During the 9th citizen conference of the third season of the ‘Science à cœur’ (Science at heart) series, l’Université Pierre et Marie Curie (UPMC) presented a passionate meeting with Philippe Walter, Co-director of the Centre for the Research and Restoration of Museums in France (C2RMF); to discover the multidisciplinary techniques offered by science to understand ‘the secrets of artworks’.
Passionate about archaeology since his college days, Philippe Walter chose a multidisciplinary approach to meld passion and professional career. Chemists, physicians, art historians and, occasionally, museum curators rub shoulders within the Centre for the Research and Restoration of Museums in France(C2RMF). Here, disciplines converge to focus on one objective: the study of museum works.
Throughout his presentation, Philippe Walter explained the chemistry and physics techniques used to analyse two artworks studied at the C2RMF. These diverse analyses allow concrete scientific data regarding these artworks to be obtained (nature of materials, age of the work and so on) by art historians in order to identify the artists, the origin of the primary materials and the techniques that allow the artist to obtain the desired affect.
Analysis of cosmetics dating from Ancient Egypt
Appearance was essential in Ancient Egypt, and as such, Egyptians of all social classes used make-up: men, women and children. Their eyes for example were underlined with black Kôhl in order to emphasize them. An example of this can be seen on the famous bust of Nefertiti that dates from XIV BC. Egyptian funeral customs demanded that the deceased be buried with objects from their daily life; a reason why numerous samples of make-up have been found in tombs.
Over a 6 year period, beginning in 1995, Philippe Walter established a partnership between the industrial group L'Oréal research and the C2RMF with the common goal of reconstructing the history of make-up. Make-up samples of about 1mm3 (limited to the extent possible) were first studied with a Scanning Electron Microscope (SEM) that provides information about the morphology of grains of powder and gives a first insight into the elementary composition of samples. Then, an analysis of diffracting x-rays was conducted at the Grenoble synchrotron (ESRF) allowing two lead-based materials to be identified.
Additional work was conducted to understand why lead compounds were used in Egyptian make-up. This work consisted of using the nature of these composites as well as the historical knowledge of techniques from the time understood following the translation of a cosmetics and remedies ‘recipe book’ written by Pliny the Elder in the 1st century*.
By using a multidisciplinary approach these studies were able to reveal a complex mechanism of immunity activation; a mechanism vaunted by modern Eastern make-up today even though its origin remains unrecognised. Lead is not toxic when weakly concentrated. However, cells submitted to the stress of individual lead particles generate nitrogen monoxide (NO). The immunity system is thus activated and vascularisation increases, helping protect sensitive eyes from the numerous infections of the time. This study was published in the review ‘Nature’ in 1999 .
Analysis of The Mona Lisa by Leonardo de Vinci (the Louvre)
Although samples are taken from certain museum pieces, scientists have had to find other techniques to study exceptional works like the Mona Lisa. One day per year, specialists are authorised to study the portrait using non-invasive, in situ techniques.
Without a sample to analyse, scientists use optical techniques to study the evolution of the painting, including its composition and the glaze that binds the powder. The thickness of different layers of an artwork can thus be studied using a non-invasive technique referred to as X-ray fluorescence analysis. This technique is mobile and can be used in the Louvre itself. Different scientific techniques allow a better understanding of the subtlety of Leonardo de Vinci’s work on the faces included in his paintings.
In the C2RMF, scientists concentrate on the study of faces, emblems of the pictorial depiction of an innovative technique from the start of the XVI century named ‘sfumato’, or smoke, in Italian. The results of these scientific analyses show that artists using the sfumato technique effected compositional changes in the glaze, the thickness of which varied according to different areas of the face.
Modelling as a technique to understand works of art
It is possible, using modelling, to make virtual cuts of the painting and do tests of cumulated layers. Using this technique, we learn that the Mona Lisa has a pictorial layer of 50 µm and a layer of varnish, also around 50 µm thick. So Leonardo de Vinci’s work is as fine as the thickness of a hair! This modelling has also shown that de Vinci painted approximately 20 successive layers, each 2 µm thick, to produce the smile of the enigmatic Mona Lisa that we know so well today. Hence, subtly imposed layers allowed Leonardo de Vinci to erase all traces of brushstrokes and make Mona Lisa’s smile so incredibly fine.
Finer analyses followed to show that the exact composition of the work included small quantities of iron oxide, manganese, and lead:
Fe203 1%, Mn02 1.4%, Pb 5%
The composition of the intense black pigment used by de Vinci to give life to the Mona Lisa is not the same as that used for others of his works. The artist made the choice to use an inorganic element in his glaze and was able to make his own artistic practises evolve.
This enormous work impressed a large number of his colleagues of the time. Giorgi Vasari claimed that Leonardo de Vinci had spent ‘four years working on the Mona Lisa before leaving it unfinished’. He was also said to be astounded by de Vinci’s tones that were ‘blacker than black’. De Vinci never delivered his work to the person who commissioned the work.
The work of Philippe Walter’s team allowed the technical aspects of de Vinci’s fine and lineless works to be a little better understood. We now know that the artist used the glaze technique, patiently superimposing numerous, almost translucent layers only a few micrometers thick to obtain different nuances of shadow.
Collaborations between scientists and museums
Beyond the analysis of an artwork conducted at a single moment, other data must be taken into account in order to guaranty the reliability of the results. One must first consider that the materials studied may have undergone transformations due to the aging of primary materials over the centuries. Secondly, the materials used can be very complex and originate from human manufacture of the time. The last aspect to take into account is that artists can change their work methods over the course of their lives. All of this suggests that there may be some room for error on the part of conservationists and art historians.
Historians and museum curators often ask for scientific artwork dating and authentication techniques to be used. Surprising discoveries can be made during such collaborations as the foundation layers of artworks are probed. One example would be Picasso’s ‘Le gobeur d’oursins’, painted during the Second World War on top of another painting of a colonel, missing from the musée d’Antibes.
Philippe Walter’s evidence opens the doors to molecular and structural archaeology, and interdisciplinary research at the juncture between physio-chemistry and social history. These studies aim to better understand the evolution of materials and techniques over time taking archaeology as a starting point, but also aim to better understand how crystals and molecular groupings are formed, then transformed over long periods, sometimes with unexpected preservations.
Concluding his presentation, Philippe Walter referred to artworks that have disappeared completely. These historical and artistic losses suggest that the scientific study of works of art can, in a present context, be enriching for the conservation of artwork in museums, but also for artists of our era.
* Translation made by Thierry Bardinet, Surgeon-dentist and doctor of historical and philological sciences of the practical school of further education (l’École pratique des hautes études).
 Making make-up in Ancient Egypt, P. Walter, P. Martinetto, G. Tsoucaris, R. Bréniaux, M.A. Lefebvre, G. Richard, J. Talabot, E. Dooryhee, Nature, vol. 997, (1999) http://www.nature.com/nature/journal/v397/n6719/abs/397483a0.html
 Revealing the Sfumato Technique of Leonardo da Vinci by X-Ray Fluorescence Spectroscopy, L. de Viguerie, P. Walter, E. Laval, B. Mottin, V. A. Solé, Angewandte Chemie International Edition, vol. 49, (2010), http://onlinelibrary.wiley.com/doi/10.1002/anie.201001116/abstract
Find out more:
1) Rencontre entre beauté égyptienne et chimie, CNRS INFO, 3 9 4, (2 0 0 1) http://www.cnrs.fr/Cnrspresse/n394/pdf/394a22.pdf
2) La chimie s’invite au musée, CNRS le journal n. 232, (2009), http://www2.cnrs.fr/presse/journal/4351.htm
3) Nouvel éclairage sur les visages de Léonard de Vinci, CNRS Communiqué de presse, (2010), http://www2.cnrs.fr/presse/communique/1942.htm
4) Unexpected materials in a Rembrandt painting characterized by high spatial resolution cluster-TOF-SIMS imaging, J. Sanyova, S. Cersoy, P. Richardin, O. Laprévote, P. Walter, A. Brunelle, Anal Chem. 83(3), (2011), http://www.ncbi.nlm.nih.gov/pubmed/21218778
5) Interview with Philippe Walter on C2RMF collaborations, (2004), http://arts.ens-lsh.fr/peintureancienne/entretien/menu1/entretien03.htm