Reflectance spectroscopy on natural dyes

Since prehistoric times natural dyes have been used for many purposes, such us to color fibers, to produce inks, watercolors and paints; their use declined rapidly after the discovery of synthetic colours. Anyway, nowadays there is a growing interest, natural dyes being neither toxic nor polluting. We selected some dyes extensively used for red (madder), yellow (weld and turmeric) and blue (woad) colours, produced by means of traditional techniques at the Museo dei Colori Naturali (Lamoli, Italy) and studied their chromatic properties through the reflectance spectroscopy technique. Indeed, it is fruitfully applied in the field of art conservation, since it permits the identification of pigments and the monitoring of chromaticity in a non-invasive way. The spectral behaviours of the dyes were investigated by comparing the spectral reflectance factors of powders and painted layers. For the purpose of supporting the laser cleaning procedures, we also evaluated the laser induced chromatic changes of the painted surfaces.