The use of infrared and Raman microspectroscopy for identification of selected red organic dyes in model colour layers of works of art

The possibilities of using infrared and Raman microspectroscopic techniques for identification of selected red dyes contained in the colour layers of oil paintings were studied and critically discussed. The samples involved aged and non-aged model materials, containing kamala, logwood, cochineal, madder and lac dyes. The specific spectral bands found for the dyes were compared with the vibrational bands of their principal colouring components. The results indicate that IR microspectroscopy permits the identification of lac (the bands at 1715, 1378 and 1309 cm−1), kamala (the bands at 1551, 1346, 1167 and 1130 cm−1), and logwood (the bands at 1503, 1476, 1293, 1209 and 1056 cm−1), even in the spectra of aged model samples. The characteristic vibrational bands of the principal colouring components of madder and cochineal are hidden under the matrices of these dyes. The Raman microspectroscopy only permits reliable identification of kamala (the bands at 1625, 1600, 1550 and 998 cm−1) and logwood (the bands at 1607, 1565, 1414, 1394 and 701 cm−1). Both the microspectroscopic methods are suitable for identification of kamala in model samples of colour layers containing inorganic whites.