Fig. 4

Detail of ER FT-IR spectra of selected samples (1500–400 cm⁻¹). The indicated bands at 1083 and 1005 cm⁻¹ are Si–O–Si (antisymm stretching); bands at 686 and 652 cm⁻¹ are Al–O stretching; band at 578 cm⁻¹ is the S₃⁻ feature of ultramarine; the band at 449 cm⁻¹ is silicate