Phthalocyanine blue at the heart of masterpieces

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1 Phthalocyanine blue at the heart of masterpieces 12/6/13 Catherine Defeyt recently defended her first thesis at the Faculty of Philosophy and Letters and the Faculty of Sciences. As a researcher at the European Archeometry Centre at the University of Liège, she has been studying phthalocyanine blue pigments (CuPc) in the field of artistic heritage using non-destructive scientific techniques, with a number of goals in mind. It is not simply a matter of providing information likely to improve the conservation or restoration of works according to the crystalline form of CuPc contained within, but also to reveal chronological markers that allow more precise dating. What's the story behind this painting? What does it represent? Why has the painter used such a bright range of colours? What was their state of mind when they created this masterpiece? These are the questions that buzz around the mind of the viewer observing a pictorial work, questions that arouse their curiosity and stimulate their imagination. But beyond these ponderings concerning the work's aesthetics, its historical existence or its artistic legitimacy, other questions exist relating to an even more mysterious dimension, invisible to the naked eye, but which are just as informative regarding the work's identity. These refer to its "guts", its "genetic code": Which pigments were used in this painting? How have they withstood the ravages of time? How do they manage to absorb the light to reflect it more intensely? Catherine Defeyt, researcher at the University of Liège's European Archeometry Centre (EAC), is one of the scientists who is endeavouring to reveal one of the most impenetrable facets of a painting, by going beyond its formal appearance to probe the depths and thus uncover the innermost secrets of its very substance. Passionate about the conservation and restoration of artworks, this young researcher has devoted her doctoral thesis to the study of copper phthalocyanine blue, a pigment that appeared in the 20th century and which is still widely used today in artists' paints. Its excellent colouristic properties have earned it a place on the palette of many well-known painters such as Kandinsky, Yves Klein, Roy Lichtenstein, etc. Catherine Defeyt chose to explore the characteristics of copper phthalocyanine within the context of one of courses taught as part of the MA in History of Art and Archeometry at ULg. She first studied copper phthalocyanine green before focusing her doctoral research on copper phthalocyanine blue (CuPc), under the direction of David Strivay and Muriel Verbeeck

2 "If a pictorial layer of a painting contains phthalocyanine blue and I use aromatic solvents commonly used in conservation/restoration, what are the risks of modifying the aspects of the work?" This question was formulated with a view to providing new indices, useful for the conservation and restoration of paintings containing this pigment, which is sensitive to the action of certain solvents. It was the starting point for the researcher's reflection, and the cornerstone of her thesis. CuPc, a resolutely modern pigment The advent of artistic modernity was accompanied by the appearance of new scientific techniques and numerous technological revolutions that completely changed the world of art. Industrialisation led to the widescale and low-cost commercialisation of a great variety of paints, whose so-called "synthetic" pigments were created in laboratories thanks to advances in organic chemistry. With the appearance of ready-to-use tubes of paint, artists no longer made their colours themselves, as their predecessors did in the 16th and 17th centuries by crushing pigments and then mixing them with a binder such as egg yolk and later oil

3 Copper phthalocyanine blue (CuPc), belonging to the family of synthetic organic pigments, made its appearance on the market in Initially intended for the printing sector, the new blue rapidly entered other fields of application, especially in the coloration of plastics, paint for the automotive and building industries and artists' paints. "As shown in the illustration in my thesis, nearly half of blue artists' paints are based on - 3 -

4 phthalocyanine blue, alone or in combination with other pigments", explains Catherine Defeyt. Its power as a colorant is superior to that of Prussian blue or ultramarine. It is much brighter than Prussian blue and less purple than ultramarine. Added to these qualities is its competitive price. According to the researcher, this particularly interesting price/quality ratio explains its intensive use in the manufacturing of blue oil paint. Copper phthalocyanine blue took the leading role, overtaking Prussian blue, ultramarine and cobalt blue. CuPc inside out Copper phthalocyanine blue is a polymorphous pigment that comes in five crystalline forms, created and patented by industry at various intervals in the 20th century. Conventionally known as alpha (α), beta (β), gamma (γ), delta (δ) and epsilon (ε) according to the Greek alphabet, the polymorphs of CuPc differ in stability, solubility and hue. However, only the alpha (α), beta (β) and epsilon (ε) forms were investigated by the scientist because these are the only three forms present in artists' paints. The alpha form is metastable. The alpha form of CuPc pigment crystals may well turn into large beta form crystals. This phenomenon, called recrystallisation, causes unwanted optical modifications, in particular, a shift in colour from blue to green. However, Catherine Defeyt noticed that when the pigments are contained in a pictorial environment, phase transition doesn't occur. "One possible reason is the immobilisation of the pigment particles in the pictorial system induced by the hardening of the interstitial binder when the layers of paint dry." Following this observation, the scientist chose to pursue the issue from a different angle and focus on the elaboration of a procedure to distinguish the alpha, beta and epsilon forms in a pictorial environment. "We know that if we detect CuPc blue in the original layers of a painting during analysis, it can't have been painted before 1935 because this pigment wasn't available on the market prior to this date. The identification of CuPc blue offers a terminus post quem likely to call into question the veracity of its attribution. Thanks to this pigment, we have been able to detect forgeries attributed to James Ensor, Max Ernst, Maurice Utrillo and Lioubov Popova. I wanted to go even further", she adds. "I wanted to draw attention to the fact that if we identify the crystalline form of the pigment, we can narrow down the chronological margin, given that the alpha form of phthalocyanine appeared in 1935, the beta form in 1953 and the epsilon form in 1962." The procedure to distinguish the CuPc polymorphs in a pictorial environment developed by Catherine Defeyt has been particularly useful in dating Léger and Pechstein forgeries, unmasked by scientists at the Rathgen Research Laboratory in Berlin, thanks to the identification of the pigment in the original layers

5 Besides the fake German Expressionist paintings, the researcher used non-destructive methods such as infrared spectroscopy, Raman spectroscopy and a scanning electron microscope to study the paintings of Paul Delvaux, such as Faubourg sold at Christie's, or La Genèse at the Aquarium Museum at the University of Liège. The researcher also had the privilege of examining The Soler Family by Picasso, and blue paintings by the American painter Sam Francis as well as Roy Lichtenstein, currently kept at the Getty Conservation Institute in Los Angeles