Fig. 1
a Absorption of synthetic indigo in chloroform (1.3 × 10−5 M) and dimethylformamide (2.1 × 10−5 M); Kubelka–Munk spectra of solid indigo (1 wt%) dispersed in BaSO4 (calibration performed with BaSO4). b Kubelka–Munk spectra of unheated and heated 24 h at 130 °C indigo 1 wt%/palygorskite mixture. Reference materials for calibration: Spectralon (solid lines), palygorskite (dash-dotted lines)