Material analysis in art historical research provides information on dating, tracing the process of origin and state of preservation of art objects. Usually XRF and optical spectroscopy are used to study pigments in paintings. However these methods suffer from poor spatial resolution and therefore can not be used for the analysis of the layer structure of paintings. In these cases high resolution methods like SEM/EDX are indispensable.
The scientific laboratory of the "Prussian Palaces und Gardens Foundation Berlin-Brandenburg (SPSG)" is engaged in the material analysis of German paintings from the 15th and 16th century, especially the works by Lukas Cranach the Elder (1472-1553) and the Younger (1515-1586). In this particular case, a portrait of Joachim II. Elector of Brandenburg (1505-1571) originated in the second half of the 16th century  by Lukas Cranach the Younger is signifying, (Fig. 1). Main subjects of research are the changes of pigments caused by ageing and the mechanisms involved in these processes. Questions like "What was the original colour of the background which is nowadays grey?" or "Why turned blue into grey?" are of special importance in art history and conservation science. To find an answer to these questions the portrait was analyzed by X-ray fluorescence (XRF) and optical spectroscopy and microscopy. Lukas Cranach the Younger used azurite (Cu3(CO3)2(OH)2), a copper mineral, for the blue colour in the finger ring of Joachim II. whereas in the grey-blue background the elements identified by XRF (Si, K, Co, Ni, As) are indicative of the occurrence of smalt. Smalt is a potassium silicate glass coloured blue due to the presence of cobalt (II)-oxid. Nickel and arsenic are components of cobalt ores. Smalt is the oldest known cobalt pigment. The main period of smalt application was between 1550 and 1700. Lukas Cranach the Younger typically used smalt for the design of the portrait backgrounds which were formerly blue but now widely grey coloured. Lukas Cranach the Elder however obviously did not use smalt. To verify smalt in the portrait background and to determine in which layer it is located and if a change of colours is involved, Scanning Electron Microscopy- and Energy Dispersive X-Ray-investigations were performed at the "Central Institute for Electron Microscopy (ZELMI)" of the Technical University of Berlin.
Preparation and Analysis
A small particle from the upper edge of the painting background was extracted.
It was embedded in resin, ground and polished on one side so that it could be analysed in cross section. It was coated with a thin carbon layer. A Hitachi-SEM (S-2700) equipped with an XFlash-SDD-Detector (Serie 1500) was used for SEM/EDX.
The accelerating voltage for the analytical measurements was 20 kV. The X-ray-mappings were accomplished with a count rate of about 100000 counts per second at an acquiring time per pixel of 10 milliseconds. A comparative observation of the light microscope image, (Fig. 2), and the EDX element mappings, (Fig. 3), enables the reconstruction of the composition of the paint layers: On the chalk ground (high content of Calcium) an inter layer was identified that contains white lead (2 PbCO3 . Pb(OH)2) and some red lead (Pb3O4, orange coloured particles in the light microscope image). White lead has been an important white pigment since ancient times. It is lightproof, and has a high covering power. Red lead or so called minium is a bright red or orange crystalline or amorphous pigment. The next layer, a grey underpaint layer, consists of white lead and plant black. Particles of white lead and widely discoloured smalt can be seen in the uppermost blue paint layer. Smalt is known for poor ageing properties in oil binding media that result in the loss of blue colour in the pigment. As can be seen in the light microscope cross section image, only a few pieces of smalt are still blue while the others appear grey. The EDX-spectra (Fig. 4) shows that the content of potassium in the grey coloured smalt particles (red spectra) is significantly lower than that in the blue ones (blue spectra). The EDX-mappings confirm this. While all smalt particles have the same silicon content the grey ones contain less potassium than the blue ones.
The discoloration of the smalt particles is probably caused by a diffusion of potassium ions from the glass matrix into the binding medium. This process is accompanied by a strong distortion of the glass matrix in general and of the local environment of the cobalt ions in particular. Even thought the concentration of cobalt in the glass does not change, the distorted local environment of the cobalt ions leads to changes in the optical absorption properties and finally to a discoloration of the glass.
Due to the good spatial resolution compared to X-Ray-Fluorescense and Optical Spectroscopy and the high speed of the SDD-detector, SEM/EDX analysis is a powerful tool for the investigation of cross-sections of paintings and other art objects. Useful information concerning the layer composition, the pigments and filling materials used by the artists as well as the deterioration processes of painting materials can be gained.
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