A study of Roman glass by reflectance and electron paramagnetic resonance spectroscopies

Reflectance and electron paramagnetic resonance (EPR) spectroscopies were used to study 25 fragments of Roman glass. Colour coordinates were used for an unbiased classification of the glasses in colour groups, which accounted for the presence of blue, blue-green, green, yellow-green, yellow and purple samples. Reflectance spectra were recorded in the 250–2500 nm wavelength range and showed absorption bands characteristic of Fe^II, Fe^III and Mn^III ions; furthermore, Co^II and Cu^II bands were observed in the spectra of the blue glasses. A decrease of the absorbance ratio of Fe^II to Fe^III ions was observed moving from blue-green to green and yellow-green glasses; however, yellow fragments still proved to be reduced glasses. EPR spectra displayed the characteristic patterns of Fe^III and Mn^II ions, with g-values in the 2–5 interval and spectral features depending on the relative content of the two elements. The characteristic pattern of the V^IV ion (g ≈ 2) and signals due to the formation of iron-sulphur complexes (g ≈ 6) appeared in the spectrum of a dark yellow glass, recorded at 77 K.