Spectroscopic and chemometric characterization of green earth pigments employed by ancient Romans in the wall decoration of buildings in the X Regio (Venetia et Histria)

In this work the study of the green pigments employed by ancient Romans in the decoration of 50 wall fragments from buildings of different ages located in seven archaeological sites of the Augustean X Regio (Venetia et Histria) is presented. The pigments are green earths-celadonite and glauconite, the latter associated to variable amounts of smectites-as found through mid-Infrared Fourier Transform Spectroscopy and as confirmed by Principal Component Analysis. The investigation was performed in suitable spectral ranges that while accounting for all the most intense absorptions of green earths allowed to minimize the influence of absorptions due to accessory minerals in the pigment ores or to the mineral species contained in the preparatory layers of the painted fragments (carbonates, quartz, kaolinite and gypsum) and sampled with the pigment. Three commercial green pigments consisting of the minerals celadonite and glauconite were also considered to check the soundness of the approach followed for the identification of the historical pigments and the reliability of the results obtained. For the first time it is shown that Principal Component Analysis is an effective tool to confirm the results of conventional spectroscopic analysis of the two green earths celadonite and glauconite.