Iridescent Art Nouveau glass – IBA and XPS for the characterisation of thin iridescent layers

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Abstract

The external proton beam of the Tandem accelerator of the Research Centre in Rossendorf/Germany was used to carry out non-destructive particle-induced X-ray emission (PIXE) particle-induced gamma-ray emission (PIGE) and Rutherford backscattering (RBS) measurements simultaneously on Art Nouveau artifacts produced around 1900 by Tiffany/USA and Loetz/Austria. These studies should proof the technology of producing an iridescent layer on a glass surface. By means of the yield ratio Y(Si-K)/Y(Si-γ) of both characteristic X-radiation (Si-K) and γ-radiation (Si-γ) of the element silicon it could be shown that a thin top layer is present on the glass surface due to the treatment of the heated artifacts (about 500°C) with an alcoholic solution of SnCl2 [1]. Combined evaluation of the PIXE and RBS spectra resulted in a thickness of 20–300 nm for this top layer. In addition, a transition region between the iridescent layer and the bulk glass was obtained by RUMP simulations. Approximately 80% of the total amount of the Sn were found to be present in this transition layer and only 10–20% in the outermost surface region. XPS studies showed that the outermost layer consists of SnO2. The formation of other Sn compounds in the outermost near-surface region based on Sn–Si–O during the manufacturing process can be excluded.

Introduction

Art Nouveau iridescent glass has been of great interest for public and private collections ever since this type of glass was produced. The iridescent effect could be achieved by spraying the hot glass surface with an alcoholic solution of SnCl2 as well as by applying mixtures of different metal salts (e.g. tin chloride, barium carbonate, strontium carbonate, cooper nitrate, etc.). Some recipes applied for a patent regarding the artificial production of the iridescent glass surface by Loetz manufacture are given in [1]. As the decoration on the glass surface consists in some cases of tiny and fine lines the application of a microbeam facility seems to be appropriate.

The characterisation of thin iridescent layers in a non-destructive manner is an important tool to examine their manufacturing process on originals of companies such as Tiffany/USA or Loetz/Austria. Previous investigations carried out on cross-sectioned samples by SEM/EDX have shown that the iridescent layer was applied directly onto the lead–silica bulk glass of Tiffany objects, whereas for Loetz artifacts potash–lime–silica glass was used as bulk material coated with a lead glass layer of 100–300 μm thickness [2]. In addition, silver was detected in the lead containing surface layers of Loetz artifacts as well as in a thin surface domain of Tiffany objects.

The combination of particle-induced X-ray emission (PIXE) and Rutherford backscattering (RBS) for simultaneous measurements allows the determination of both the elemental composition and the thickness of the thin iridescent surface layer. The results obtained by GUPIX (layered target) [3] were used as input for a first-order simulation of the measured RBS spectra. Additional measurements were carried out with XPS in order to state the chemical composition of the outermost near-surface region more precisely. It should be proved whether SnO2 or other tin containing compounds (e.g. SnSiO4) have been formed when producing iridescent layers of Tiffany or that of Loetz.

Section snippets

IBA equipment

Simultaneous PIXE, particle-induced gamma-ray emission (PIGE), and RBS analyses were performed using the external proton beam of the 5 MV Rossendorf Tandem accelerator. X-rays of the elements with an atomic number in the range of 11<Z<27 were measured at 50° backward angle by means of an Si(Li) detector (PIXE1: 12mm2 active area, ΔE=140eV). An aperture of 0.9 mm in diameter was positioned in front of PIXE1 detector to reduce the count rate for that detector. X-rays of elements Z>25 were

Results and discussion

The element tin on the iridescent surfaces of both Tiffany and Loetz glass fragments, recently found by X-ray fluorescence analysis [2], could be detected by PIXE as well. In addition, the yield ratio of Y(Si-K)/Y(Si-γ) measured simultaneously by PIXE (ESi-K = 1.740 keV) and PIGE (ESi-γ=1.779 MeV) served as an indicator for the presence of a thin absorbing surface layer on the glass [6]. As described in [6] this method makes use of the different attenuation of low-energy X-radiation (Si-K) and

Summary

The combination of PIXE, PIGE and RBS has shown that the iridescent layers of Loetz glass were produced using a SnCl2 solution and not by applying mixtures of different metal salts. This is because only Sn was detected in the top region of the glass. A thin SnO2 layer (ca. 50 nm thick) covers the outermost glass surface. In the near future additional measurements on Tiffany glass fragments showing a fine and tiny decoration will be carried out by means of a microbeam to proof the similarities

Acknowledgements

The authors gratefully acknowledge the New York Historical Society, “Passauer Glasmuseum”, Museum of Applied Arts Vienna, as well as Christian Clausen for providing us with original glass fragments of Tiffany and Loetz. Ion Beam Analysis performed at the Research Centre Rossendorf was supported by the European Community FMGE-CT98-0146 (Large Scale Facility AIM Rossendorf).

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