Non-destructive Low Field EPR and Solid State NMR for the Characterization of Paramagnetic Components in Cultural Heritage Objects 1 2 3 Nicholas Zumbulyadis , Brian J. Antalek , William J. Ryan , Joseph P. Hornak 1 2 3 3 Independent Researcher, Eastman Kodak Co., and Rocheser Institute of Technology, Rochester, NY We report the first use of low-field electron paramagnetic resonance (LFEPR) as a non-destructive technique for the characterization of cultural heritage objects using single turn solenoid (STS) coils or spiral surface coils. LFEPR spectra of transition metal ions can be obtained at 300 MHz. Examples include the potential of estimating the firing temperature (TF) of archaeological ceramics, and detecting the presence and composition of paramagnetic inorganic pigments. In parallel 29 29 experiments, differences in the Cu(II) – Si hyperfine coupling detected by Si MAS NMR provide a deeper understanding of the different features observed in the LFEPR spectra of Egyptian blue vs. Han blue. 1 The LFEPR spectrometer previously reported was modified by the use of a three-port circulator, a new magnet power supply, a digital lock-in amplifier, and LabView interface. Two, glass free, sample coils were used. The original STS was used with samples in a 15 mm diameter styrene tubes. The second coil was a 7-turn, two cm diameter, spiral surface. Magnetic field sweeps were calibrated with a hall probe gaussmeter. Terracotta clay samples fired in air using an electric, resistive oven. Solid state NMR was performed on a Varian Inova 400MHz, wide bore spectrometer with a 4mm CPMAS probe running VnmrJ 2.2D software. The LFEPR spectra of clay samples fired at 100 < TF< 1200 ºC showed three unique spectral peaks: a broad g≈4, a narrow g≈2, and a broad g≈2. The original sample has the narrow g≈2 and broad g≈4 peaks. The narrow g≈2 peak rapidly disappears on heating, while the g≈4 peak is more persistent. At 800 °C the broad, strong g≈2 peak starts to grow, reaching its maximum peak-to-peak signal (SPP) at 1000 °C. This broad g≈2 peak diminishes to approximately 5% of its 1000 °C size by 1200 °C. Fig. 1 summarizes the change in SPP of these three g-factor absorptions with temperature. With these three temperature dependent spectral peaks it is possible to estimate the TF. For example, the ratio of SPP for the narrow g≈2 to g≈4 peaks can be used as a temperature marker between 100 and 500 ºC. The presence of only the g≈4 peak indicates 500 < TF< 800 ºC. The ratio of the broad g≈2 to g≈4 peaks can be used between 900 and 1200 ºC. Fig. 1. SPP vs. TF for the clay components. Fig. 2. LFEPR spectra of blue pigments. Fig. 3. LFEPR spectra of coin and flower pot. The three blue pigments studied possessed unique LFEPR spectra. (Fig. 2.) The Cu paramagnetic center in Egyptian blue possesses a broader peak-to-peak linewidth (ΓPP) than Han blue. Since the two pigments are isostructural except for the presence of Ca or Ba in the lattice, the ΓPP difference must be related to the cations. Ultramarine blue is a modern day synthetic version of the natural mineral lapis lazuli. This pigment contains a S3 radical anion that is responsible for the LFEPR signal. Its ΓPP value is narrower than the other two blue pigments and more asymmetric. The uniqueness of the three spectral peaks should allow discrimination of these pigments from other related phases in a ceramic artifact. 29 +2 A comparison of the Si solid state MAS NMR spectra of Han and Egyptian blue indicate that the substitution of Ba ions +2 29 by Ca increases the silicon paramagnetic shift due to Cu(II) - Si hyperfine coupling by 224 ppm (17,804 Hz). This is due +2 to changes in the Cu(II) spin-orbit coupling by the larger Ba and can potentially explain the differences in the photoluminescence and LFEPR spectra. The utility of the surface coil system was demonstrated with a 1921 Saxon Notgeld (emergency money) coin and a red clay terracotta flower pot. (Fig 3.) Signals were detected with a sufficient signal-to-noise ratio that allowed differentiation of these two different samples. These surface coil results open up the possibility of developing a unilateral LFEPR system 2 3 similar to ones proposed for MRI and NMR. The unilateral NMR and LFEPR systems could provide complementary results for some cultural heritage objects. 1. J.P. Hornak, M. Spacher, R.G. Bryant, A modular LFESR Spectrometer. Meas. Sci. Technol. 2:520-522 (1991). 2. C.L. Bray, J.P. Hornak, Unilateral MRI using a Rastered Projection. J. Magn. Reson. 188:151-159 (2007). 3. B. Blümich, et al. Noninvasive testing of art and cultural heritage by mobile NMR, Acc. Chem. Res. 43:761–770 (2010).