Subtracted shifted Raman spectroscopy of organic dyes and lakes

Resonance Raman spectroscopy has been utilized to identify vegetal and animal dyes and lakes. These compounds have been used by artists since antiquity, and their identification has important implications in art history and conservation. The resonance condition is useful when working with real ancient objects so as to enhance the Raman scattering of chromophores with respect to the strong signal of the matrix, such as the textile or parchment over which the colorant is supported. The strong fluorescence that generally characterizes the resonance Raman spectra was eliminated by using subtracted shifted Raman spectroscopy (SSRS). A systematic study on reference organic dyes was primarily carried out to evaluate the suitability of the method and to estimate the strengths and limitations of the spectrum reconstruction process. The method was then applied to the noninvasive identification of madder on ancient dyed silk textiles such as an altar table cloth and a manutergium attributed to the Egyptian artistic textile production of the 10th-11th Century A.D.