The possible use of EPR spectroscopy for paint pigment identification

Full text: Visible and near-infra-red reflectance spectroscopy are now routinely used for the identification of paint pigments in Renaissance painting, thus playing a part in authentication and restoration. Since most of the pigments are minerals, and many minerals either contain traces of paramagnetic ions, or have paramagnetic ions in their main components (e.g., chromic oxide, haematite), it seemed logical to determine whether EPR could distinguish between different pigments. 14 pigments of different colours were tested in a Varian E-12 EPR spectrometer, at a frequency of ∼ 9.1 GHz. Measurements were made at room (∼ 20 deg C) and liquid N2 temperatures, in the standard special quartz sample tubes. The active volume is 0. 15 ml, but at most, a volume of sample (powder) one tenth of this was used. The spectra (to be shown) clearly demonstrate that EPR can distinguish between different pigments. The power was 1 milliwatt, the modulation amplitude 4 gauss, the total field sweep 10,000 gauss, centred 5000 gauss and the amplification between 102 and 104, as indicated on the charts. The signal to noise ratio is excellent, and sensitivity could be increased (if necessary) by a factor of ∼ 103, thus allowing much smaller samples to be tested. Because the signals are so strong it should be possible to scan at least small painted canvases by the following non-destructive technique, using existing equipment. All that is required is a modified resonant cavity; it must have a high Q, and a slot where the magnetic field is maximum, and the electric field zero. The canvas can thus be placed flush with the waveguide, to interact with the microwave magnetic field emanating from the slot