Indigo carmine: Understanding a problematic blue dye

The history, analysis, and use of the semi-synthetic dye indigo carmine is described by means of a study of historical sources including patents, dye identification using high performance liquid chromatography (HPLC), and the examination of dyed textiles. Indigo carmine, the acid dye 'Saxon Blue', was invented by Johann Christian Barth in 1743. Indigo carmine is a water-soluble blue acid dye with the advantage of an easy dyeing process. Its disadvantages are poor lightfastness and washfastness. For the identification of indigo carmine by HPLC, a gradient using water, methanol, and 0.5 mM tertiary butyl ammonium hydroxide can be used to give an unambiguous analytical result. Analyses of dated textile objects prove that indigo carmine was in use after 1770 until the beginning of the twentieth century. This study provides practising conservators with chemical, technological, and historical information which can help in the identification of textiles dyed with indigo carmine, and suggests ways of responding to poor washfastness.