A photochemical study on the blue dye indigo: from solution to ancient Andean textiles

Abstract

The degradation of indigo and its water soluble derivative indigo carmine was investigated under light excitation in the presence and absence of molecular oxygen in solution (homogeneous) and gels (heterogeneous) media. Collagen and carboxymethylcellulose (CMC) aqueous gels were chosen to simulate a natural textile environment, wool and cotton, respectively. Isatin was found to be the major degradation product of indigo. In solution, the photodegradation quantum yields (Phi(R)) were in the order of 10(-4), with the exception of aqueous media (Phi(R) = 9 x 10(-6)), and dependent on the irradiation wavelength. In the case of indigo carmine the Phi(R) values were found to suffer a 2-fold increase upon going from water to gels. The results indicate the absence of degradation products involving singlet oxygen and suggest peroxides, or other oxygen based radicals, to have a key role in the degradation of indigo. Finally, the relevance of the simulation is discussed by comparing the main degradation products to those found in the blues of millenary Andean textiles.

High-performance liquid chromatography-diode array detector-mass selective detector analysis of major natural dyes with the application of H2O2/ultraviolet treatment as a way to simulate burial degradation of textiles

“…46 It was found that such molecular cleavage resulted in the production of lower molecular weight species, mainly isatin. 42,46 In the present investigation, the H 2 O 2 /UV degradation of indigotin in DMSO solution progressed rapidly, and consistent with previous findings in the literature, isatin was produced as the major degradation product. It is suggested that isatin was produced as ÁOH addition occurred to the central double bond of indigotin upon H 2 O 2 /UV degradation ( Figure 11).…”

Characterization of the Excited States of Indigo Derivatives in their Reduced Forms

“…[25][26][27] However, in these works the yields of degradation are not reported and keto indigo is known to be a stable molecule that only suffers some degradation in extreme conditions, such as the presence of nitric acid or under light excitation, and even at these conditions with very low quantum yields of reaction (10 À4 or lower). [28] The significantly different photophysical behaviour shown by the leuco forms strongly suggest that this form is more prone to reactions and subsequently to degradation.…”

Bright lights: disclosures from the optical, spectroscopic and chromatographic characterization of a 19th century Portuguese sedan chair

“…Textiles recovered from archaeological excavations allowed the identification of natural dyes used for colouring, such as purple and red insect dyes identified in Roman Egyptian archaeological textiles [2], or the application of indigo for the production of the Paracas mantles used for covering the deceased [3]. Other natural dyes were used for textile decoration, like cochineal and brazilwood lakes.…”