Identification of Indigo Dye (Indigofera tinctoria) and Its Degradation Products by Separation and Spectroscopic Techniques on Historic Documents and Textile Fibers

Understanding the use of colorant materials throughout history is now the objective of multiple studies centered on the identification and characterization of these materials. The main objective of this study is to increase knowledge of the degradation pathways of indigo blue dye (Indigofera tinctoria) and the detection of degradation products. This will enable the identification of the dye on historical objects, even when it is present in a degraded state. To this end, different analytical techniques were used for the identification of indigo blue and its degradation products. Capillary electrophoresis with diode array detection (CE-DAD) and fluorescence spectroscopy were used for characterization and detection of the colorant and its degradation products caused by the aging process. Nuclear magnetic resonance (NMR) spectroscopy was used to confirm the presence of degradation products. The evaluation of color variations (lightness, chroma, and tone) in indigo blue on paper samples was made with colorimetric study, while the changes in indigo solutions were evaluated with UV-visible spectrophotometry. Additionally, changes in the alkalinity and acidity of the colorant were made with pH measurements. Lastly, the main agents responsible for the modifications that resulted in the production of degradation products of indigo blue were also identified. These methods were applied to historic samples from the Collection of Drawings, Maps, and Plans (sixteenth to nineteenth centuries) and to the thread used in the binding of a notarial record book (Registro Notarial de Torres) (fifteenth century) held by the Royal Chancery Archive in Granada (Spain). This paper provides deeper insights into the materials used in artworks and helps in the identification of indigo blue despite the degradation caused by external agents or by its natural aging process.

Automated summary

This study aims to investigate the degradation pathways of indigo blue dye, as well as the detection of its degradation products, to enhance the identification of the dye on historical objects. Various analytical techniques such as spectroscopy, colorimetry, and pH measurements were employed to study the stability and changes of indigo blue in different materials.