Renaissance enamels - colouring and state of preservation Non-destructive studies by external PIXE–PIGE–RBS


Renaissance enamels - colouring and state of preservation Non-destructive studies by external PIXE–PIGE–RBS

Neelmeijer, C.; Hasselmeyer, L.; Eulitz, J.

The work mechanism of the orbit clock, a masterpiece of Eberhard Baldewein in the 1560s [1] and permanent exhibit in the Math.-Phys. Salon of Dresden, houses in a fire-gilded brass body carrying rich silver-made ornamental strips plus decorative ornaments of coloured enamel. Recently, this unique object of art was under general restoration. PIXE analysis of dismount metal pieces searched for differences in composition possibly revealing former coaction of several workshops. Substantial studies of enamels used PIXE, PIGE and RBS simultaneously at the external 3.85 MeV proton beam in He-atmosphere, allowing both the detection of elements Z ≥ 5 and, in particular by RBS, the identification of surface deterioration [2].
The detailed enamel artworks show opaque but also translucent layers lying upon another for achieving visual depth effects or being singly arranged on/between gold (sometimes silver) ornamentation. Chromaticity of the vitreous materials was induced due to embedding of specific metallic ions. In accordance with ref. [3] the enamels under analysis – red, green, brown, blue, black, white – show the corresponding accompanying elements – Cu, Cu plus Fe, mainly Fe, Co plus Cu, Mn plus Fe, Pb plus Sn – respectively. Enamel composition analysis (wt% of elemental oxides) took place in consideration of small adjacent gold or silver filigrees casually hidden by the proton beam. Maximum MnO concentration of 9% was obtained for black enamel improved by 1% CoO. The translucent red material (~2% CuO) includes ~1% MnO for plastic shading effects. Tints of green enamel were tuned by the addition of both ~10% Cu- and ~6% Fe-oxide. A brown-orange detail shows 22% Fe2O3 plus 0.4% CuO. Just about 1% CoO proves quite enough for blue enamel colouration, shaded by adding ~6% CuO.
For blue, black and green enamels the concentrations of the glass network former SiO2 are in the order of 60% or above, just sufficient for resistance to humidity (hydrolysis, leaching) [2]. High concentration of 10-20% Na2O (fluxing agent) along with only 2-8% K2O (network modifier) contribute to the enamel long-term stability. These criteria, ensuring durability, are diminished in case of the red material characterized by CSiO2 < 60% and CK2O > 16%. Therefore, conservation activities should mainly concentrate on decorations made from red enamel. Altogether, not one of the RBS spectra identifies oxygen enrichment in the vicinity of the enamel surface as a characteristic signal of progressing leaching effects [2].
Two positions were found pointing to potential earlier restoration work. The small blackish detail of a complex ornamentation shows 1% MnO only but nearly twice of the current concentrations for both Fe2O3 and CoO. In addition, the RBS spectrum shows a striking carbon surface-signal reflecting probably carbon containing material for blackening. On a peculiar opaque detail of CuO-based light blue enamel Co is missing and Pb originates from potential lead white pigment.

[1] http://www.tschirnhaus-gesellschaft.de/e_g_mps.html
[2] M. Mäder, C. Neelmeijer, Nucl. Instrum. Methods Phys. Res., Sect. B 226 (2004) 110-118.
[3] K. H. Wedepohl, Die Herstellung mittelalterlicher und antiker Gläser, F. Steiner, Akademie der Wissenschaften und der Literatur, Mainz, Stuttgart, 1993

Keywords: Eberhard Baldewein; orbit clock; enamel; nondestructive analysis; external proton beam; PIXE-PIGE-RBS

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