From mining waste to mosaic tesserae: Evidence for metallurgical slag reuse in Roman mosaics

Black tesserae used in Roman mosaics are commonly interpreted as products of specific glassmaking recipes or, in some cases, as obsidian. The material analysed in this study differs markedly from these established categories. We examine black tesserae from Roman mosaics dated between the late first and fifth centuries AD at three archaeological sites in the Upper Guadalquivir region (southern Spain), an area closely associated with ancient lead and silver metallurgy. A multi-analytical approach combining Raman spectroscopy, micro X-ray fluorescence (µXRF), and lead isotope analysis shows that the tesserae consist of lead–silicate glass with very high lead contents and negligible alkali levels, allowing diagnostic indicators to be defined. Their mineralogical, chemical, and isotopic characteristics closely match those of local lead-smelting slags, indicating that metallurgical by-products were deliberately selected and transformed for mosaic production. These results provide new insight into material circulation, reuse, and resource management in Roman technologies.