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
White components based on calcium-carbonate: chalk, limestone dust, synthetic calcium-carbonate, eg. St. John's white, lime white (can be identified as calcite; limestone dust may occasionally contain dolomite, synthetic products aragonite), ground shell (can be identified as aragonite, calcite, or a mixture of the two) [2]. Therefore calcite may originate from the setting coat, can be a white additive or a binder (from the reaction of lime water added to the pigment and the carbon-dioxide of the atmosphere). Aragonite may be a supplementary component in the levelling plaster (rarely), but in most cases it appears as an additive. It cannot derive from the binder since it develops from lime water at a higher temperature only. White components based on calcium-magnesium-carbonate: dolomite dust (can be identified as dolomite - in some cases containing calcite as well). Dolomite may be due to the levelling plaster or was used as a white additive [3]. White components based on calcium-sulphate: anhydride, gypsum (can be identified as anhydrous gypsum or crystallizable gypsum — by adding crystal water of varying amounts). Gypsum may come from the levelling plaster but it may have been added as a white additive, too. 3. Interfering effect of the white component 3.1. Dilution of the sample Substances originating from the intonaco or the levelling piaster may amount to half of the pigment sample that has been scraped or flaked off the grounding, but in many cases their proportion is even higher. If the binding medium used was lime water and white paint was also added to the pigment to obtain the required hue or in order to increase its covering capacity, the concentration of the component providing the colour itself may be very low . For example, the reflections of red ochre (ferric oxide), which is so often utilized for wall paintings can be of so low intensity for a mixture of light red lime-white and red ochre containing 2 mass per cent of the paint that the red ochre component can no longer be identified with certainty (Fig. 1). For the preparation of the experimental mixtures sufficiently crystallized, synthetic ferric oxide was used. In the case of red clay containing ferric oxide the detection limit is reached at a lower rate of dilution. 3.2. Masking effect of white components In some cases the reflections of the white components mask the reflections of the pigment. In such instances, if the pigment is present only in low concentration or if owing to its composition and crystalline structure the rest of its reflections appears with sjnall intensity compared with its maximum (100%) reflection, identification will remain uncertain. For example, the two strongest reflections of mercuric sulphide (cinnabar) are masked by the strongest reflections of quartz and dolomite, likewise the two strongest reflections of lead oxide (minium, red lead) are blotted out by the strongest reflection of mercuric sulphide and dolomite (Table 1).
