Investigation of pigment degradation due to acetic acid vapours: Raman spectroscopic analysis

The preservation of moveable cultural heritage objects inside museums includes trying to keep these objects under the best conditions as possible. This is done, amongst others, by controlling the atmospheric conditions under which the objects are preserved: the amount of light, temperature and relative humidity, etc. should be carefully monitored in exhibitions as well as in storage rooms. However, other atmospheric components may as well be important for the optimal preservation of cultural heritage materials, such as acetic compounds derived from wood package. A study of the degradation of malachite, leadwhite, lead-tin yellow type I, red lead and pigment orange 36 is performed using Raman spectroscopy. The degradation of these pigments was analyzed after exposure to acetic acid vapours. Three different acetic acid concentrations in the atmosphere were tested: ca. 150 ppm, ca. 300 ppm and ca. 500 ppm. In all conditions, the relative humidity was kept constant at ca. 75 %. A possible colour change, which can go hand in hand with the degradation process, was evaluated by observing the change of the RGB values of the sample colour. Besides pigment orange 36, which showed to be unreactive towards the acid atmosphere, acetate species were generated for each pigment: malachite reacted towards a verdigris variety; red lead, leadwhite and lead-tin yellow degraded towards lead acetate. At high doses, red lead alters towards a black pigment, which is presumed to be plattnerite.