Influence of coloring agent (copper) on the crystallization of calcium antimonates: Studies on Roman turquoise glass tesserae

White and turquoise Ca-antimonate tesserae from Noheda (Spain) were characterized using LA-ICP-MS, XRD, SEM, and XANES spectroscopy to investigate their composition, crystalline phases, and opacification mechanisms. White tesserae contain a single Ca-antimonate phase (CaSb 2 O 6 or Ca 2 Sb 2 O 7 ), controlled by MgO content and thermal conditions, while most turquoise tesserae exhibit both phases and more heterogeneous compositions. XANES analysis reveals that antimony occurs predominantly as Sb 5+ in turquoise glasses, while copper redox state varies with concentration and source, showing limited interaction with Sb. Experimental reproductions demonstrate that in situ crystallization and crystal addition produce distinct microstructures, with copper selectively inhibiting crystal growth only via the in situ pathway. These findings offer new insights into Roman glass production practices and opacification technologies.