Study on the colouration of Bulgarian late-antique and medieval archaeological glass finds by a validated total reflection x-ray fluorescence procedure

A study on the colouration of late-antique and medieval glass finds excavated on the territory of present Bulgaria was performed. Optimized total reflection x-ray fluorescence (TXRF) procedure was used for determining the concentrations of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper and zinc in the glass fragments. The samples were analyzed as slurries in Triton X 114 and the certified reference material BAM-S005 Type A (soda lime silica glass) was used for method validation. The precision of the determinations is characterized by a relative standard deviation (RSD) of less than 10% in most cases. It was found that the majority of the existing colour variations in the analysed late-antique and medieval glasses are due to the deliberate addition of cobalt, copper, manganese and iron to the glass melt under appropriate melting conditions. In other cases the colouration is a result of the presence of impurities in the raw materials. The study throws light on the technologies of preparing coloured and colourless glass for different uses in the ancient and medieval glass workshops.