Towards building a Cathodoluminescence (CL) database for pigments: characterization of white pigments

Paintings and painted surfaces are considered to be extremely complex due to their multitude of materials and thus form the basis for particularly intricate Cultural Heritage studies. The combination of Scanning Electron Microscopy (SEM) with Cathodoluminescence (CL) can serve as a powerful tool for the identification of individual pigments. SEM/CL has the potential of identifying both organic and inorganic pigments and can focus on a micrometer or even nanometer scale. The combination with Energy Dispersive Spectrometry (EDS) allows for robust, cross-checked, elemental and mineralogical characterization of pigments. In order to apply SEM/CL in a routine-based way for the identification of pigments, it is necessary to have a robust, open-access database of characteristic CL spectra of pigments. A large project has been undertaken to create such a database, focusing primarily at the pigments, both organic and inorganic, which were most commonly used from antiquity until today. In the present paper, the CL characterization of common white pigments is presented. White pigments were selected, due to their significance and frequency of use, since they were also present on the ground layers or mixed with other pigments in most of the painting layers. More specifically, the CL spectra of samples in pure form of calcite, kaolinite, lead white, zinc oxide, barium sulfate, lithopone and titanium white are presented. In all cases, the CL spectra present characteristic bands, which allow for a secure identification of the pigments. In order to facilitate comparison with other databases, EDS and RAMAN spectra are also presented. Additionally, the effect of weathering on the CL spectra was evaluated, by comparison to naturally and artificially aged samples and to pigments identified on areas of two paintings, of the 19th and 20th c., respectively. Finally, the effect of binding media was also studied, using combination of pigments with four common media: egg yolk, linseed, walnut and poppy oil. Overall, both weathering and binding media appear to cause minor differences in the occurring spectra, without preventing the identification of pigments.

Study of the Red Iron Paints for Rubrics and Miniatures: Accelerated Aging and Analytical Data

In this study, the red iron paints used in ancient manuscripts for rubrics and miniatures were fabricated and investigated. The commercial three different iron pigments (red ochre (Fe2O3), red bolus (Fe2O3.xSiO2.yAl2O3), and hematite (Fe2O3)) and three binding media (gum Arabic, fish glue and parchment glue) were used for the preparation of analogous to historical red iron paints. The obtained model red iron paints were analyzed with the aim to create a short data library which could be used for the characterization of different model compositions of red paints as well as real historical and archaeological red paints. The obtained red paints and binding media were characterized using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDX), X-ray diffraction (XRD) analysis and thermal (TG/DSC) analysis techniques. The accelerated aging test was also applied for the analogous to historical red iron paints. These results are useful to develop red paint guidelines for the storage and display for improved conservation and accessibility of manuscripts.

Senųjų rašytinių šaltinių dažų ir rišiklių tyrimas

Investigation of paints and binding media of ancient written sources

Formation of zinc oxalate from zinc white in various oil binding media: the influence of atmospheric carbon dioxide by reaction with 13CO2

The formation of metal oxalates in paintings has recently gained a great deal of interest within the field of heritage science as several types of oxalate compounds have been identified in oil paintings. The present work investigates the formation of metal oxalates in linseed oil in the presence of the artists’ pigments zinc white, calcite, lead white, zinc yellow, chrome yellow, cadmium yellow, cobalt violet, and verdigris. The oil paint films were artificially photo-aged by exposure to UVA light at low and high relative humidity, and afterwards analysed by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). The results showed that, compared to the other pigments investigated, zinc white is especially prone to metal oxalate formation and that high humidity is a crucial factor in this process. Consequently, the reactivity and photo-aging of ZnO in various oil binding media was investigated further under simulated solar radiation and at high relative humidity levels. ATR-FTIR showed that zinc oxalate is formed in all oil binding media while X-ray powder diffraction (PXRD) revealed it was mainly present in an amorphous state. To examine whether atmospheric CO2(g) has any influence on the formation of zinc oxalate, experiments with isotopically enriched 13CO2(g) were performed. Based on ATR-FTIR measurements, neither Zn13C2O4 nor Zn13CO3 were formed which suggests that the carbon source for the oxalate formation is most likely the paint itself (and its oil component) and not the surrounding atmosphere.

Electrochemical assessment of pigment-binding-pigment interactions in oil paint deterioration: a case study on indigo and Prussian blue

The degradation of synthetic oil paint film specimens containing indigo and Prussian blue pigments and pictorial samples from the Sant Francesc de Paula painting from the museum of Cardedeu (Catalonia, Spain) has been studied by voltammetry of immobilized particles. This technique, combined with SEM-EDX, ATR-FTIR and CG-MS techniques permits to propose a dual scheme for the degradation of the pigments when submitted to natural, UV and UV solar light aging. Under ‘ordinary’ conditions, Prussian blue acts as a radical scavenger moderating the production of ROS produced in the binding media by the action of UV radiation, resulting in the formation of Berlin green and indigo adducts with radicals. In the case of localized areas of the Sant Francesc de Paula paint, strong degradation occurs where Prussian blue acts as a promoter of the indigo oxidation to isatin, thus resulting in a considerable chromatic shift.