The last two decades have witnessed a growing interest in non-destructive analyses for the investigation and determination of painting materials. The sample volume is typically kept to a minimum while analysing museum objects. In such cases, the preferred method is a spectroscopic technique.
This article discusses the photo-physical analysis of an ancient pigment (cuprorivaite, CaCuSi4O10 (Egyptian blue)) and a modern pigment (barium manganate (VI) sulphate (manganese blue)).
The analysis consists of quantitative information, i.e., the photoluminescence quantum yields (PLQY) acquired in the NIR region and qualitative that is emission and excitation spectra.
Methods and Materials
An FLS980 Fluorescence Spectrometer (Figure 1) featuring a 450 W Xe lamp with double excitation and emission monochromators was used to measure the excitation and emission spectra. The sample detection was performed by NIR detectors (Hamamatsu).
A barium sulphate coated integrating sphere was used to obtain corrected emission spectra, which were then used to calculate PLQY as per the procedure outlined by De Mello et al. For emission quantum yields, experimental uncertainties were calculated to be ±20%, and ±5 nm and ±2 nm for emission and absorption peaks, respectively.
Figure 1. The FLS980 Fluorescence Spectrometer
Results and Discussion
The excitation and luminescence spectra of the two samples are illustrated in Figure 2. Two types of electronic transitions (2B1g-2Eg and 2B1g-2A1g) are shown by Egyptian blue shows and can be related to Cu2+ ions, which are anticipated to be the only luminescent elements of cuprorivaite. Conversely, those of Manganese Blue can be assigned to ligand-field (LF) transitions (2E-2T2, c.a 800 nm – 900 nm) and to ligand-to-metal charge-transfer bands (600 nm-800 nm) of the MnO42- unit.
Figure 2. Excitation and emission spectra of ancient, Egyptian blue, and modern, manganese blue, pigments. The energy levels corresponding to each transition are also displayed.
The peak of the luminescence profile of Egyptian blue and manganese blue is at 920 nm and 1300 nm, respectively. Subsequently, the associated PLQYs have been acquired. A quantum yield of Φ=0.5% was shown by manganese blue that is attributed to luminescence quenching factors occurring at lower energy.
Conversely, Egyptian blue was shown to be a very strong NIR emitter with a quantum yield of 10.5%, which is considered to be the optimal quantum efficiency for a molecule-level chromophore in the range of 800 nm – 1100 nm.
In this article, fluorescence spectroscopy was used to analyse photo-physical properties of an ancient and a modern art pigment. Moreover, the measurement of the PLQY of the pigments helped to quantitatively determine their photoluminescence properties, demonstrating the effectiveness of this technique to non-destructively measure samples of cultural significance.
This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.
For more information on this source, please visit Edinburgh Instruments.