J.CAC VOLUME 32 (2007)
Identification of Indigo and its Degradation Products on a Silk Textile Fragment Using Gas Chromatography-Mass Spectrometry
This article describes a new technique for the analysis of natural dyes on textile fibres. The use of m-(trifluoromethyl)- phenyltrimethylammonium hydroxide (TMTFTH) extraction and derivatization, followed by gas chromatography-mass spectrometry (GC-MS), has proven to be a simple and fast technique for the analysis of indigo dye on textiles. Not only does the procedure allow for the identification of the main dye component, indigotin, but it also provides information on the degradation of indigo on textiles, a topic that has received limited attention in conservation literature. This paper discusses compounds formed through the reactions of TMTFTH with indigotin and the main degradation products of indigo, 2-aminobenzoic acid, isatin and isatoic anhydride. In addition, a new degradation product has been tentatively identified as 2-benzyl-3-indolinone. The occurrence of these degradation products was investigated in samples obtained from an Indian textile from the Sultanate period (13th-15th centuries) belonging to the Fine Arts Museums of San Francisco, woven, in large part, from indigo-dyed blue silk fibres. During a period of storage, pale yellow discolorations gradually appeared on tissue paper used to wrap the textile. Analysis of the tissue paper determined an abundance of indigo degradation products, in particular anthranilic acid (2-aminobenzoic acid). These compounds formed on the textiles fibres through oxidative degradation and subsequently volatilised to the surface of the paper, and also to the neighbouring fibres.
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