This book forms a sequel to the first volume which appeared under the editorship of John Chalmers and Howell Edwards in 2005, comprising some twenty-five contributed chapters from eminent authors and researchers over a wide-ranging field of topics, which all had the common theme of the applications of Raman spectroscopy to the characterisation of materials of relevance to art history and to archaeological artefacts. The idea for this first volume was generated by a ground-breaking meeting on the same topic held at the British Museum in London in November 2001, sponsored jointly by the Royal Society of Chemistry (Molecular Spectroscopy Group, Analytical Division) and the British Museum under the joint chairmanship of Dr Ian Freestone of the British Museum Research Laboratories and Professor Howell Edwards of the Chemical and Forensic Sciences Department of the University of Bradford. It was apparent at this meeting that a synergy existed between analytical Raman spectroscopists who were leading the application of their technique towards the rather novel area of art materials and archaeological artefacts and the special sampling considerations and data interpretation that these demanded and the conservation scientists, museum curators and archaeologists who required novel information to facilitate the restoration and preservation of the objects in their curacy.

It is certainly the case that most vibrational spectroscopic studies of artwork and archaeological artefacts carried out up to the last quarter of the 20th century were 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 (of up to 3 kW power output), operating mainly at 435.8 nm, and photographic plates, 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 with sensitive photon detectors revolutionised the range 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. In a recent survey of the literature in the decade 1997–2007, which has addressed Raman spectroscopic applications to art and archaeology by Vandenabeele et al. (2007), 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 where an almost exponential increase in Raman papers published in art and archaeology should be noted (Figure 1).

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 Raman spectroscopic techniques, and also complementary data, for studying artworks have appeared in journals which hitherto had not attracted research work of this kind, for example, Studies in Conservation, Journal of Archaeological Science, Archaeometry, and Antiquity as well as in the more mainstream spectroscopic literature. The growth of work specifically in the area of Raman spectroscopy applied to art and archaeology has stimulated the acquisition of novel spectroscopic instrumentation and trained specialists by museums and a new focus directed at the communication of the results forthcoming from these studies. In addition, the analytical information derived from optical, spectroscopic and diffraction experiments on artwork is being used increasingly to provide evidence for the authentication of high-value artwork in museum collections and in the private domain and also for the scientific provenancing of unknown artworks; the success of this approach is dependent upon the acquisition of data derived from the materials comprising the art object or artefact at both the elemental and molecular levels.

In the twelve years that have passed since the publication of the first volume on Raman Spectroscopy in Archaeology and Art History in 2005, a steady growth in output in this field has occurred: from the first conference mentioned earlier in the British Museum in 2001, dedicated international meetings on Raman Spectroscopy in Art and Archaeology (RAA) have been held in Ghent (2003), Paris (2005), Modena (2007), Bilbao (2009), Parma (2011), Ljubljana (2013), Wroclaw (2015) and Evora (2017). Other mainstream spectroscopy and art analysis conferences now hold dedicated sessions on Raman spectroscopy in art and archaeology, for example, the International Conference on Raman Spectroscopy, GeoRaman, InArt, Technart and IRUG (formerly the Infrared Users Group and now the Infrared and Raman Users Group). Additionally, several topics which were presented as novel examples or as typical case studies in the 2005 publication have now themselves become more widely applicable: a particularly apposite example of one of these is the adoption of non-destructive, handheld or mobile Raman spectrometers for the interrogation of artworks and artefacts in situ and in the field.

In this second volume, the original idea of offering full peer-reviewed chapters supported by individual case studies that describe the application of Raman spectroscopy to specific examples selected from the area or artworks and archaeology has been maintained. This book will give the reader a measure of the important contribution that the Raman spectroscopic technique is making currently to the provision of information about the analytical molecular composition of material relevant to artworks and archaeological artefacts. The major questions that archaeologists, conservators and art historians have regarding the characterisation of their specimens can be summarised as follows:

• There is an artefact that requires analysis; what is the composition of the material present?

• How much of that material is present – are there any other materials present that we should know about?

• Where did this material originate?

• What, if anything, has happened to it in the burial environment, if appropriate? Is there evidence of environmental or biological degradation and, if so, is this still ongoing?

• Are there “unusual materials” present which warrant further study? Is there any evidence that the specimen has undergone unrecorded restoration?

• Is the specimen or artefact genuine or a fake?

• The specimen is subject to strict protocols of preservation and only non-destructive analysis is acceptable – can this be assured?

Whereas most of these can be addressed by the adoption of analytical Raman spectroscopy, the main purpose of any data acquisition must be incorporated in a holistic forensic approach that necessarily involves the consideration of historical data and all available documentation in a science/arts amalgamation. For example, although the establishment of a chronological database of pigments used in art is well established, the recognition of a particular pigment in an out-of-context situation in an artwork does not imply that the artwork is a fake: Prussian blue, synthesised by Diesbach in 1706 was a mainstay of blue pigment colours over the next two centuries but its presence detected in an otherwise Renaissance painting from the 16th century does not condemn that artwork to be a forgery. Unrecorded restoration is a major reason for the discovery of such materials in these situations. Another example is that of chrome yellow synthesised by Vauquelin in 1807, but its presence on an ancient Egyptian cartonnage does not imply a restorative procedure carried out in the 19th century, as lead(ii) chromate is a naturally occurring mineral which has a Raman spectrum that is indistinguishable from that of synthetic chrome yellow. Hence, the interpretation of Raman spectral data should always be accomplished with an interdisciplinary awareness of the background provenancing or history.

Finally, the question has sometimes been raised in meetings as to the provision of an all-encompassing database of Raman spectral data that would facilitate the interpretation of the often complex data produced in the analytical interrogation of artworks. A major advantage of Raman spectroscopy is the ability to provide spectral data from complex mixtures containing inorganic and organic moieties and species without effecting their separation chemically or mechanically. A particular advantage arising from this is the recognition of characteristic biomarkers that indicate unambiguously that biological degradation of an artwork or artefact has taken or is taking place. This is especially valuable information for curators, who need to assess the integrity of their specimens for storage and display: it has been shown on many occasions that Raman spectroscopy can provide an early warning system for the presence of ongoing biological degradation in a sensitive biological sample such as a human mummy, textile, or vellum manuscript before that degradation becomes visible to an observer.

This volume serves to illustrate a selected number of examples of the information that Raman spectroscopic analysis can supply to assist the objective understanding of the material composition of artefacts and artworks: this information has been obtained through the micro-destructive or non-destructive sampling of the specimens concerned and the increasing role that mobile instrumentation has played in the acquisition of the necessary spectral data.

Peter Vandenabeele and Howell G. M. Edwards