M. Járó - L. Költő szerk.: Archaeometrical research in Hungary (Budapest, 1988)

Analysis - KRISTON László, JÁRÓ Márta: Impact of acid treatment of pigment samples from wall paintings on the result of paint identification by X-ray diffraction

KRISTON László*, JÁRÓ Márta** IMPACT OF ACID TREATMENT OF PIGMENT SAMPLES FROM WALL PAINTINGS ON THE RESULT OF PAINT IDENTIFICATION BY X-RAY DIFFRACTION Abstract - Identification by X-ray diffraction of pigment samples taken from wall paintings is sometimes very difficult or impossible because of the presence of white components (used for the intonaco or as additive orbinding media). The aim of the experiments was to determine whether the dilution (by acid) of the white components would change the structure of pigments commonly used for murals. The result of the investigation was: the sturcture of some pigments did not change (red ochre, cinnabar, green earth, Egyptian blue) and detectability improved; the structure of the minium changed but the detectability improved indirectly; the structure of the yellow ochre did not change, but detectability did not improve; malachite became unidentifiable after treatment. 1. Introduction Fjtamination by X-ray diffraction of pigment samples taken from wall paintings is often sufficient for identifying the inorganic, crystalline pigment or pigments providing the colour and consequently - especially if the analysis is of informative nature - there is no need for further element analytical procedures. As a result, the analyses will be faster and simpler. In many cases, however, identification will be rendered difficult or, in fact, impossible by the presence of white components used for the intonaco, or the levelling plaster (the upper layer of the intonaco if it has more than one), or as additives and binding media. The distoring effect of the white components may be manifested in two ways: 1. they dilute the sample and thus the concentration of the pigment will fall below the detection limit [1], 2. their reflections will mask the reflection of the pigment on the diffractogram. The aim of our experiments was to determine whether a dilute acid solution applied to dissolve the white components, which make identification more difficult, would change the structure of pigments used for murals and if the efficiency of X-ray diffraction can be improved by acidification. In the course of the tests described below the behaviour of the most frequently occurring pigments was examined after the dissolution in dilute hydrochloric acid of the white components based on calcium-carbonate and calcium-magnesium-carbonate. 2. The most frequently occurring white components in the pigment samples taken from murals The white components have been grouped according to their chemical composition and, further, we give the minerals they may be identified as in the course of the X-ray diffraction test. * Institute of Criminology H-1028 Budapest, Maros u. 6/a. ** National Centre of Museums H1087 Budapest, Könyves K. krt. 40.