the researcher analyzes several spots in order to characterize the paintings and to avoid later touch-ups inducing errors. “If we see large areas with a pigment that is more recent than the alleged date of production of the piece of art, we know it’s a fake”, she tells us. Repaired torn canvases are also telling: they are restored with a mixture of calcium carbonate, or calcite, and fish glue. Over this, one applies a layer of white paint before touching up the painting. The white paint helps one to date the restoration, because some white pigments are highly typical of certain times. This is what is revealed by the titanium white that Cristiane found in the pictures The last Tamoyo and Bust of Mrs. Amoedo, painted by Rodolfo Amoedo in 1883 and 1892, respectively. According to a 2009 article in X-Ray Spectrometry, only in 1921 did a suitable paint appear on the market, for artistic purposes, made from this pigment, indicating that the paintings were touched up in the twentieth century.

Cristiane characterized the palette of pigments used by eight nineteenth century painters in 12 paintings. The results have already been accepted for publication in the specialized journal Applied Radiation and Isotopesand they show, for example, that Eliseu Visconti and Henrique Bernardelli used cobalt blue, whereas Pedro Peres employed Prussian blue. They also confirm some things that were already known informally, such as the fact that nineteenth century Brazilians made shades of red by mixing ochre red and vermilion. Knowing this will be fundamental for future restorations to use pigments similar to the original ones, whenever they can still be acquire.

Exposed layers
Rather more intriguing findings resulted from the examination of the picture Gioventú, which earned Eliseu Visconti a silver medal at the Paris Universal Exhibition in 1900. A computerized X-ray, similar to those radiologists use to investigate broken bones, revealed a previous work of art beneath the girl in the painting. It is, undoubtedly, a study for another award winning painting, the Reward of Saint Sebastian, which got the gold medal at the Saint Louis International Exhibition of 1904, in the USA. In the X-ray images, the angel placing a laurel wreath on Saint Sebastian, who is tied to a tree, stands out even more clearly than the Gioventú girl. The painter seems to have changed his mind after the study, because the finished painting shows an angel placing a halo around the saint’s head.

Cristiane Calza/COPPE-UFRJEliseu Visconti: beneath the paint of Gioventú, an outline of the painting Reward of Saint SebastianCristiane Calza/COPPE-UFRJ

The X-ray, taken jointly with colleagues Davi Oliveira and Henrique Rocha, also highlighted the wear and tear of the canvas at some points near the frame. “This is quite common, as this area gets more worn because of how the fabric is stretched and  friction with the wood of the canvas frame and  the picture frame. Such areas are invisible to the naked eye because the painting was restored previously, when the little holes were filled with paste and paint,” explains Cristiane. Detecting faults covered by paint can be of great help in the conservation and restoration of the painting.

The X-ray fluorescence analysis of this painting made it possible to characterize the palette used by Visconti, regarded as an artist that bridged the nineteenth and the twentieth centuries because he was a pioneer of impressionism in Brazil. In the yellow veil that covers the girl, the iron and lead content reveals that he used white lead  paint, which artists stopped using in the twentieth century, mixed with ochre yellow. The background greenery reveals the mixtures with which Visconti created different shades of green: viridian, chrome oxide, ochre yellow and cobalt blue.

Diving into a more distant past, Cristiane has also assessed the Marajoara thongs and Egyptian pieces from the collection of the National Museum of Rio de Janeiro. Marajoara ceramic pieces are among the most sophisticated representations of pre-Columbian art and, along with Renato Freitas, at the time a master’s degree candidate, the chemist from Coppe examined 400 fragments of the ceramic thongs that covered the pubis of the women from the island of Marajó. The most abundant element in these ceramic pieces is iron, which explains why the clay that was used was red. The characterization, published in 2009 in X-Ray Spectrometry, indicates that there is raw material from more than one source. Perhaps the Marajoaras used clay from several sources to produce their thongs; alternatively, the differences might indicate that the studied collection was made by different tribes. One would have to associate the chemical data with archeological information to gain a better understanding of the history of this people about whom we know so little.

The analysis of fragments of an Egyptian sarcophagus and of the tissue used to wrap a mummy also revealed the potential usefulness of the X-ray fluorescence technique. Besides characterizing the pigments used, which proved to be in line with what was available at the time, Cristiana indicates, in an article from 2008 in Applied Physics A, that the fabric, whose origin is well documented, was from the same time as the sarcophagi fragments that she examined.

As she can help to elucidate mysteries from the past, Cristiane now has her schedule booked solid with requests from museums and historical buildings that are being renovated. She is aided by Renato Freitas, who is now working on his PhD, and who may start sharing the work when the new piece of equipment, even smaller than the current one, is ready.

Scientific articles
CALZA, C. et al. Characterization of Brazilian artists’ palette from the XIX century using EDXRF portable systemApplied Radiation and Isotopes. In press
CALZA, C. et al. Analysis of the painting Gioventú (Eliseu Visconti) using EDXRF and computed radiographyApplied Radiation and Isotopes. In press

Republish