Non-destructive vs. Microchemical Analyses: the Case of Dyes and Pigments

Non-destructive tests (NDT) of historical artifacts are the ideal scientific tools for the analyses of culturally important heritage objects. However, for the investigations of certain materials, especially natural organic dyes and pigments, it is important for curators, conservators, and scientists to be aware that such analyses are not always practical or prudent. At best, NDT can definitively detect the presence of the major component of a dyestuff or pigment, but will have inherent problems in positively identifying other colorants present in the natural pigment or dyestuff. The identification of these secondary components is necessary for the determination of the botanical, entomological, or malacological provenance of the dyestuff. As such, these techniques will only provide a global picture of the coloring material on an object and will have difficulty in identifying the specific species that produced the colorant. At worst, in some dye analyses, NDT may produce artificial or even erroneous results. Microchemical tests (MCT), especially high-performance liquid chromatography (HPLC) coupled with photodiode array (PDA) and/or mass spectrometric (MS) detection, have pushed the limits of detection of organic colorants to picogram levels. Thus, only miniscule sampling of the material being examined is necessary, which affords the experimentalist an advanced instrumental tool that is nearly non-destructive and does not affect the structural or aesthetic integrity of the object. Such MCT can provide a wealth of multicomponent information vital to the determination of the biological provenance of the dyestuff, which may also shed light on its geographical point of origin. Several case studies of archaeological colorants, primarily from ancient Israel, are discussed in terms of the above arguments, and they include the dye sources mentioned above, including the famous rare royal purple pigment. ORGANIC DYES AND PIGMENTS: WHAT'S THE DIFFERENCE? This work will not discuss inorganic pigments, which are composed of various metal–nonmetal ionic compounds that have been used in the painting of the surface of a substrate, such as a wall, sarcophagus, pottery, canvas, or even a textile (notably burial shrouds). In this work, only organic colorants (dyes or pigments) used for the true dyeing of textiles are addressed [Koren 1993]. Various definitions have been given in the literature for these two types of colorants. The difference between them relates to the relative solubilities of these colorants in water, whether neutral, acidic, or basic (alkaline). A dye, without being complexed to another chemical species, is a textile colorant that is generally soluble in aqueous solution. A pigment is relatively water-insoluble and can thus also be used as a paint colorant. In any textile dyeing process the colorant must first be dissolved in water, and in this way the individual dissolved molecules can penetrate into the interiors of the fibers and attach themselves to them via physico-chemical bonds. A substance that is categorized as a dye readily dissolves in water in its original chemical form, by definition. For a waterinsoluble pigment to be used as a textile "dye", it must first be transformed to a chemically soluble form. That is the case with the famous dark blue indigotin pigment that is produced via a complex reduction and oxidation process from the precursor molecules present in the 1 9th International Conference on NDT of Art, Jerusalem Israel, 25-30 May 2008 For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=65