The source of blue colour of archaeological glass and glazes: The Raman spectroscopy/SEM-EDS answers

The founding of lapis lazuli to give blue colour to glass and glazes is more and more coming out from the most recent studies of pottery and glassware of different epochs and provenances. The recent discovery in the south of Italy (Frederick II, Melfi castle) of enamelled glass pieces including blue enamels questioned the real nature of the blue colour raw materials because the castle is built over and not far from outcrops of two volcanic rocks - haüynophire and phonolite - that contain haüyne, mineral belonging to sodalite group as lazurite, that can be blue or become blue after heating. Raman microscopy and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy were tested in order to identify the rock used as raw material and to understand if the chromophore-bearing mineral in the artefacts could be other than lazurite. To do this, the two volcanic rocks, lapis lazuli and archaeological glass with blue enamels, were studied. In particular, temperature-dependent Raman measurements and Raman mappings both on single lazurite and haüyne crystals and on the rocks and the enamels were carried out also in order to investigate the colouration mechanism of haüyne. The results obtained allowed to make hypotheses about the transformations occurring in haüyne with heating and about the procedures of distinguishing the raw materials.