Mathilda Larsson Coutinhoa,^b,c , Teresa Pereira da Silva^d, José Mirão^a, João Pedro Veiga^b 

^aLaboratório Hercules,Universidade de Évora, 7000-809 Évora, Portugal 

^b CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal ^cVicarte, VICARTE - Vidro e Cerâmica para as Artes ,Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal

^dLNEG (National Laboratory for Energy and Geology), Unity of Mineral Resources & Geophysics, Estrada da Portela-Bairro do Zambujal, Apt. 7586, 2610-999 Amadora, Portugal 

Roman mosaics are unquestionable manifestations of historical and artistic heritage. Their artistic value is directly related to the depicted themes and decoration techniques. Bichrome black and white mosaics are typically present in a larger number in a Roman city. However, coloured mosaics are greatly important, given the amount of details they can depict, as well as the different types of materials and techniques involved in their production. These can give much information on the level of technology and knowledge associated with the respective civilisations.

A set of Roman tesserae of different tones of white, grey, yellow and red was studied by X-ray diffraction (XRD), fibre optics reflectance spectroscopy (FORS) and Raman spectroscopy (Raman).

Data acquired during conventional laboratory-based XRD experiments carried out in reflection mode only show diffraction peaks associated with calcite, suggesting that limestone materials were used for the production of these tesserae samples. Nevertheless, particularly in the case of the yellow and red tesserae, their colours imply the presence of other compounds. FORS data suggested the presence of semi-conductors like haematite and goethite in the yellow and red toned tesserae, respectively. These results were corroborated by synchrotron radiation-based XRD (SR-XRD) and supported by Raman results. The SR-XRD experiments were performed in transmission mode at the High Energy Materials Science beamline HEMS at the storage ring PETRA III at the Deutsches Elektronen-Synchrotron DESY, Germany. These techniques confirmed the presence of haematite and goethite in the red and in the ochre-yellow tesserae, in that order. Additionally, SR-XRD data also showed the presence of quartz and dolomite in some of the tesserae samples, contributing to a better characterisation and understanding of these artefacts.

Abstract presented in the Conference Materiais 2019, 14-17 Abril 2019, Lisboa, Portugal