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

Malachite (Sample M-2) On the diffractogram representing the state before acid treatment malachite, gypsum and calcite can be identified (Photo 3a). After the acid treatment neither malachite nor its transformation product could be detected in the sample (Photo 3b). This implies that in the case of malachite the acid treatment brought about structural changes and the evolving product was washed out from the sample while it was being rinsed in distilled water. But the relative concentration of the glauconite present in the sample increased and its reflections appeared on the diffractogram. 5. Conclusions When examining paint samples taken from murals after the decomposition by dilute hydrochloric acid of calcite, aragonite and dolomite, which are present in the sample as additives, transformation products of binding media or as impurities left behind in the levelling plaster, the following observations were made: — the structure of the pigment did not change and detectability improved for hematite (red ochre), mercuric sulphide (cinnabar), glauconite (green earth) and calcium-cupricsilicate (Egyptian blue); — the structure of the pigment did not change, but detectability did not improve for goethite (yellow ochre); - the structure of the pigment changed but detectability improved indirectly for lead(II,III)-oxide (minium); - the sturcutre of the pigment changed and became unidentifiable in the case of malachite. The increase in the relative concentration of quartz and the distorting effect resulting from it may be reduced by the mechanical removal of the quartz grains. The performance of informative X-ray diffraction tests carried out without preliminary (or subsequent) element analysis will in the majority of cases improve through the decomposition by dilute acid of the calcium-carbonate or calcium-magnesium-carbonate based white accompanying substances. It is intended to carry out further tests empoying less frequently used pigments which are identifiable by X-ray diffraction, and attempts will be made to remove other interfering white components (e.g. white lead, gypsum, barium white). Notes and bibliography 1. Fitz suggests the removal of the pigment from the base before the diffraction tests in order to increase detectability: S. Fitz: Identification of pigments in paintings with X-ray diffraction method — possibilities and limits, 5th Triennial Meeting, ICOM, International Committee of Conservation, Zagreb, 1978, Preprints 20/7. Given the fact that the samples involved crumble easily, this could be done only very infrequently. 2. The list is based on the discussion in the article by R.J. Gettens, E.W. Fitzhugh, R. E. Feller:Calcium Carbonate Whites, Studies in Conservation, 19 (1974) 157-84. 3. In the special literature dolomite is only rarely referred to as a white pigment, however it was widely used in the Roman province of Pannónia: M. Járd: Preparation of wall paintings in the Roman province of Pannónia according to recent investigations,