Research and Optimization of Raman Spectroscopy Applied to the On-site Analysis of the Artistic Heritage.

(Spanish Government Research Project)

An essential information for a curator/researcher of the artistic heritage is to know the constitutive materials with which the artworks were made. Raman spectroscopy, this is to say, the molecular information in the way of optical frequencies, in additon to the incident, that any material provides (for instance, a pigment), allows us to obtain this information without damage, manipulation, or phisical sampling of the artwork that we want to analyze. The knowledge of this photonic technology must be transferred and applied to artistic enviroments and, by extension, to any professional involved in the world of the art.

So that, an important challenge of this interdisciplinary project is to research the possible advantages, and mainly the problems, that a portable Raman spectrometer can offer when an artwork is directly analyzed, that is to say, without realising a previous cleaning of the artwork, without extraction of microsamples and without transferring it from its natural environment to the laboratory (to insure its transferred, in the case of heritage artworks, is more expensive than the budget of this project). This is, therefore, to advance a step more in the artistic pigment analysis with Raman spectroscopy. It will require to confront and to overcome new and diverse problems, since with such work premises the detected  information will be, generally, weak, null or, at least, very confusing due to the great amount of existing pigments, mainly from the last third of XIX century. The main cause of the degradation of the Raman spectrum is the high level of intensity which presents the fluorescence generated by the binders (especially, in oil and tempera paintings) and by possible varnishes more or less oxidized. This fluorescence, besides limiting the acquisition time of a spectrum (by saturation of the CCD), can present a fluctuation level (shot noise) which masks the desired signal (Raman bands of pigments).

A fundamental objective of the project is to be able to detect spectra with a sufficient SNR guarantying the objective identification of any pigment. In order to achieve such aim it is proposed: firstly, to use a Raman equipment with IR laser excitation; in this way, minimum levels of fluorescence are obtained in comparison to those obtained with visible laser (also the intensity of the Raman bands will be smaller). And, secondly, to make use of the UV pulsed radiation in those cases in which the quality of the obtained spectrum is not sufficient, that is, to locally remove, in a controlled way, the binder and/or varnish concentrations taking advantages the non thermal photoablation effect induced by the UV pulses. In this sense, another objective of this project is the design and implementation of an optical head with a triple input (IR continuous laser, UV pulsed laser and white light source) to guarantee the impact of both lasers on the same point of the surface of the analyzed artistic object. The creation of a new collection of reference pigments is considered essential whose elaboration will be carried out following the old recipes, original processes of syntheses, adulterants and, when there is, the registered patent of each one. In this way, once they are spectrally characterized, it will be possible to have an authentic global data base (documented library of Raman spectra) that will provide both molecular and documentary information about the key pigments used in the history.