Analytical strategies for natural dyestuffs in cultural heritage objects

The identification of natural organic materials in works of art and objects of cultural heritage can be one of the most challenging tasks an analyst is asked to perform. Substances applied as adhesives, varnishes, dyes and consolidants can be particularly demanding. Their ingredients may include natural colouring materials, resins, waxes, proteinaceous materials, gums and oils, all of which can be quite complicated chemically. In addition, they have frequently deteriorated as a result of ageing and the effects of environmental factors, making analysis even more difficult.

 

All those concerned with the care of the object benefit from being given advice on the application of analytical techniques to the examination of organic materials. If a tiny sample can be taken from the object, many widely-used techniques can give information on the constituents of the substance and how it may have aged; the results may even tell us something about the technology of its production or application. Methods of non-destructive examination have also been developed that may provide valuable information on some organic materials without the need for sampling.

 

One of the objectives of the Sharing Knowledge and Resources Networking Activity of the Eu-ARTECH project is to evaluate methods for the investigation of organic materials used in cultural objects and to disseminate appropriate, well documented procedures for their analysis. For greater efficiency, it was decided to focus on one class of substance. Natural dyestuffs are an ideal subject for study: they have been used for colouring textiles, wood, leather, paper, bone and so on, and have also been used for the production of pigments for painting. Some, like the blue dye indigo (from woad, Isatis tinctoria L., Indigofera species and other plants) are relatively easy to identify by several instrumental methods; other dyes are more difficult. Some, such as the anthocyanin dyes and the ‘soluble’ redwood dyes (obtained from species of Caesalpinia) are particularly fugitive so that their presence on the object may not be easily recognised.

 

Several methods for dyestuff analysis have been published, but not all are suitable for cultural heritage objects, perhaps because of the sample size required, or because a non-invasive method must be used. In addition, not all laboratories working in the field of cultural heritage are familiar with the investigation of colouring matters. The five laboratories involved in this networking project – the Instituut Collectie Nederland, Amsterdam (ICN), the Royal Institute for Cultural Heritage, Brussels (KIK/IRPA), the National Gallery, London (NGL), the ‘ORMYLIA’ Art Diagnosis Centre, Ormylia (OADC) and the Department of Chemistry, Centre SMAArt, University of Perugia (UNI-PG) – have experience in the analysis of dyes by several different techniques. The project on analytical strategies for the examination of natural dyestuffs in cultural heritage objects thus had two principal aims. The first was to provide a set of analytical protocols for the examination of dyestuffs, following a comparison of the analytical procedures used by these five laboratories; the second was to characterise a selection of reference material for use as analytical standards.