Fig. 2

a Reflectance spectra of unheated (black line) and heated (24 h at 130 °C, 160 °C and 180 °C) indigo–palygorskite mixtures (indigo 1 wt%). Reflectance spectra of solid indigo (1 wt%) dispersed in BaSO₄ (calibration performed with BaSO₄) is also reported for comparison (dash-dotted black line). b Projection in the a*b* space of the colorimetric measurements and L* values of the samples before and after heating (empty symbols: samples heated at 130 °C after the Soxhlet extraction; ind: indigo). c Reflectance spectra of unheated (black lines) and heated (24 h at 130 °C, 160 °C and 180 °C) indigo–palygorskite mixtures (indigo 0.3 wt%). d Reflectance spectra of unheated (black lines) and heated (24 h at 130 °C, 160 °C and 180 °C) indigo–palygorskite mixtures (indigo 5 wt%)