A CRITICAL ANALYSIS OF COMMERCIAL PIGMENTS

Transcription

1 A CRITICAL ANALYSIS OF COMMERCIAL PIGMENTS M. Pérez 1, K. Castro 1, Mª.D. Rodríguez 2, MªA. Olazabal 1 and J.M. Madariaga 1 University of the Basque Country, Dept. Analytical Chemistry 1 and Dept. Painting 2, P.O. Box 644, Bilbao, Spain. qabpealm@lg.ehu.es. Web: The spectroscopic analysis of artworks has become a useful tool in the understanding of cultural heritage, in dating works as well as in investigating artistic techniques. If two pigments are detected mixed in a layer of paint, it could be thought that the artist mixed the two pigments, but it might be that the commercial pigment used was actually a mixture of pigments and/or fillers. These authors have found, by FT-Raman and FT-IR spectroscopy, that commercial pigments like cadmium yellows are in origin a mixture of cadmium sulphide and barium sulphate. Moreover, in some cases there is no correspondence between the commercial name of the pigment and its chemical composition. For instance, one commercial lead white, supposedly 2PbCO 3 Pb(OH) 2, was actually PbSO 4. On the other hand, the different origin or method of synthesis of a given compound can provide pigments spectroscopicaly different, as found in the case of ultramarine blues, and cobalt blues. To accomplish a good pigment analysis of a sample (interpreting unknown spectra) it is necessary to have a spectra database not only of pure pigments but also of pigments and fillers from different trademarks and origins, used in the current and past pigment industry. Introduction The scientific examination of the pigments and binders used in different objects of art, such as paintings, pottery, manuscripts or wallpapers, can provide very useful information in the understanding of the cultural heritage [1,2,3]. This kind of studies has great importance not only in the identification of the techniques used, but also in processes of conservation and restoration of the artwork. Among the different analytical techniques used to perform these analysis, vibrational spectroscopies (IR and Raman mainly) are quite advantageous because non-destructive analysis can be carried out. At the time of interpreting the spectra obtained in the analysis of the pigments and binders, it is necessary to use a database that includes spectral characterisation, name and synonymous, as well as technique and period of application for each pigment. This can be of interest due to the presence of confusions found in the literature: similar names have been used to describe more than one pigment. A classic example is provided by minium, applied to both cinnabar (HgS) and red lead (Pb 3 O 4 ) in the antiquity [4]. Another case is the presence of adulterations or impurities in pigments; even C. Cennini in the 15 th century noticed that cinnabar sold as pure pigment, was mixed with minium or with brick dust [5]. Nevertheless, these problems can also appear in the analysis of current industrial pigments. Authors have found in some cases no correspondence between the commercial name of the pigment and its chemical composition. On the other hand some pigments supposedly pure, are mixture of the pigment with a cheaper one, or with another chemical substance used in the method of synthesis. 1

2 In this study some problematic examples related to several trademarks are described: (1) Pigment with the same name but different chemical composition, (2) Some pigments with spectral differences because of their various origins (ultramarine blue, chalk and dragoon's blood), and (3) A list of pigments mixed with CaCO 3, BaSO 4 or gypsum to get the desired colour. Samples Several pigments from eight trademarks, which are nominated as A, B, C, D, E, F G and H, are involved in this work. The following list collects the analysed pigments with their respective commercial names. - A: Genuine flakes white, lead white; Cobalt blue dark; Cobalt blue greenish; Cobalt blue light; Cobalt blue medium; Cobalt blue pale blue; Cobalt cerulean blue; Chalk from Bologna, Chalk from Sabnitz; Chalk from Champagne; Sarti Chalk, natural greyish chalk; Sarti Chalk, natural yellowish chalk; Dragon s blood; Ultramarine blue black; Ultramarine blue greenish extra; Ultramarine blue greenish light; Ultramarine blue light; Ultramarine blue reddish; Ultramarine blue very dark; Cadmium orange; Cadmium orange deep vermilion; Cadmium orange light; Cadmium orange very light; Cadmium red bluish purple; Cadmium red deep; Cadmium red light; Cadmium red light F; Cadmium red medium; Cadmium red medium F; Cadmium red medium pure English; Cadmium yellow deep, Cadmium yellow F; Cadmium yellow lemon; Cadmium yellow light; Cadmium yellow medium; Cadmium yellow medium deep; Cadmium yellow very light; English red dark; English red light; German ochre, light warm brown; Iron oxide yellow; Fawn ochre German very light umber; Italian ochre AVANA; Italian gold ochre; Umber Cyprus, raw dark; Umber raw greenish, German olive green hue; Vagone green Earth; Verona green Earth, Venetian red; Terre Ercolano; Terre Pozzuoli. - B: Cadmium red deep; Cadmium lemon; Cadmium red; Cadmium yellow deep; Cadmium yellow pale. - C: Ultramarine blue; Iron oxide red mine; Chrome orange. - D: Dragon's blood. - E: Lead white. - F: Lead white. - G: Cobalt blue. - H: Ultramarine blue. Instrumentation The FT-Raman spectra of the samples were collected with a Nicolet FT-Raman 950 spectrophotometer, with an excitation wavelength of 1064 nm (Nd: YAG laser), and an 2

3 InGaAs detector. To avoid thermal decomposition of the samples, powers between 12 and 17 mw were used. To obtain a good signal-to-noise ratio 2000 scans were accumulated for each spectrum with a resolution of 4 cm -1. Data acquisition and analysis were carried out with the Omnic software provided by the Nicolet Instrument Corp. The FT-Infrared spectra of the samples were collected with a Nexus spectrophotometer from Nicolet. The pigments were analysed from KBr disks formed by grounding and homogenizing mg of the sample with KBr (150 mg total), in an agata mortar, and pressing the powder to 1 Mpa to get a pellet. Data acquisition and analysis were carried out with the Omnic software provided by the Nicolet Instrument Corp. Results and Discussions Lead white Lead white is historically the most important of all the white pigments until the introduction of zinc oxide in the 19 th century. The common lead white of commerce is the basic carbonate 2PbCO 3 Pb(OH) 2 (CI: Pigment white 1). Although the mineral cerussite, PbCO 3, occurs as an impurity in hydrocerussite (lead basic carbonate), rarely appears in paintings due to the artificial origin of lead white. Nevertheless there are references [6] where lead white is mixed with other pigments, such as calcium carbonate, gypsum or chalk for making opaque watercolour. Three samples of lead whites have been analysed by FT-IR and FT-Raman spectroscopy. Figure 1 shows the FT-IR spectra of the three samples and Table 1 summarizes the wavenumbers of the different IR and Raman signals. All of them have the same commercial name but in fact their chemical composition are different. Only the F trademark corresponds to pure lead white (2PbCO 3 Pb(OH) 2 ), whereas the A trademark is PbCO 3 mixed with PbSO 4 and the E one has actually only a small quantity of PbCO 3 mixed with PbSO 4 and another salt containing sulphate. Figure 1: Infrared spectra of lead white samples. The trademarks to whom correspond each spectra are a) A: Genuine flakes white, lead white, b) F: Lead white, c) E: Lead white. 3

4 Table 1: Characteristic Infrared and Raman spectra bands for lead white pigments. Commercial name IR wavenumber Raman wavenumber A: Genuine flakes white, lead white 3541w, 3415w br, 1737w, 1404vs, 1128w, 1112w, 1074w, 1045vw, 846vw, 762w, 682s, 617vw, 598vw F: Lead white 3436w br, 1728w, 1436vs, 1405vs, 1051m, 838s, 678s E: Lead white 3510m, 2920vw, 2852vw, 1618w, 1420w, 1120vs, 1074s, 968w, 808w, 696vw, 619s, 598m, 519w, 488w, 441w Chalk 1377m, 1049vs, 975m, 960m, 827w, 817w, 679w, 438vw, 426w, 398w, 193sh, 170sh, 145s, 129s, 111sh 1476w, 1374w, 1363w, 1053vs, 837vw, 694vw, 681vw, 672vw, 224w, 175w, 151w, 132sh, 117sh, 98sh 2923vw, 2882vw, 2841vw, 1153vw, 1070w, 1051vw, 975vs, 640vw, 617vw, 601w, 448w, 438m, 426w, 332w, 284w, 185w, 146s, 127sh, 121sh, 104sh Chalk is one of the several types of calcium carbonate pigments more usually used in artworks apart form calcite, lime white, shell white and coral. It is composed of fossil remains of unicellular algae [7]. Sometimes chalk has been mixed with lead white and zinc white to get a denser and whiter aspect. Several samples of the A trademark have been analysed by FT-IR and FT-Raman. Chalk is a clear example of pigment with spectral differences due to the origin of the material. The principal components in each Chalk, as reported by the supplier, are summarized in Table 2. The results obtained by FT-IR and FT-Raman spectroscopies agree completely, except in Chalk from Bologna where in the Raman spectra appears a band at 1023 cm -1 which can not be explained by the only presence of calcium carbonate and gypsum (see Figure 2). Figure 2: Raman spectra of chalk samples. The trademarks to whom correspond each spectra are a) A: Chalk from Champagne; b) A: Sarti Chalk, Natural yellowish chalk; c) A: Chalk from Bologna. 4

5 Dragon s blood Table 2: Characteristic Raman spectra bands for chalks. Commercial name Composition from the supplier. A. Chalk From Champagne CaCO 3 A. Chalk from Sabnitz CaCO 3 A. Chalk from Bologna CaCO 3 + gypsum A. Sarti chalk, Natural greyish Chalk CaCO 3 + gypsum (small quantity) A. Sarti chalk Natural yellowish Chalk CaCO 3 + gypsum (small quantity) A. Chalk from Bologna CaCO 3 + gypsum The two samples of dragon's blood have been analysed by using FT-IR spectroscopy. In both samples the characteristic infrared bands appear at the same wavenumbers (see Figure 3). Nevertheless there are differences in the relative intensity of the signals at 1702 and 1384 cm -1. Moreover, the intensity of the bands at 2951 and 2935 cm -1 is stronger in the A trademark. A simple process of polymerization can explain the decrease of the intensity in these bands corresponding to C-H bonds in D sample. In this case the ageing of the pigment could be the responsible of the changes in the IR spectra. Figure 3: Infrared spectra of dragon's blood samples. The trademarks to whom correspond each spectra are a)d: Dragon's blood; b) A:Dragon's blood Cobalt blue Cobalt blue is an artificial pigment discovered around 1800, used as imitation of artificial ultramarine blue. Its chemical composition corresponds to a mixture of CoO Al 2 O 3 and H 3 PO 4. The seven commercial samples of cobalt blue have been studied by FT-IR spectroscopy. Even though six of all cobalt blues belong to the same trademark, with only slightly changes in colour, there is no coincidence in their FT-IR spectra, which concludes that they do not share with the same chemical structure. Three of the spectra are shown together in Figure 4, where the spectral fingerprint are clearly different. Table 3 summarizes the IR wavenumbers of the seven samples analysed. 5

6 Figure 4: Infrared spectra of cobalt blue samples. The trademarks to whom correspond each spectra are a) A: Cobalt blue dark; b) G: Cobalt blue; c) A: Cobalt blue light. Table 3: Characteristic Infrared spectra bands for cobalt blue pigments. Commercial name IR wavenumber (nm -1 ) G: Cobalt blue 758 sh, 663s, 557s, 513s A: Cobalt blue dark 1179sh, 1116sh, 1060sh, 971s, 927vs, 896vs, 867s, 612s, 578s, 461m A: Cobalt blue greenish 644vs, 534vs, 469sh A: Cobalt blue light 673sh, 650sh, 597s, 585sh, 554sh, 522sh, 463m A: Cobalt blue medium 808sh, 669s, 560s, 514s A: Cobalt blue pale blue 795sh, 755sh, 666m, 636w, 615sh, 651m, 487m, 447m, 417sh A: Cobalt cerulean blue 675sh, 656s, 596vs, 556sh, 523sh, 463vs Ultramarine blue In the literature two kind of ultramarine blues are described. The natural one is obtained from the mineral lapis lazuli, a complex rock mixture, in a process of several extractions. But it is important to emphazise that although the extraction process is carried out in the most carefully way, some impurities of calcite can remain in the pigment. This can be used to distinguish between the natural pigment and the synthetic one (without calcium carbonate). In painting books of all ages there are warnings about adulterated natural ultramarine blues due to its high cost. [7] Among the eight samples of ultramarine blue analysed by FT-IR spectroscopy it can be appreciated changes in the proportion of silicates, as a consequence of the process of extraction, or the origin of the samples. The most remarkable case corresponds to "Ultramarine blue light" from the A trademark, where an important quantity of CaCO 3 has been found (see Figure 5). 6

7 Figure 5: Infrared spectra of ultramarine blue samples. The trademarks to whom correspond each spectra are a) A: Ultramarine blue light; b) C: Ultramarine blue, c) A: Ultramarine blue black. Cadmium pigments The colour index distinguishes in the case of cadmium yellow between CdS pure PY 37 and lithopone variety (a mixture of CdS with BaSO 4 ) PY 35. As a consequence of the possible confusion the trademark should advice about the cadmium pigment variety involved in the selling. As well as with cadmium yellow the same situation exist for cadmium oranges and red, which are in fact sulfoselenides Cd(S, Se). As CdS has no signal in the mid infrared region the use of FT-IR spectroscopy allows to determine even small quantities of BaSO 4 in cadmium yellow pigments. In all cadmium pigments referred in this paper BaSO 4 has been found even when the pigment has been sold as a pure one, as seen in the example shown in Figure 6. Figure 6. Infrared spectra of a) A: Cadmium red medium pure English, b)a: BaSO 4. Pigments with fillers As it has been mentioned before, in some cases pigments are mixed with some fillers to decrease cost or with the aim of changing the colour shade. Some of the most used 7

8 fillers are calcium carbonate, barium sulphate and gypsum. A list of pigments analysed in this work, where these fillers, appear are given: - List of pigment that contains CaCO 3 : Trademark A: English red dark; English red light; German ochre, light warm brown; Iron oxide yellow; Fawn ochre German very light umber; Italian ochre AVANA; Ultramarine blue light; Umber Cyprus, raw dark; Umber raw greenish, German olive green hue; Vagone green Earth (small quantity); Verona green Earth (small quantity). Trademark C: Iron oxide red mine (small quantity). - List of pigments that contains BaSO 4 : Trademark A: Cobalt blue greenish (small quantity), cobalt green. Trademark B: Cadmium red; Cadmium lemon; Cadmium red deep. Trademark C: Chrome orange - List of pigment that contains gypsum: Trademark A: Italian gold ochre; Italian ochre AVANA; Terre Ercolano, mixed orange Earths; Terre Pozzuoli, mixed red Earths, Vagone green Earth; Venetian red; Verona green Earth. Bibliography [1] R.J.H. Clark, L. Curri and G.S. Henshaw Characterisation of Brown-black and Blue Pigments in Glazed Pottery Fragments from Castel Fiorentino (Foggia, Italy) by Raman microscopy, X-Ray Powder Diffractometry and X-Ray Photoelectron Spectroscopy, J. Raman Spectrosc., 28, 1997, [2] H.G.M. Edwards, C.J. Brooke and J.F. Tait, Fourier Transform Raman spectroscopic Study of Pigments from English Medieval Wall Paintings, J. Raman Spectrosc., 28, 1997, [3] K. Castro, M.D. Rodríguez-Laso, L.A. Fernández, J.M. Madariaga, Fourier Transform Raman Spectroscopic Study of Pigments Present in Decorative Wallpapers of the Middle Nineteenth Century from the Santa Isabel Factory (Vitoria, Basque Country, Spain), J. Raman Spectrosc., 33, 2002, [4] H.G.M. Edwards, D.W. Farwell, E.M. Newton and F. Rull Pérez Miium; FT-Raman non-destructive analysis applied to an historical controversy, The Analyst, 124, 1999, [5] C. Cenninni, Tratado de la Pintura, Manuales Meseguer, 1979, Barcelona 4 th edition. [6] R.J. Gettens, H. Künh and W.T. Chase, Lead White, Studies in conservation, 2, 1967, [7] "Artist's pigments". A Handbook of their History and Characteristics; Volume 2, Oxford University Press, New York, Acknowledgements M. Pérez is grateful to the Basque Country Government for her pre-doctoral fellowship. K. Castro is grateful to the Spanish Government (MECD) for his pre-doctoral fellowship. This work was partially funded by the project BHA of the Spanish R+D+I Program