Raman spectroscopy of inorganic materials in art and archaeology: spectroscopic analysis of historical mysteries

Howell G.M. Edwards DOI: 10.1039/b715002b

Introduction

The application of chemical analysis to art and archaeological specimens can be traced back to the late 18th and early 19th Century; in 1815, Sir Humphry Davy read a paper entitled 'Some experiments and observations on the colours used in painting by the Ancients' to the Royal Society and thereby probably laid claim to the first scientific publication in this field. Davy's analytical work comprised a study of the pigments on wall-paintings excavated at Pompeii and several palaces in Rome using classical wet chemical methods. In this paper, Davy refers to previous analytical studies made by savants, that had not been published hitherto but which had been communicated verbally to him, including Klaproth's discovery of cobalt in an ancient blue glass specimen, and tantalisingly refers to his minimal sampling of these precious archaeological specimens which were in the care of his friend, Canova. Hence, he recognised that the destructive sampling required for the provision of analytical chemical information using the standard methods of qualitative and quantitative analysis pertaining at that time, although undesirable, was nevertheless necessary for the provision of novel information about the chemical composition of the artefacts which was not accessible by other means.

The extent of the problem facing the analyst can be gauged from the classic studies of English porcelains carried out by Eccles and Rackham and by Church in the late 19th and early 20th Centuries, when whole items were consumed for the gravimetric determination of ceramic body compositions in attempts to classify the articles as belonging to certain factories — in this respect, the records of early porcelain factories of the composition of their wares was vital for the attribution of the specimens. It is not surprising, therefore, that the renaissance of chemical analysis applied to archaeological and art specimens has been driven in the last decade by the application of non-destructive spectroscopic micro-sampling techniques and this has reinforced the scientific authentication and provenancing of specimens as a central theme at the interface between history, art and science.

It is certainly true that most vibrational spectroscopic studies of art work and archaeological artefacts carried out up to the last quarter of the 20th Century was exclusively the preserve of the infrared spectroscopist; there are several reasons for this, but the most important are that earlier Raman spectroscopic instrumentation used Toronto mercury arc excitation, operating mainly at 435.8 nm, and photographic, or later, photoelectric recording. This required rather large quantities of pure samples that were stable to high-energy visible radiation and the total absence of fluorescence emission, which could swamp the much weaker Raman scattering intensity. In the late 1970s, however, the classic marriage of a laser Raman spectrometer with an optical microscope developed by Delhaye and Dhamelincourt, with sensitive detectors, revolutionised the scope of applications that could be undertaken using the new laser Raman microscopy, and almost immediately resulted in the first brief description of artefact analysis using this novel instrumentation. So it was that the first publications of Raman spectroscopic studies of materials relevant to art and archaeology appeared from the mid- 1980s, from Guineau and his collaborators in Paris. In a recent survey of the literature in the last decade which has addressed Raman spectroscopic applications to art and archaeology by Vandenabeele et al., the growth of the technique can be clearly seen expressed as a proportion of the total number of papers published in art and archaeology, and this is reflected in data presented in the Web of Science (Fig. 1), where a ten-fold increase in Raman papers published in art and archaeology between 1998 and 2007 should be noted.

Also, a greater awareness is now apparent across several disciplines at the arts/science boundaries, and particularly in the field of scientific conservation and restoration, because publications using the Raman spectroscopic technique for studying art works are now appearing in journals which hitherto had not attracted research work of this kind, for example, Studies in Conservation, J. Archaeological Science, Archaeometry, and Antiquity as well as in the more mainstream spectroscopic literature. The growth of work in the area of Raman spectroscopy applied to art and archaeology has stimulated the acquisition of spectroscopic instrumentation and trained specialists by museums and a new focus directed at the communication of the novel results forthcoming from these studies; this has itself generated a growth of specially themed topics at Raman spectroscopy conferences, such as ICORS and GeoRaman. A whole new conference series on the topic of Raman in Art and Archaeology(RAA), which started first as a one-day meeting in the British Museum in 2001 attended by 120 delegates from 17 countries, will attain RAA5 in Bilbao, Spain in a five-days meeting scheduled for 2009. Finally, the first book dedicated to the applications of Raman spectroscopy in art and archaeology appeared in 2005, published by the Royal Society of Chemistry, containing 25 chapters written by specialists working at this interface and covering a host of topics including, textiles, mummies, biomaterials, prehistoric art, dyes and pigments,...