A STUDY OF THE DEGRADATION OF NATURAL ULTRAMARINE BLUE IN OIL PAINTINGS DUE TO THE INFLUENCE OF THE LOW SULFUR CONTENT CHROMOPHORE IN THE COMPOSITION BY RAMAN ANALYSIS

Abstract

: The discoloration of natural ultramarine is related to the change in the molecular structure and crystal arrangements of the minerals due to the release of sulfur in acidic conditions. The acid breaks the Al-O bands and causes the cage to open permanently to allow access to the chromophore responsible for the color, released from the crystal structure and the loss of aluminum. Thus, a color change allows the release of the color-bearing radical anions. Thus, it is due to the natural ultramarine that has a stimulating effect on the dissolution of the binder (linseed oil), which leads to the appearance of a white dotted surface or heterogeneous white lines on the surface of the blue color or darkening dark gray more homogeneous. The study aims to study the mechanism of color change to grey whiteness in one of the oil paintings dating back to the late nineteenth century from the Gezira Museum in Cairo. It was carried out several examinations using stereo microscopy, polarized microscopy, spectroscopy photo by (VIS-UV-IR-IRFC), and analyses by color photometer, Raman and XRD methods, and related techniques (such as FTIR ART and SEM-EDX).