Formation of an outer borosilicate glass layer on Late Bronze Age Mycenaean blue vitreous relief fragments

Ancient glass samples from Greece were studied by a combination of SEM/EDX to determine their chemical composition and infrared (IR) and Raman spectroscopy for their glass structure. The archaeological samples consisted of three blue vitreous Mycenaean relief fragments from the Late Bronze Age. The chemical composition of the samples is consistent with typical soda-lime-silica glasses, with the possible use of plant ash instead of soda in preparation. The deep blue colour of the samples is due to tetrahedrally coordinated Co2+ ions. The infrared spectra of the Mycenaean relief fragments deviate from all other previously measured ancient samples as the spectra resemble those of highly polymerized silica or highly polymerized low alkaline borosilicate glasses. Only spectra taken on relatively fresh cuts are consistent with the analyzed soda-lime-silica glass composition. Measurements on cuts on the sides of the fragments show that only a very thin layer (<1 mu m) on the front and back of the samples has a fully polymerized glass network. Strong background fluorescence hindered Raman spectroscopy in the highly polymerized surface layer. Boron could not be detected by SEMI EDX (scanning electron microscopy/energy dispersive x-ray spectroscopy) because of the thinness of the outer layers, whereas reflectance infrared measurements were not impacted by the bulk glass composition. Possible chemical reaction mechanisms, conditions and likely boron sources leading to the formation of this outer layer are discussed.