X-ray and ion irradiation effects on azurite, malachite and alizarin pictorial models

Résumé

XPS was used to analyze X-ray effects on pellets of azurite, malachite and alizarin pure pigments and on azurite and alizarin tempera paint mock-ups, where the pigments were mixed with egg yolk. X-ray radiation damage was assessed by comparing successive high-resolution spectra acquired on the same spot. The spectra of the pure pigments show low radiation sensitivity and reactivity under the selected X-ray irradiation conditions, although some differences among them were detected. Whereas alizarin does not experience surface chemical changes during the prolonged irradiation experiment, a slow progressive chemical reduction of azurite and malachite (copper-based pigments), together with the formation of cuprite, is observed. On the other side, tempera paint mock-ups are sensitive to X-rays and alterations of the egg yolk binding medium are displayed in a shorter time scale than the induced chemical modifications of the pure pigments. Besides, XPS allows to determine that the azurite pigment and the binder interact chemically. Azurite is partially reduced in the tempera paint from the beginning of the irradiation experiment and the subsequent X-ray exposure induces further reduction.