Pixe-a Characterisation of Painted Surfaces Using the LNS PIXE-alpha Spectrometric System
Painted surfaces (paintings, frescoes, ceramic pigments) exhibit some common characteristics that can be summarized as follows: a superficial layer with a thickness ranging from 5 to 20 mm that contains the coloured pigments, a substratum below the superficial layer, the backing that supports the whole masterpiece. Each of these constituents has a definite role in the process of historic and artistic knowledge. Their characterization is of fundamental importance for the restoration and conservation.
The LANDIS (the INFN Laboratory that works in collaboration with the Physics and Astronomy Department of Catania University and the IBAM CNR) has developed new non destructive methods aimed to the characterization of the first layer, the coloured one, by using portable instrumentation.
The PIXE (Particle Induced X-Ray Emission) seems to be the most appropriate analytical method to characterize the painting surfaces. In fact:
The analysed depth is well defined and limited to a few microns;
The ionization of the charges particles is not affected by resonant processes or selective transparency effects as in the cases of electromagnetic induced analytical processes.
Due to the sharpness of the analysed layer no contribution from the substratum affects the results.
A limitation to the use of the PIXE technique arises from the fact that only elemental analysis can be performed i.e. chemical compounds cannot be determined.
PIXE analysis is generally performed by using charge particle accelerators.
The LANDIS Laboratory has developed a portable system based on the use of a 210Po as charged particle emitter. The system is shown in Fig.1. The radioactive source (alpha particles of 5.1 MeV outgoing energy) is contained in the head of the spectrometer visible on the right of the figure. An Helium flux reduces the absorption of the fluorescence X-rays emitted by the sample under analysis.
The detector is a 10 mm2of active surface SDD with 138 eV energy resolution. PIXE analysis is performed by approaching the head of the instrument to the painted surface and by acquiring the X-ray fluorescence energy spectrum as seen in the PC shown in the figure 1.
Several analyses have been carried out during the last years.
One of the most important result concerns the characterization of the gold preparations in a Salerno Pontificale conserved for a restoration at the Istituto Nazionale della Patologia del Libro inRome. Fig.2 shows an analyzed point where it has been possible to carry a complete quantitative characterization of the gold preparation. In this case the preparation is composed by gypsum mixed to white-lead in a proportion of 8 to 1. This value has been compared to the one reported in the medieval recipes. Quantitative analysis has been possible by the use of GUPIX code in the “layered option”.
b) Ancient ceramics coloured surface.
Several examples of intervention at museums or deposits can be presented. One of the more interesting measurement campaign refers to the systematic study of ceramics pigments of ancient wares conserved at the Festos (Crete) deposits of the Scuola Italiana di Atene. As an example the large use of talc has been evidenced through the elemental composition and has been confirmed through the complementary use of the portable LNS diffractometer which evidenced the Enstatite. An example is given by the ware in Fig. 3.
c) Fresco surfaces
The PIXE – alpha system and the LNS portable diffractometer have been used for the quantitative characterization of the fresco surfaces at the Nestor Palace in Pylos (see fig.4): Quantitative data enabled the individuation of Egyptian blue and talc.
Recently, an extensive work program has been undertaken to check the limits of quantitative analysis on roman frescoes by the use of the PIXE and diffractometer portable systems.
Staff: Francesco Rizzo