A spectroscopic characterisation of the dyes extracted from Iris germanica and their identification on illuminated manuscripts

Iris flowers have been used for their dying properties at least since the Middle Ages and for centuries later in Europe. In particular, as regards the Iris germanica species, the colours that can be obtained from the petals are violet (the pure juice), dark blue (upon addition of alum) and green (upon alkalisation). The ancient technical treatises are rich with recipes describing the preparation of these dyes. Nevertheless, the dyes of Iris germanica have rarely, if ever, been identified on paintings. This could be due to a lack of diagnostic information, useful for the selective identification of these dyes. In fact, normally the identification of organic colourants throughout non-invasive methods is complex and generally harder than for inorganic colourants. In this study, non-invasive spectroscopic techniques, such as UV–visible diffuse reflectance spectrophotometry with optical fibres (FORS), FT-IR spectrophotometry in reflectance mode and Fibre Optic Molecular Fluorescence (FOMF), as well as micro-invasive techniques such as Surface Enhanced Raman Spectroscopy (SERS) and FT-IR spectrophotometry in transmittance mode, have been used to characterise the dyes extracted from the petals of Iris germanica. In the end, the methods exploited in this work have been built to move from laboratory-based information retrieved through non-invasive and micro-invasive techniques, to the in-situ study of manuscripts in which we identified selectively the dyes and, in particular, the iris green cited by the ancient technical treatises. Moreover, the combined application of these techniques could allow to identify its presence even in cases when the dye is mixed with verdigris or superimposed on it.