Fig. 6

a FORS spectrum acquired from a pink area of plate 23 (improved positive), where an inflection point at 580 nm and an apparent absorbance maximum at 850 nm indicate the presence of iron(III) oxide. b Raman spectrum collected from a pink area of the same photograph, where peaks at 224, 290, 408, 494, and 608 cm⁻¹ are attributed to iron(III) oxide