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.

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.