Fig. 3

a Reflectance spectra of unheated (black lines) and heated (24 h at 130 °C, 160 °C and 180 °C) indigo–sepiolite 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 sepiolite-containing samples before and after heating (ind: ind). c Reflectance spectra of unheated (black lines) and heated (24 h at 130 °C, 160 °C and 180 °C) indigo–sepiolite 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–sepiolite mixtures (indigo 5 wt%)