Indigoid dyes in artworks

Polish researchers have devised a new technique for identifying and distinguishing between indigoid dyes, to improve the restoration and conservation of works of art.

Historical artworks are susceptible to degradation by environmental factors such as exposure to light and changes in pH, as well as physical damage. Their restoration requires an intimate knowledge of the dyes and pigments used in the original, to enable the proper reconstruction of spoilt areas. In the case of natural organic pigments, identification can be problematic because similar shades are produced from different sources, like shellfish or plants.

The indigoid dyes are especially difficult to analyse due to their inherent instability and low solubility. Moreover, the indigoid members indigo (blue), indirubin (red), isoindirubin (red) and isoindigo (brown) are all structural isomers of 3H,3?H-bisindolylidene-3,3?-dione. Maciej Jarosz and co-workers from the Warsaw University of Technology distinguished between the indigoid pigments and their synthetic precursors with HPLC followed by mass spectrometric detection using electrospray ionisation complemented by UV/visible detection.

The pigments were dissolved in dimethyl sulfoxide, a solvent that causes severe ionisation suppression in mass spectrometry, but the HPLC step successfully separated solvent from analytes to prevent interference. Retention times and UV/visible spectra were sufficient to identify pigments with characteristic chromatographic profiles. Mass spectrometry was employed for analytes that were colourless in the visible region, or for those that coeluted, and was also available when structural confirmation was required.

The approach was used to identify the pigments in 0.5mg of fibre removed from a 19th century Japanese tapestry entitled Cranes in the landscape. The fibre was frozen and ground before pigment extraction in dimethyl sulfoxide. The combined use of both detection techniques revealed the presence of indigo and methylene blue.

Although the technique is destructive, the method sensitivity ensures that very small fragments are sufficient for analysis. Jarosz told Chemistry World that samples were always taken by the art restorer to ensure minimal damage. His group has now turned its attention to organic dyes from the original palette of the Polish painter Leon Wyczolkowski.

Steve Down