Raman microscopy as a structural, analytical and forensic tool in art and archaeology, RJH Clark

Tags: pigments, pigment, spectroscopy, Burgio, L., UCL, New Zealand, Raman spectroscopy, King George III, J. Raman Spectrosc, RM, The British Library, sherds, Copyright Elsevier Limited, Earliest Industrial Processed Platinum, Christ's College, Czech Spectroscopy Association, William Ramsay, physical inorganic chemistry, American Philosophical Association, London WC1H 0AJ, Otago University, scientific study, Sir William Ramsay Professor of Chemistry, Lindisfarne Gospels, 9th Century Abbasid art, scientific studies, 15th century, kaolinite clay, black pigment, traditional materials, Century Maestro, high pressure chemistry, 9th Century AD, Jorge Ingles, mid-20th Century, New Zealander, Chemical Heritage Foundation, Raman Microscopy, verdigris, Jean Bourdichon, Cotton Nero, Bibliotheque Nationale de France, manuscripts, Eton College, Pope Damasus, British Library, Gutenberg Bibles, The Logic of Scientific Discovery, Johann Gutenberg, Karl Popper, R. J. H. Spectrochim, Clark R. J. H., illuminated manuscripts, Robin Clark, University of Canterbury, Canterbury University College, University College London, R. J. H. J. Cult, Clark, R. J. H. J. Mol, Smith, D. C. Spectrochim, Richard Ernst, Clark Christopher Ingold Laboratories
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Chemistry in New Zealand January 2011 Raman Microscopy as a Structural, Analytical and Forensic Tool in Art and Archaeology Robin J. H. Clark Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WC1H 0AJ, UK (e-mail: [email protected]) About the Author Robin Clark, CNZM, FRS, is Sir William Ramsay Professor Emeritus of Chemistry at University College, London (UCL). He was born in New Zealand and educated at Christ's College and the University of Canterbury, before moving to the UK and UCL in 1958 for PhD study in chemistry. He joined the faculty at UCL in 1962, rose to become Dean of Science and then, for 20 years, Sir William Ramsay Professor of Chemistry, now emeritus. He has lectured in many countries (NZ included), acted as visiting professor in numerous universities sharing his expertise in transition metal chemistry, mixed valence chemistry, metal-metal bonding and spectroscopy. He work is recorded in more than 500 publications and he remains remarkably active. He has made major contributions to most aspects of Raman and resonance Raman spectroscopy, but it is his recent pioneering applications of these and other techniques to the characterization of pigments in artwork and archaeology that has gained him widespread international recognition. Recent awards include the Czech Spectroscopy Association's Marci medal, the RSNZ Sidey medal and the inaugural Franklin-Lavoisier Prize of the Maison de la Chimie (Paris) and Chemical Heritage Foundation (Philadelphia). He was elected Hon. FRSNZ in 1989, FRS in 1990 and Companion of the New Zealand Order of Merit (CNZM) in 2004, and International Member of the American Philosophical Association in 2010, the third New Zealander since its founding in 1743. He is the 2011 RSNZ Distinguished Speaker and will be opening the touring portrait exhibition The Science of Art in March in Wellington and then undertaking a national lecture tour.
Introduction Having left New Zealand in 1958 with research experience in diffusion-controlled reactions (Walter Metcalf, Canterbury University College) and high pressure chemistry (Bill Fyfe, Otago University) to study for a PhD on the chemistry of the early transition metals (Ron Nyholm, UC-London), it would not necessarily be clear to readers as to why I might be in a position to write an article for CiNZ on the seemingly unrelated topic above. In brief, my research at UCL over the past ca. 50 years kept evolving from my PhD topic through different aspects of physical inorganic chemistry (ligand-field theory, mixed-valence chemistry, metal-metal-bonded species, linear-chain complexes, etc.) to spectroscopy (electronic, infrared, Raman, resonance Raman, electronic Raman) and many other topics. Some of these, notably Raman spectroscopy (along with yet further techniques), I chose to bring together in the last decade or so for a common purpose after recognising the serious lack of scientific study on pigments, dyes, etc. used on artwork and archaeological artefacts. Specifically, it was clear that there was a great need to be able, rapidly and unambiguously, to identify such materials at micrometre sizes on artwork of all sorts and to establish artists' palettes in different regions and at different periods. Not only would this contribute in a major way to scholastic knowledge in the area, but it would provide a firm scientific basis for the identification of art forgeries. The following outline illustrates how these studies have developed and the nature of some of the long-standing problems solved. The Raman Effect, first detected in 1928 in Calcutta by C.V. Raman, has developed into a powerful method for
the study of gases, liquids, glasses, solutions and solids, whether organic or inorganic. The changes in frequency of the incident photons scattered inelastically from materials of all sorts are unique to each material and are collectively referred to as its Raman spectrum. This is a unique fingerprint of the material on which the laser beam from the source is incident. Raman spectroscopy was used traditionally, in conjunction with infrared spectroscopy, to obtain the vibrational frequencies of virtually all normal modes of a compound and, by use of group theory, to establish the symmetry and then the force constants of the molecule. Large numbers of studies of these sorts have been published, from the early classic works of Herzberg, Wilson, Decius and Cross, to the compilations of data on inorganic materials by Nakamoto1 and many others. The theory of the Raman Effect has been well set out by Long.2 Many compounds crystallise in more than one polymorph, each having different properties, hence it is vitally important to be able to distinguish these apart. Each crystallises in a different space group, with different point or site groups for each atom, and so the infrared and Raman spectra of each polymorph are distinct. Obvious cases in point are calcite and aragonite (each CaCO3), rutile, anatase and brookite (each TiO2), etc.3-6 These extensive studies over several decades have led to the realisation that Raman spectroscopy, and especially its newer variant in which the laser beam is focussed onto the sample via a microscope, Raman microscopy (RM), would be an ideal, precise technique with which to probe the identities of pigments and dyes used in art and archaeology.3-10
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Raman Microscopy as an Analytical Technique The identification of pigments on manuscripts, paintings, postage stamps, enamels, glasses, ceramics, stuccoes, icons, polychromes, papyri and archaeological artefacts is critically important to understanding the history of an object and in the resolution of problems related to restoration, conservation, dating and attribution of artwork. Many techniques, both molecular and elemental, have been used for such studies, but the former are the more important since they lead to the identification of the pigment, which is usually a molecule rather than an element. Raman microscopy (RM), significantly a molecular technique, has emerged as a consequence of major recent advances in optics and detector technology as probably the most suitable of such techniques on account of its high spatial (ca. 1 m) and high spectral (ca. 1 cm-1) resolution, its molecular specificity, its excellent sensitivity by way of chargecoupled device (CCD) detectors, and the fact that most items may be analysed non-destructively and in situ. Furthermore, the introduction of small, portable spectrometer systems, albeit of lesser performance (Raman microprobe systems) has been advantageous for the study of immovable or awkward items forming part of our cultural heritage, such as cave paintings and statues in galleries. The identification of pigment degradation products on artwork is also a matter of great interest because studies addressing possible degradation pathways give insight into the nature of the environment in which the artwork or archaeological items have been held. Some geological conversions, such as that of malachite to azurite, are very slow, but many chemical reactions taking place on artwork, such as that of hydrogen sulfide with lead or copper pigments, are very fast. The source of the hydrogen sulfide is atmospheric or via bacterial attack on sulfur-containing binders.
Scientific Investigation of Artwork A few highlights of recent Raman studies at UCL on high profile manuscripts, artwork and artefacts follow. All such studies involve the critically important establishment of palettes, which in some cases lead to resolution of longstanding ambiguities. Lindisfarne Gospels (ca. 715 AD) and Other Early Manuscripts The British Library contains one of the world's foremost collections of Anglo­Saxon manuscripts, many of which have been very well studied from a palaeographical standpoint. However, until recently, little had been known as to the materials used in their construction and illumination ­ their pigments and binders. No Anglo-Saxon recipe books for artists' pigments are known to exist, and later ones ­ even price lists ­ were unreliable, being subject to mistranslation; moreover, the terms used were often ambiguous, being used for more than one pigment. A particularly notable codex is the Lindisfarne Gospels, a work of art which, to many, represents the pinnacle of artistic achievement in manuscript illumination (Fig. 1). Considered to have been created around 715 AD by Eadfrith, the then Bishop of Lindisfarne in Northumbria, in honour of St. Cuthbert who was himself Bishop of Lindisfarne from 685­687 AD, the major pages display fantastic complexity of zoomorphic interlace ornament as well as contrastingly simple evangelist portraits. The most significant discovery15 was that the blue pigment used is indigo (C16H10N2O2; derived from the woad plant indigenous to
The immense literature on pigments is now well documented in substantial articles and reviews3-10 as well as in libraries of pigments and minerals.11-13 Most minerals are long established and have well defined structural and spectroscopic properties, such as those relating to colour as a function of particle size, density, physical and chemical stability, etc. Obviously, synthetic pigments have known first dates of manufacture, a fact that is key to the possible dating of illuminated manuscripts and paintings. That is, a synthetic pigment cannot appear on a work of art that supposedly predates the year of first manufacture of the pigment. If it does, then either the artwork is a forgery or it has been restored at a later date, a matter made evident by whether or not there is under paint.
It is important to emphasise that it is not possible to authenticate artwork; thus in The Logic of Scientific Discovery, originally published in 1934, three years before he emigrated from Vienna to Christchurch, Karl Popper postulated that scientific hypotheses can never finally be confirmed as true, and are acceptable only in so far as they manage to survive numerous attempts to falsify them.14 The art world and especially auction houses often leave much to be desired in this context, seemingly offering artwork and artefacts for sale without first having carried out perceptive scientific tests for forgery.
Fig. 1. St. Jerome's prefatory letter to Pope Damasus on his Latin Vulgate translation of 405 AD. Copyright The British Library Board, Cotton Nero iv; reproduced with their permission and that of the John Wiley & Sons, J. Raman Spectrosc. ­ see ref. 15.
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England) and not lazurite (Na8[Al6Si6O24]Sn) as had until then been believed. Lazurite in 715 AD was known to have been found only in the Badakshan mines in the Hindu­Kush area of Afghanistan and so, to have been used on the Gospels, would have required the existence, improbably, of a trade route at that date between these mines and Northumbria. The correct identification of the blue as indigo rather than lazurite removed the need for such an unlikely proposition. Several other detailed studies of Anglo­Saxon and early English manuscripts have also been carried out successfully by RM. In addition, the semi-precious gemstones used to decorate the plates of embossed silver attached to the oak cover of the Carolingian manuscript, the Tours Gospel, Evangelia Quatuor, were identified to be silica (SiO2, white, cat's eye), emerald [Be3Al2(Si6O18), beryl, green, coloured with traces of Cr3+], carbuncle [Fe3Al2(SiO4)3, iron garnet, red-brown], and sapphire [Al2O3, very pale blue, trace amounts of Cr3+].16 Until recently, many unsubstantiated identifications made by purely visual means had become accepted as fact, having been repeatedly quoted uncritically. Thus, the traditionally accepted triptych of the insular palette, red lead (Pb3O4), orpiment (As2S3) and verdigris [basic copper(II) acetate, (MeCO2)2Cu.Cu(OH)2, and a suite of closely related compounds] was by no means always adopted by artists at the time, as shown by RM; thus, the green pigment verdigris was shown to have been substituted in many cases by a mixture of orpiment and indigo, the mixture being known as vergaut.
Gutenberg Bibles (ca. 1455 AD) Johann Gutenberg in Mainz (Germany) was responsible in the middle of the 15th century for the invention of European printing using movable metal type, an invention which had a profound impact on Western culture. Of the ca. 180 copies of the bibles he produced, about 135 were printed on paper and about 45 on vellum, some 48 copies having survived until the present day. The palettes of eight of these immensely valuable (multimillion pound) Bibles have been established by RM, viz. the King George III in the British Library (London) and others held at Eton College (Windsor) and Lambeth Palace (London), the Bibliotheque Mazarine and the Bibliotheque Nationale de France (both in Paris), and the Staatsbibliothek zu Berlin and the Niedersдchsische Staats und Universitдtsbibliothek, Gцttingen.18 Most of these are brilliantly illuminated codices, the red, yellow, black, blue, green and white pigments on the King George III version, for instance, consisting of vermilion (HgS), lead tin yellow type I (Pb2SnO4), carbon black, azurite [2CuCO3.Cu(OH)2], malachite [CuCO3.Cu(OH)2], verdigris, chalk (CaCO3), gypsum (CaSO4.2H2O) and white lead, in agreement with the instruction contained in the accompanying model book. The illuminations in the British Library and Eton College Bibles are similar to one another, consisting of brilliantly interwoven flora and fauna around the columns of printed text. (Fig. 2).
Arabic Treatise Containing Early Maps and Celestial Diagrams (ca. 1200 AD) The Arabic treatise The Book of Curiosities of the Sciences and Marvels for the Eyes at the Bodleian Library contains a series of early maps and celestial diagrams that are of great importance to the history of medieval cartography. The treatise consists of two books, the first pertaining to celestial (astrological and divinatory) matters and the second to terrestrial (descriptive and historical) matters. Considered to date from ca. 1200 AD in Egypt, the work includes: two world maps, one circular and one rectangular; maps of the Indian Ocean, Mediterranean and Caspian Seas; maps of the Mediterranean islands of Sicily and Cyprus and of the cities of al-Mahdiya (now Mahdia, in Tunisia) and Tinnis in the Nile delta; and unique maps of the Nile, Euphrates, Tigris, Oxus (now referred to as the Amu Darya) and Indus rivers. The pigments used for the illustrations were shown by RM17 to consist of cinnabar (HgS), orpiment, lazurite, indigo, carbon black, white lead [2PbCO3.Pb(OH)2], and a copper-based blue (possibly a form of verdigris), red ochre (essentially haematite, Fe2O3) and gold, additional shades being obtained by way of numerous pigment mixtures. Some pigments had partly degraded. The analysis provides a starting point for building up knowledge about the production at this time in Egypt of secular, illuminated Arabic manuscripts as distinct from Korans. The identification of light-sensitive orpiment and hydrogen sulfide-sensitive white lead may be important for the development of future conservation strategies for such manuscripts. Many other studies of Korans have now been carried out.
Fig. 2. Illumination on the prologue page of the King George III version of the Gutenberg Bible. Copyright The British Library Board, C.9.d.3 f4v-f5; reproduced with their permission and that of the American Chemical Society,­ see ref. 18
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Bourdichon Miniatures (ca. 1499 AD) Studies of miniatures taken from the Book of Hours of Louis XII by Jean Bourdichon, painter at the Royal Court of France from 1499 to 1521, by RM and X-ray fluorescence have revealed that the extensive palette includes 13 traditional pigments and dyes, along with the unusual artists' materials specular haematite (Fe2O3), iron pyrite (FeS2) and mosaic gold (SnS2). Possibly the most interesting feature of Bourdichon's palette is, however, the presence of metallic bismuth, found to have been used for only a few decades round 1500 AD to convey the impression of a soft mid-grey (wismuthmalerei, see Fig. 3). It is characterized by first-order Raman lattice modes at 70 and 97 cm-1 and their overtones.19
Vermeer; in consequence of this, when sold at Sotheby's auction house on 6 July 2004, it realised GBP 16.2M.20 Many stylistic and other arguments are in favour of this re-attribution, including the discovery that the thread count of the canvas, 12/cm in both the warp and the weft is identical to that on Vermeer's long accepted painting The Lacemaker held in the Louvre gallery in Paris. On the scientific side, three pigments were identified by RM (Fig. 4), lead tin yellow (type I), lazurite, and vermilion, all of which were available to Vermeer and known to have been used by him in ca. 1670. Lead tin yellow (type I) is a crude date-marker pigment in the sense that it is understood to have been used only in the period ca. 1450­1700 AD. Moreover, the finding of the rare and expensive mineral lazurite, rather than the much cheaper synthetic equivalent (Ultramarine blue), itself suggests that the painting may predate 1828, the year in which the latter version was first synthesized. The attribution is, however, still subject to the strictures of Popper.14
Fig.3. The Nativity, Bourdichon, ca. 1499 AD; copyright the Victoria and Albert Museum, E.949-2003 and reproduced with their permission - see ref. 19. Vermeer Painting: Young Woman Seated at a Virginal (ca. 1670 AD) Until 2004 there were considered to be only about 35 authentic paintings by the famous Delft artist, Johannes Vermeer (1632­1675), but in that year the art world accepted the case for a 36th. The painting Young Woman seated at a Virginal had originally been regarded as having been painted by Vermeer, but this attribution became unclear in 1947 when Hans van Meegeren revealed that he had painted a number of `Vermeers' himself and sold seven to unwitting museums and collectors (including one to Hermann Goering) during the period 1937­1945. However, extensive art historical and scientific research in the late 1990s and early 2000s has led to persuasive evidence consistent with a re-attribution of this particular painting to
Fig. 4. Painting Young woman seated at a virginal under study by Raman microscopy to identify the pigments present. The results provided evidence consistent with the attribution of the painting to Vermeer and none to the contrary. Copyright the American Chemical Society and reproduced with permission ­ see ref. 20. Icons Extensive RM studies have been carried out on, and on cross-sections from, many Russian, Albanian and Greek icons, often in conjunction with laser-induced breakdown spectroscopy (LIBS) and other techniques. These reveal details of the stratigraphy of overpaintings on icons (Fig. 5).9 Detection of Forgeries Many studies have led to, or contributed to, the realisation that a significant proportion of works of art throughout the world are forgeries. These are illustrated with the four examples that follow.
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the lines showed that the yellow-brown material forming the background to the lines contains substantial (up to 50%) anatase (TiO2). This material was shown to have a particle size (ca. 0.15 m) and particle size distribution (very narrow) that is characteristic of the synthetic post ca. 1920 product, and not of natural anatase. The clear indication is that the Vinland Map is a 20th century product.23 RM confirmed that anatase is present in the ink alone of the Map and that the black component is carbon black, not an iron gallotannate24 as some had supposed. Despite controversy, much of it perpetuated by the scientific popular press and Nordic protagonists, the situation remains clear that the Vinland Map is a forgery.25
Fig. 5. A. Cross-section of a monastic habit painted on a Greek icon; photography with a microscope in reflected light. B. Spectra of pigments taken with a Raman microscope. Identification: (1) underlayer: caput mortuum, lead white, azurite, red lake and yellow ochre (2) highlight: lead white and grains of caput mortuum; (3) varnish; (4) overpainting: ultramarine blue, minium, lithopone and carbon black. Copyright John Wiley & Son and reproduced with permission ­ see ref. 9. Egyptian Papyri (supposedly ca. 1250 BC) Six Egyptian papyri, supposedly dating to the time of Ramses(II) and Nefertari (ca. 1250 BC) were shown to have palettes which include six modern pigments, viz. Prussian blue (Fe4[Fe(CN)6]3 14-16H2O), ultramarine blue, a Hansa yellow, pure white anatase (TiO2), phthalocyanine blue [Cu(C32H16N8)], phthalocyanine green [Cu(C32H16-nClnN8)], and two -naphthol reds; the dates of first manufacture of these modern pigments are 1704, 1828, ca. 1910, ca. 1920, 1936, 1936, 1939 and possibly ca. 1950, respectively. Thus, the papyri studied are clearly 20th century forgeries. Authentic papyri such as those held in the Petrie Museum at UCL were found to have been painted solely with mineral pigments such as Egyptian blue (CaO.CuO.4SiO2 or its mineral equivalent cuprorivaite), orpiment, haematite, malachite, carbon and pararealgar (As4S4).21 The Vinland Map The Vinland Map, held at Yale University (New Haven) is a world map on parchment that includes, significantly, representations of Iceland, Greenland and the north-eastern seaboard of North America. On the bases of cartographical, palaeographical and philological analyses the map was originally thought to have been drawn from two different prototypes around 1440 AD, implying that it predates, by some 50 years, the discovery of America by Christopher Columbus in 1492 AD. However, detailed studies by McCrone22 and others of the black ink defining
Postage Stamp Forgeries: Mauritius 1847, Hawaii 1851 RM offers an effective, rapid and non-destructive way of identifying the pigments and dyes used in the inks, paper, and cancel marks of postage stamps. Thus, the rare and valuable (up to GBP 0.5 M) so-called Hawaiian Missionary stamps (2, 5 and 13 cents, issued in 1851) from the Tapling Collection at the British Library were shown to have been printed using Prussian blue as the blue pigment. In addition, the paper fibres of the stamps were shown in the case of the 13 cent stamp to have been interspersed with ultramarine blue to act as an optical brightener to counteract the effects of the yellowing of paper or cloth fibres with time, cf. Reckitt's blue/dolly blue added several decades ago into copper urns during the washing of white clothing for this reason. Distinctions between genuine and forged or reproduction stamps can sometimes be drawn on the basis of the pigments used.26 Raman microscopy at UCL contributed to the final opinion expressed by the Royal Philatelic Society in 2006 that the rare so-called Grinnell Hawaiian Missionary stamps are forgeries dating from the late 1800s/early 1900s and not ca. 1851, thereby closing a matter which has been controversial since 1918 when the rediscovery of 72 Grinnell stamps was first reported. Similar studies of the earliest Mauritian stamps (1847) have been carried out, notably on an extremely rare one penny stamp (1d, orange-red; used), a rare 2d stamp (blue; unused), as well as a reproduction stamp (1905), early forgeries, and Britannia-type stamps (1858­1862). No numerical value is expressly printed on any stamp, the nominal value being designated solely by its colour. The pigments used were identified by RM to be red lead on the 1d stamp, Prussian blue on the 2d stamp, chrome green ­ a mixture of chrome yellow (PbCrO4 and Prussian blue ­ on the 4d stamp, and vermilion on the 6d (orange) stamp. The blue 2d stamps were known to have been converted illegally to the much rarer, and thus more valuable, green 4d ones, evidently by painting an aqueous slurry of chrome yellow over the surface of 2d stamps and allowing them to dry. Such forgeries could, however, be detected by RM examination of the stamp edges. The technique of RM has great potential for the expertizing of stamps, i.e. distinguishing between genuine and forged or reproduction stamps.27 The New Zealand Chalon issue stamps of 1855, in particular the Dwarf issues, have also been examined recently in London by RM.
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Ingles (15th Century Maestro) and the Spanish Forger (mid-20th Century) Some miniatures believed to be authentic work by the 15th century Spanish maestro Jorge Ingles became suspect forgeries in the mid-20th century on stylistic and compositional grounds. However, no systematic study of the identities of the materials used on these miniatures (now believed to number over 300) had been carried out until 2009, when five were subjected to RM and X-ray fluorescence analyses. These revealed that the pigments used on each of the Spanish forger's miniatures include not only traditional materials of Ingles' period, e.g. vermilion, carbon black, red lead, lead white and indigo, but also four much later ones (chrome yellow, Scheele's green, emerald green and ultramarine blue). These results were, obtained easily and provided a firm scientific basis for recognising the modern forgeries.28
Iraqi Stuccoes (9th Century AD) The archaeological finds (which numbered over 1100) from Samarra made by Sarre and Herzfeld (1911­ 1913) have recently been studied scientifically for the first time. Six 9th century Iraqi stuccoes from Samarra (Fig. 6) have been analysed by RM and the following pigments identified: carbon black, haematite, gypsum, lazurite, lead white, orpiment, pararealgar, red lead, vermilion, and the chi-phase of As4S4.32 Fig. 6. Stuccoes, Samarra, 9th Century Abbasid art. Copyright Elsevier Limited. and reproduced from J. Archaeol. Sci. with permission ­ see ref. 32. Puebloan Ceramics from the American Southwest (ca. 1100­1300 AD) The Ancestral Puebloan (Anasazi) period (ca. 900­1300
Scientific Investigations in Archaeology Pioneering Raman and other scientific studies have opened up many new fields of research in archaeology.6 Recent examples are discussed according to the supposed dates of each artefact.
Chinese Sherds (ca. 4200 BC) The white background layer (slip) on painted sherds (Yangshao culture, Henan, China, ca. 4200 BC) has been shown by RM to contain anatase, leading to claims that this material had been used as a white pigment for over 5000 years. However, native anatase is relatively rare, impure and expensive and its Raman scattering crosssection is extraordinarily high, perhaps 1000 times that of kaolinite clay, the assumed prefiring host material of the sherds.29 SEM/EDX analyses of further Yangshao sherds from another site (Yiquanma) indicate that their white slip contains just 1 wt% of TiO2 (made up of anatase, rutile and other titanium ores). Hence, the anatase component is not present in sufficient proportions to determine the pigmentary properties of the slip, despite the fact that its scattering dominates the Raman spectrum of the sherds.30 The detection of anatase in Chinese sherds and also in Roman ceramic pots and sherds from the East Midlands in these trace amounts does not imply, as has been suggested, that native anatase had been sourced and used as a white pigment for thousands of years. It is a component at the ca. 1 wt% level in kaolinite from many parts of the world, notably Georgia, USA, and it has been detected by RM in countless sherds based upon kaolinite matrices in similarly very low proportions. Punic Make-up Materials (4th-1st Century BC) Ten archaeological Punic make-up materials from Tunisia dating from the 4th to the 1st centuries BC have been analysed by RM and synchrotron X-ray diffraction techniques in order to identify the materials used by Carthaginians for cosmetic purposes on the living and for ritual purposes on the dead.31 Haematite and cinnabar (HgS) were the most commonly found red pigments, in both cases along with quartz and sometimes calcium carbonate, clay and possibly madder. However, no clear distinction could be drawn between living and ritual uses for these materials.
AD) of the American southwest has yielded much pottery of great interest. Raman and infrared spectroscopy has been used to characterise the black pigments on blackon-white potsherds referred to as Mesa Verde Whiteware, which date to ca. 1100­1300 AD. This study has led to conclusive evidence for the use of carbon-based paints.33 Maghaemite (­Fe2O3) and magnetite (Fe3O4), found alternatively or mixed with a carbonaceous pigment, were also found on some sherds. Infrared measurements indicated that little, if any, organic material from biogenic precursors of the black pigment or from pigment binding agents remained in the paints. RM could potentially indicate the nature of the original raw materials, including clays, tempers and pigment phases, as well as the firing temperatures and atmospheres employed in the production of these artefacts. Identification of Iron Oxide Impurities in Earliest Industrial Processed Platinum (1842) A detailed investigation of the impurities present in a
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Chemistry in New Zealand January 2011
19th century Russian platinum coin (3 rouble) reveals the presence of three types of iron oxide, viz. magnetite, iron deficient magnetite and haematite.34 These oxides had been formed during the heating of the impure platinum metal powder as part of the refining process in operation in Russia from 1828 to 1843, and were found by RM to be dispersed in the matrix of metallic platinum containing ca. 1 wt% iron. The presence of these inclusions reflects the incomplete refining practice for native platinum at that time. The state of oxidation of the iron in iron oxides with the magnetite structure directly influences the paramagnetic properties of these coins, and may also be related to the wavenumber of the most intense Raman band (650 - 700 cm-1) of the oxide. The study has made it possible to understand the limitations of platinum technology during its first large-scale production in the first half of the 19th century. The refinement and production process was a major industrial and metallurgical achievement at this time, when not all the platinum group metals to be removed from the native platinum ore were actually known, and the melting point of platinum (1769 єC) could not be reached industrially. Pigment Degradation to Produce Lead and Copper Sulfides Galena (PbS) is an important raw material for the lead smelting and photoconductor industries, a model material for quantum dot research, an ingredient in ancient cosmetics and also in modern hair dyes, e.g. Grecian Formula,35 and a visually offensive black degradation product of white lead in artwork. In the last context, PbS has now been identified successfully by RM on partly degraded manuscript illuminations, despite its black colour which is not conducive to efficient light scattering.36 Much consideration has been given to procedures for reversing the unwanted chemical change of lead pigments. CuS behaves similarly.37 Detailed Raman studies have now also been carried out on millimetre-sized crystals of PbS under resonance Raman conditions and the phonon modes identified and assigned. In order to confirm the vibrational nature of the features observed, Raman spectra were obtained on natural crystals (mineral and synthetic with natural isotopic abundances) as well as on crystals highly enriched (99%) with 34S.38,39 Conclusion The artificial separation of the arts and the sciences is now being reduced largely by the efforts of scientists throughout the world: the scientific study of artwork and artefacts being an area which is expanding rapidly. Many exciting new avenues are opening up, especially in the use of surface-enhanced Raman and resonance Raman spectroscopy for the faster and more effective identification of dyes; such materials are frequently very difficult to identify by RM alone on account of their high background fluorescence.40 Collaboration between the arts and sciences has improved, largely through the initiatives of scientists as now recognised by the Proceedings of the US National Academy of Sciences41 and the entire June 2010 issue of Accounts of Chemical Research.42 The field
has also recently attracted the active attention of Richard Ernst, 1991 Nobel Laureate in Chemistry (NMR spectroscopy), who has developed a gantry for the RM study of large paintings, notably central Asian ones.43 On the wider front, many pigments have been found also to be important in areas outside of colour technology.44 Thus, anatase is photoactive, leading to its role in smart glass, catalysis, nanotechnology, and the study of the wavenumber dependence of Raman bands on nanoparticle size.45 References 1. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley: New York, 5th edn.1997. 2. Long, D. A. The Raman Effect, Wiley: Chichester, 2002. 3. Best, S. P.; Clark, R. J. H.; Withnall, R. Endeavour 1992, 16, 66-73. 4. Clark, R. J. H. In Scientific Examination of Art, National Academies Press: Washington DC, 2005, pp. 162­185; Burgio, L.; Ciomartan, D. A.; Clark, R. J. H. J. Mol. Struct. 1997, 405, 1-11. 5. Clark, R. J. H. J. Mol. Struct. 1995, 347, 417-428. 6. Smith, G. D.; Clark, R. J. H. J. Archaeol. Sci. 2004, 31, 1137-1160. 7. Best, S. P.; Clark, R. J. H.; Daniels, M. A. M.; Porter, C. A.; Withnall, R. Stud.Conserv. 1995, 40, 31-40. 8. Burgio, L.; Clark, R. J. H.; Stratoudaki, T.; Doulgeridis, M.; Anglos, D. Appl. Spectrosc. 2000, 54, 463-469; Burgio, L.; Melessanaki, K.; Doulgeridis, M.; Clark, R. J. H.; Anglos, D. Spectrochim. Acta B 2001, 56, 905-913. 9. Daniilia, S.; Bikiaris, D.; Burgio, L.; Gavala, P., et al. J. Raman Spectrosc. 2002, 33, 807-814. 10. Clark, R. J. H. Chem. Soc. Rev. 1990, 19, 107-131; 1995, 24, 187196; C.R. Chimie 2002, 5, 7-20. 11. Bell, I. M.; Clark, R. J. H.; Gibbs, P. J. Spectrochim. Acta A 1997, 53, 2159-2179. 12. Burgio, L.; Clark, R. J. H. Spectrochim. Acta A 2001, 57, 14911521. 13. Bouchard, M.; Smith, D. C. Spectrochim. Acta A 2003, 59, 22472266. 14. Popper, K. R. The Logic of Scientific Discovery, Hutchinson: London, 1962. 15. Brown, K. L.; Clark, R. J. H. J. Raman Spectrosc. 2004, 35, 4-12, 181-189; 217-223. 16. Clark, R. J. H.; van der Weerd, J. J. Raman Spectrosc. 2004, 35, 279-283. 17. Chaplin, T. D.; Clark, R. J. H.; McKay, A.; Pugh, S. J. Raman Spectrosc. 2006, 37, 865-877. 18. Chaplin, T. D.; Clark, R. J. H.; Jacobs, D.; Jensen, K., et al. Anal. Chem. 2005, 77, 3611-3622. 19. Burgio, L.; Clark, R. J. H.; Hark, R. R.; Rumsey, M. S., et al. Appl. Spectrosc. 2009, 63, 611-620. 20. Burgio, L.; Clark, R. J. H.; Sheldon, L.; Smith, G. D. Anal. Chem. 2005, 77, 1261-1267. 21. Burgio, L.; Clark, R. J. H. J. Raman Spectrosc. 2000, 31, 395-401. 22. McCrone, W. C. Anal. Chem. 1988, 60, 1009-1018. 23. Towe, K. M. Acc. Chem. Res. 1990, 23, 84-87. 24. Brown, K. L.; Clark, R. J. H. Anal. Chem. 2002, 74, 3658-3661. 25. Towe, K. M.; Clark, R. J. H.; Seaver, K. A. Archaeometry 2008, 50, 887-893. 26. Chaplin, T. D.; Clark, R. J. H.; Beech, D. R. J. Raman Spectrosc. 2002, 33, 424-428.
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27. Chaplin, T. D.; Jurado-Lopez, A.; Clark, R. J. H.; Beech, D. R. J. Raman Spectrosc. 2004, 35, 600-604. 28. Burgio, L.; Clark, R. J. H.; Hark, R. R. J. Raman Spectrosc. 2009, 40, 2031-2036. 29. Murad, E.; Koster, H. M. Clay Minerals 1999, 34, 479-485. 30. Clark, R. J. H.; Wang, Q.; Correia, A. J. Archaeol. Sci. 2007, 34, 1787-1793. 31. Huq, A.; Stephens, P. W.; Ayed, N.; Binous H., et al., Appl. Phys. A 2006, 83, 253-256. 32. Burgio, L.; Clark, R. J. H.; Rosser-Owen, M. J. Archaeol. Sci. 2007, 37, 756. 33. van der Weerd, J.; Smith, G. D.; Firth, S.; Clark, R. J. H. J. Archaeol. Sci. 2004, 31, 1429-1437. 34. van der Weerd, J.; Rehren, T.; Firth, S.; Clark, R. J. H. Mater. Charact. 2004, 53, 63-70. 35. Walter, P.; Welcomme, E.; Hallegot, P.; Zaluzec, N. J., et al. Nano Lett. 2006, 6, 2215.
36. Smith, G. D.; Derbyshire, A.; Clark, R. J. H. Stud. Conserv. 2002, 47, 250-256. 37. Smith, G. D.; Clark, R. J. H. J. Cult. Heritage 2002, 3, 101-105. 38. Smith, G. D.; Firth, S.; Clark, R. J. H.; Cardona, M. J. Appl. Phys. 2002, 92, 4375-4380. 39. Etchegoin, P. G.; Cardona, M.; Lauck, R.; Clark, R. J. H., et al., Phys. Stat. Sol. (b) 2008, 245, 1125-1132. 40. Leona, M.; Stenger, J.; Ferloni, E. J. Raman Spectrosc. 2006, 37, 981-992. 41. Burgio, L.; Clark R. J. H.; Hark R. R., Proc. Nat. Acad. Sci. 2010, 107, 5726-5731. 42. Acc. Chem. Res. 2010, 43, June issue. 43. Ernst, R. R. J. Raman Spectrosc. 2010, 41, 275-287. 44. Price, L. S.; Parkin, I. P.; Hardy A. M. E.; Clark. R. J. H. Chem. Mater. 1999, 11, 1792-1799. 45. Zhang, Z.; Brown, S.; Goodall J. B. M.; Weng, X., et al. J. Alloys Compounds 2009, 476, 451-456.
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