Sic 55,3 2010 pollock

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S t u d i e S i n c o n S e rvat i o n 5 5 ( 2 0 1 0 ) pag e S 2 0 4 – 2 1 5

204

Received December 2009

A Technical Analysis of Three Paintings Attributed to Jackson Pollocknarayan Khandekar, carol Mancusi-ungaro, Harry cooper, christina rosenberger, Katherine eremin, Kate Smith, Jens Stenger and dan Kirby

INTRODUCTION1

according to his own claim, in 2002 alex Matter, son of photographer Herbert Matter and painter Mercedes Matter, found 32 works of art wrapped in brown paper in a Long island storage facility that his father had owned. a photograph provided by alex Matter shows the brown paper inscribed with the following words: ‘pollock (1946–49) / tudor city (1940–1949) / 32 Jackson experimental / works (gift & purchase) / Bad condition. / 4 both sides. all / drawing boards. / robi paints. / Macdougal alley, 1958’ [1].

alex Matter believes that Jackson pollock (1912–1956) completed these works in Herbert Matter’s studio in new York city between 1946 and 1949 and that he might have used paints developed by robert (robi) rebetez, a Swiss art supplier and Herbert Matter’s brother-in-law [1]. correspondence by the authors with

Three paintings from a group of thirty-two attributed to Jackson Pollock (1912–1956) were examined at the Harvard Art Museums. Paint samples were examined using Raman spectroscopy, scanning electron microscopy–energy dispersive X-ray spectrometry (SEM-EDX), laser desorption ionization time-of-flight mass spectroscopy (LDI-MS), Fourier transform infrared spectroscopy (FTIR), pyrolysis–gas chromatography–mass spectrometry (py-GCMS) and carbon-14 (C-14) dating. Carbon-14 dating detected no ‘bomb’ carbon from atmospheric atomic testing. All three paintings contained pigments and media available only after Pollock’s death, including PR 254, PY151, and an emulsion copolymer of n-butyl methacrylate and iso-butyl methacrylate.

one of robert rebetez’s daughters revealed that neither she nor her sister recalled paints being sent to relatives in the uS and that their father’s store stocked only standard brands of artist’s paints.

upon finding the works, alex Matter put the paintings under the care of Mark Borghi Fine art in new York city, and a conservator was found to treat the paintings in 2002 [1]. Many of the paintings and the brown paper wrapper were featured in an exhibition at the McMullen Museum of art, Boston college [1]. From their discovery to the present, the paintings have been the centre of vigorous discussion and public speculation surrounding their attribution to pollock [e.g. 2–5].

the Harvard art Museums conducted an inde-pendent, pro-bono analysis of 3 of the 32 paintings. the investigation was conducted between September 2005 and december 2006. this study is part of a broader ongoing investigation into the materials and techniques used by twentieth-century artists and fits within the art Museum’s mission of object-based teaching and critical inquiry.

the three works (presumably named using a com-bination of initials from Mark Borghi and Jackson

1This paper is based on a report initially posted to the Harvard Art Museums website from 31 January 2007 until August 2009.

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a technical analysis of three paintings attributed to jackson pollock 205


their materials: raman spectroscopy, scanning electron microscopy with energy dispersive X-ray spectrometry (SeM-edX), laser desorption ionization time-of-flight mass spectrometry (Ldi-MS), Fourier transform infrared spectroscopy (Ftir), pyrolysis–gas chromatography–mass spectrometry (py-gcMS) and carbon-14 (c-14) dating. the research team requested a diverse sampling of the 32 paintings, but not all of the works were available and substitutions were made at the discretion of the owner, resulting in the final group of 3 examined in this study.

VISUAL/MICROSCOPIC INSPECTION

the first work, MBJp09 (Figure 1), executed on blue-coated cardboard laid on canvas, has extensive cracking, primarily in the black paint, of a type associated with fast drying. Microscopic observation revealed drips of white paint inside both the cracks and the losses in the black paint (Figure 4). this observation suggests that the work was made in at least two separate painting sessions, perhaps closely spaced, as it is not known how quickly the cracks formed. Some areas of loss show evidence of an abraded blue-coated cardboard substrate.

the second work, MBJp14 (Figure 2), has fine hairline cracks in the brown paint that was applied as a uniform ground over blue-coated cardboard. Wider cracks can be seen in the white, grey and red paints. this work has a simple layered structure, and there is no evidence of multiple painting sessions.

the third painting, MBJp29 (Figure 3), executed on thin, blue-coated cardboard, is badly damaged and has been heavily consolidated and restored. it seems the work was made in at least two painting sessions separated by a period of time long enough for cracks to form in paint from the first session: there are sprinkles of black paint on top of losses and inside cracks in the white paint (Figure 5); scratches run below the top paint layer; dried paint has been scored with a sharp tool in straight lines creating a raised burr on either side of the cut (Figure 6) (although the owner asserts that these marks were made by his cat when left alone over a four-day period with the painting [6]); and there is gray paint in the cracks of the white paint (Figure 7). on top of the paint layer a thick transparent consolidation material, probably mixed quickly resulting in many air bubbles, covers much of the surface (Figure 8). Many loose chips of original paint have been randomly incorporated into this consolidation material implying that the work has undergone significant restoration. in addition, Mark

Figure 1 Painting MPJP09 (41.0 × 44.8 cm) showing sample locations.



pollock) MBJp09, MBJp14 and MBJp29 (Figures 1–3) were analyzed using a variety of techniques with a view to determining the age and composition of

black (29.8, 34.8)

violet (16.5, 26.0)

orange (20.1, 16.7)

blue (6.5, 11.5)

blue-gray (0, 8.5)

brown (0.0)

yellow (32, 16)

black (34.5, 4.0)

orange (42, 5)

dark orange (43, 15.5)

white (36, 14.5)

red (43, 15.5)

blue(0.7, 9.2)

pink(7.5, 11.6)

silver(0, 0)

brown(5.4, 8.4)

yellow(13.9, 15.1)

orange(14.9, 15.2)

red(20.0, 0.7)

white(21.4, 12.8)

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206 n. khandekar et al.


Figure 4 Detail of MBJP09 showing white paint applied over losses and cracking in the underlying black paint. Please note that the lighting was angled to show the salient points.

Figure 5 Detail of MBJP29 showing black paint speckles in the craquelure of the white paint, and into losses that have been retouched (blue). Please note that the lighting was angled to show the salient points.

Figure 6 Detail of MBJP29 showing raised burrs either side of the slice made by a sharp blade, especially noticeable through the red paint. Note that some slices are partly covered by white and black overpaint, which also is sliced. Please note that the lighting was angled to show the salient points.

Figure 7 Detail of MBJP29 showing grey and black overpaint in the craquelure of the white paint, which also has black paint speckles over it. Please note that the lighting was angled to show the salient points.

Borghi claims to have altered the painting again after its restoration:

after the cat destroyed the work in question, it was given to Franco Lissi [sic] to see if he could put any of the work back together. When the painting was returned to alex, it had been completely repainted by Lissi. i removed some of the repaint with an exacto [sic] knife to give an idea of the original condition [7].2

2Franco Lisi is the person referred to here.

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RESULTS

Identification of pigments and binding media

raman spectroscopy, SeM-edX and Ldi-MS were used to identify pigments, Ftir was used to identify pigments and binding media, and py-gcMS was used to identify binding media. in choosing which areas to sample, areas of restoration (except where noted) were avoided. pigments were identified against standards from the Forbes collection of pigments (Straus center, Harvard art Museums), tate (assembled from pigments obtained directly from the manufacturers), and the infrared and raman users group (irug, www.irug.org).

results are tabulated according to the analytical technique used (table 1). Figures 1–3 show the locations on each painting where samples were taken for analysis.

Carbon-14 dating

Samples of the support of MBJp14, of MBJp29, and of a blue-coated cardboard from the pollock/Krasner House and Study center (samples aa69434-69436) were sent to the accelerator Mass Spectrometry (aMS) Facility at the university of arizona at tucson to be tested for elevated levels of carbon-14 resulting from atmospheric atomic bomb testing, which began in 1955 [8–9]. the results (table 2) indicate that all three boards contained no detectable levels of ‘bomb’ carbon-14. although these results cannot be considered an accurate indication of age (see discussion below), it is apparent based on the relative ages that the blue board from the pollock/

Figure 8 Detail of MBJP29 showing overpaint that contains trapped air bubbles. Please note that the lighting was angled to show the salient points.

Krasner House and Study center is considerably older than the supports from the two works tested (MBJp14 and MBJp29).

DISCUSSION

Pigments

the use of raman spectroscopy, Ftir and SeM-edX to identify pigments is well reported in the conservation literature. the use of Ldi-MS is relatively new, but has proven to be extremely effective for the identification of synthetic organic pigments. Ldi-MS determines the presence of a chemical compound or molecular species based on their mass and fragmentation patterns. once determined, the mass of each compound or molecular species is compared to pigment standards (Figure 9). although good spectra were obtained, some pigments were unidentified by Ldi-MS because the appropriate standards are not yet available for comparison; however, the m/z of the parent ion is reported.

Many of the pigments detected in these works are traditional artists’ materials that have been used in paint from the nineteenth and early twentieth centuries, such as synthetic yellow ochre, mid-nineteenth century (MBJp14); synthetic ultramarine blue, 1828 (MBJp14, MBJp29); titanium white as the rutile phase, 1937 (MBJp14, MBJp29); barium- and chromium-based materials, early nineteenth century (MBJp29) [10]; one azo pigment, late nineteenth–early twentieth century (MBJp14) [11]; cadmium red, early twentieth century (MBJp29) [10]; and pBk 1 or aniline black, 1860–1863 (MBJp09) [12, p. 577]. Somewhat more modern materials were found on MBJp09: pv 39 (as early as 1913 BaSF developed cationic (‘basic’) dyes with heteropolyacids; this class of complex salt gained importance between the First and Second World Wars [12, p. 549]) and pB 15 (phthalocyanine blue, 1936 [11]). pY 14, a diarylide yellow pigment commercially available from 1935, and in the united States through e.i. du pont de nemours & co. in 1936, was also detected on MBJp09 [11; 12, p. 243]. other components identified, such as calcite and barite, were commonly used as pigment extenders in paint since well before the twentieth century [10].

Still other materials found by the research team were not available until 1950 or later. po 43, found on MBJp29 and MBJp14, is a perinone. the earliest commercial use of this class of compound was 1924; however, it was not recognized as a pigment until 1950 [12, p. 483] and was industrially produced only after 1953 [11]. pY 151, a benzimidazolone pigment found

http://www.irug.org

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Table 1 Results of analysis of samples taken from MBJP09, MBJP14, MBJP29 by Raman Spectroscopy, SEM/EDX, LDI-MS, FTIR and py-GCMS.

Color Raman SEM/EDX LDI-MS FTIR py-GCMS

painting Mbjp09Black PB 15 PBk 1, gum-based medium, acrylic

resinpoly(n-butyl methacrylate)poly(iso-butyl methacrylate)

Orange PR 9 PY 14, gum-based medium poly(n-butyl methacrylate) m/z 380 (unidentified) poly(iso-butyl methacrylate) poly(styrene) phthalic anhydride (plasticizer) violet PV 39

painting Mbjp14Black synthetic ultramarine carbon-based black Orange unidentified azo pigment PY 151 PY 151 White titanium dioxide (rutile) calcite, drying oil, acrylic resin poly(ethyl methacrylate) poly(styrene)Yellow synthetic yellow ochre synthetic yellow ocher, calcite Blue synthetic ultramarine Blue-gray synthetic ultramarine titanium dioxide (rutile) Dark Orange PO 43 PR 9 PO43, acrylic resin poly(n-butyl methacrylate) phthalic anhydride (plasticizer)Red gum-based medium Brown calcite, gum-based medium

painting Mbjp29White titanium dioxide (rutile) poly(vinyl acetate) Blue synthetic ultramarine Red PO 43 Cd PR 112 PR 112, calcite, gum-based medium Cd Yellow lead chromate Pb, Cr,

Ba, Schromium oxides, Pb, Ba

m/z 371 (unidentified) Orange PR 4 PR 4 or PR 6 lead chromate Pb, Cr chromium oxides, Pb

barium sulfate Ba, S Ba Pink titanium dioxide (rutile) m/z 457 (unidentified) Brown PR 254 PR 254 Silver poly(styrene) poly(styrene) poly(acrylonitrile) poly(acrylonitrile) poly(methyl methacrylate) poly(methyl methacrylate) poly(n-butyl methacrylate) poly(iso-butyl methacrylate)light gray-blue poly(vinyl acetate)(retouching) poly(butyl methacrylate) poly(n-butyl methacrylate)adhesive poly(vinyl acetate) Clear coating poly(n-butyl methacrylate)

on MBJp14, has only been available since 1971 [12, p. 353] although the patent application by Farbwerke Hoechst dates to 1969 [13]. it can be inferred that anything on top of this pigment must have been applied later than 1969/1971. through careful examination of the overlapping paint layers on MBJp14, it can be determined that pY 151 is in the lowest layer, except for the brown priming. therefore, the brown priming

Table 2 Results from carbon-14 dating

Sample Radiocarbon age

Pollock/Krasner Blue Board Pol 1 AA69434 288 ± 39 bp

MBJP29 Pol 2 AA69435 143 ± 38 bp

MBJP14 Pol 3 AA69436 127 ± 52 bp

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is the only material that could have been applied prior to 1969/1971. pr 112, found on MBJp29, was patented in 1939 but has only been marketed ‘for a few decades’, according to the most recent reference on the subject, Herbst and Hunger, 2004 [12, p. 295]. the preface to the first edition of Herbst and Hunger was dated december 1992, so this would put their cited marketing date for pr 112 around the early 1960s. a review of chemical abstracts, whilst not conclusive, supported this observation. pr 254, found on MBJp29, is a diketopyrrolopyrrole (dpp) pigment that was first reported by Farnum in 1974 [14] and was subsequently developed by ciba-geigy in the early 1980s [15], coming onto the market in 1986 [16]. in the literature pr 254 is recorded as having been introduced as an artist’s paint by royal talens Bv in 1996 [11], although the technical director of golden paints stated in a telephone conversation that his company had used the pigment since 1994 [17].

Binding media

the black paint in MBJp09 was found to consist of a polymer binder composed of an almost equal mixture of n-butyl methacrylate and iso-butyl methacrylate (Figure 10). the finding by Ftir, also, of a gum-based (polysaccharide) material (Figure 11), mixed with the synthetic polymer, implies that the form in which the polymer existed in the wet state was as a water-based emulsion; the polysaccharide/gum being dissolved in

the water phase. although n-butyl methacrylate and iso-butyl methacrylate polymers have been reported individually as varnishes since the 1930s [18] (acryloid F10 and acryloid/paraloid B67, respectively, are made of these polymer types), the combination of n-butyl methacrylate and iso-butyl methacrylate is an unusual pair of monomers to be found in a synthetic polymer

Figure 9 Positive ion LDI spectra from MBJP29 dark red/brown (upper) and Tate reference pigment PR254 (lower).

Figure 10 Pyrolysis gas-chromatogram of black paint from MBJP09 showing the peaks from the copolymer medium made up of iso-butyl methacrylate (inset mass spectrum A) and n-butyl methacrylate (inset mass spectrum B)

Figure 11 FTIR spectrum of black paint from: (A) MBJP09 with comparative library spectra of (B) gum arabic, (C) Acryloid™ B-67 and (D) Pigment Black 1 (PBk1)

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paint medium. a search through the chemical literature revealed that solvent-borne copolymers of this type were not reported earlier than 1965 [19–21]; and the literature on emulsion copolymers of these substances contains very few references, mostly considerably later [e.g. 22]. the implications from the literature are that copolymer products of the n- and iso-butyl methacrylate type were not available until the mid-1960s, which suggests therefore that the black paint and anything applied over it must date from after that period, leaving only the blue board to have been available during pollock’s lifetime. croll [23] reports that in a typical emulsion paint, polysaccharide/cellulosic material used as thickener/rheology modifier is present in the order of 0.78%; however, the Ftir spectrum of the black paint sample from painting MBJp09 suggests that polysaccharide/gum material is a substantial component of the paint.

the orange paint from the same painting, MBJp09, contains a similar combination of monomer substances, but with the presence also of styrene, a common con-stituent of synthetic polymer binders [23]. again, the Ftir results indicate the presence of a gum. it is likely that the water-soluble gum was added to the emulsion, but it is difficult to determine the exact origin of the particular n-butyl methacrylate/iso-butyl methacrylate/styrene terpolymer medium in the orange paint. evidence of the addition of a gum to an emulsion medium was not found in the literature, and strongly suggests that the gum and emulsion were mixed by the artist just prior to painting. the addition is also reminiscent of the activities of the well-known forger John Myatt, who reportedly added KY jelly, a carboxymethylcellulose gel, to alter the handling and drying properties of the household emulsion paint he chose to use for his works [24], but it is also possible that the addition was made to alter handling properties for aesthetic reasons alone, without any attempt to create deceptive effects such as aging.

an experiment was conducted to determine the miscibility of emulsion paints and a polysaccharide additive, such as gum arabic. although it is not possible to determine quantitatively the amount of gum in the paint by Ftir, the polysaccharide signal is the strongest of any vibrations in the spectrum (Figure 11), suggesting that a there is a significant amount of gum present. using this information one of the authors (Khandekar) decided to approximate the paint under study by mixing gum and emulsion paint at a ratio of 50:50. it was found that gum was easily incorporated into the wet emulsion paint, and when applied in an ‘action-paint’ style it dried over a weekend with the formation of considerable

craquelure similar to that observed in the paintings being studied (Figure 12). this suggests that the cracking seen on the paintings under examination could have been formed much more quickly than one would normally assume for an artist’s paint.

on painting MBJp14, Ftir analysis revealed that a traditional gum-based medium was used for two of the paints [25]. poly(n-butyl methacrylate), the binder of paints marketed as ‘Magna’ by Bocour artist colors from 1949 [26], was also found in the painting’s dark orange paint.

a third medium was also detected in MBJp14, a copolymer of styrene and ethyl methacrylate. it is difficult to determine the exact date that this copolymer was commercially introduced as a paint medium, but a patent from 1963 suggests that it was not available before that year [27]. Significant research on this copolymer did not take place until the mid-1970s and peaked again in the late 1980s, suggesting that it was not commercially available until the mid-1970s.

Five monomer compounds were detected in the silver paint from MBJp29: styrene, acrylonitrile, methyl methacrylate, n-butyl methacrylate and iso-butyl methacrylate (Figure 13). poly(butyl methacrylate) was found on top of the silver paint as a clear coating indicating that it was not entirely present as part of the medium of the silver paint. the conservator confirmed in a telephone conversation that he used this material as a consolidant and that his work did not include inpainting except for the blue paint used to inpaint the areas of abraded blue ground on MBJp29 [28]. it is impossible

Figure 12 Detail of a 50:50 mix of gum arabic and commercial acrylic emulsion paint after drying for a weekend. Note the formation of drying cracks. (Image width: 112 mm.)

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to determine the exact combination of polymer con-stituents in the silver paint – it could be a mixture of any permutation of mono-, co- and ter- polymers.

it is worth noting that the unusual combinations of polymers with the addition of gum that were observed in these three paintings suggest that they are related.

Support – carbon-14 dating

MBJp14 and MBJp29 were not lined and offered the best opportunity for investigating the support by sampling from damaged areas on the edge or verso. MBJp09 is canvas-lined, and therefore the support could not be sampled without damaging the image surface. during a telephone conversation about Jackson pollock’s materials, Helen Harrison, the director, of the pollock/Krasner House and Study center noted that a blue-coated board was found during an inventory of the house and studio after the death of Lee Krasner (1908–1984) [29]. the exact date that the board entered the pollock/Krasner house and studio therefore cannot be determined. the research team obtained the board because of its marked resemblance to the supports of the three works studied.

Samples of the support of MBJp14, of MBJp29, and of the blue-coated cardboard from the pollock/Krasner House and Study center (samples aa69434-69436) were sent to the aMS Facility at the university of arizona

at tucson to be tested for elevated levels of carbon-14 resulting from atmospheric atomic bomb testing, which began in 1955 (table 2). the results indicate that none of the three boards contained detectable levels of ‘bomb’ c-14. on first inspection it appears that the carbon-14 results show that the cardboard supports of MBJp14 and MBJp29 were manufactured prior to 1955. However, further research is required to refine the application of this technique to the study of cardboard, which is derived from materials of various dates [30], and also to determine the time lag until ‘bomb’ c-14 becomes detectible in paper products. it is also worth noting that the results show that the organic material from the pollock/Krasner Foundation blue board is considerably older than the supports from the paintings, but how much older could not be quantified by these results. even though the dates all show that the carboard predates 1955, new old stock can be obtained at any point after its manufacture; the support does not reveal the artist’s hand or the moment of paint application.

two independent studies by newman and derrick, Museum of Fine arts (MFa), Boston [31] and James Martin, orion analytical [32] of alex Matter’s paintings were carried out at the same time as this study. the MFa study examined 9 works and the orion study 17. Both studies found materials that were only available after pollock’s lifetime in many of the works, including supports and lowest layers of paint. detailed discussions of results are included in their respective papers, which support the findings presented here.

CONCLUSION

Some of the pigments identified in this study raised questions about the proposed date of creation, 1946–1949, of the three works analyzed. on MBJp29 red paint was found containing pr 112, which as of 1992 had only been marketed for ‘a few decades’. dark orange paint on MBJp14 and red paint on MBJp29 were found to contain po 43, which was not industrially produced before 1953. MBJp14 contained a pigment, pY 151, in the orange paint that was not available until 1971. the red pigment, pr 254, included in the brown paint from MBJp29 was discovered in 1974 and came onto the market in 1986. Some media raised similar questions. MBJp09 and MBJp14 contained media that were most likely not available until the mid-1960s or 1963, respectively.

this work shows the importance of material studies, and especially establishing detailed timelines of pigments and media to supplement provenance and art historical discourse, in investigating authorship and dating of works

Figure 13 Pyrolysis gas-chromatogram of silver paint from MBJP29 showing peaks identified as: (A) propene; (B) 2-methyl propene; (C) acrylonitrile; (D) 1,5-hexadiene; (E) butanal; (F) methacrylonitrile; (G) 2-methyl-2-propenol; (H) butanol; (I) triethyl amine; (J) methyl methacrylate; (K) cyclopentanone; (L) butyl-2-propenoate; (M) styrene; (N) iso-butyl methacrylate; (O) n-butyl methacrylate.

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of art. it also confirms that Ldi-MS has an important place in the conservation laboratory to identify modern organic pigments.

ACKNOWLEDGEMENTS

thanks to alex Matter and Mark Borghi for allowing access to the paintings. thanks to richard newman, head of the Scientific research Laboratory, Museum of Fine arts, Boston, for access to his SeM-edX and raman spectrometer. thanks to dr tom Learner for samples of tate’s collection of synthetic organic pigments and also to dr greg Hodgins from the university of arizona, nSF-arizona-aMS Laboratory for his help in interpreting the carbon-14 results. Finally, thanks to James Martin of orion analytical for sharing his knowledge of the science and issues involved in such studies.

APPENDIX: EXPERIMENTAL

Raman spectroscopy

the raman spectrometer used was a Bruker optics Senterra dispersive raman microscope with an olympus BX51M microscope. the raman spectrometer is equipped with 532, 633 and 785 nm excitation laser sources. there are three gratings for the 785 nm laser, covering the 70–3283 cm−1 range, four gratings for the 633 nm laser covering 60–3532 cm−1, and three gratings for the 532 nm laser covering 65–3700 cm−1. the spectrometer resolution is 3–5 cm−1, dependent on the wavenumber. the systems use an andor idus ccd detector, operating at −55°c. the five settings for the power of each laser are 1, 10, 25, 50 and 100%. the estimated actual power on the sample at the 100% setting is c. 8.5 mW for the 532 nm laser, 10.9 mW for the 633 nm laser, and 37.5 mW for the 785 nm laser. the microscope has 20×, 50×, and 200× objectives, with the laser spot size approximately 5, 2 and 1 μm, respectively. raman analysis was carr ied out at the analytical Laboratory of the Museum of Fine arts, Boston.

Scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX)

a JeoL JSM-6460Lv scanning electron microscope was used, equipped with an oxford instruments incax-sight energy dispersive X-ray spectrometer (edX) operated by the inca software platform. SeM conditions were: 20 kv, beam current optimized for edX analysis, working distance 10 mm. Samples were

examined in low-vacuum mode, with SeM chamber pressure set at 35 pa. SeM-edX analysis was carried out at the analytical Laboratory of the Museum of Fine arts, Boston.

Laser desorption ionization time-of-flight mass spectroscopy (LDI-MS)

a perSeptive Biosystems voyager-de Biospectrometry Workstation time of flight mass spectrometer was used to obtain laser desorption ionization mass spectra. the instrument was equipped with a pulsed nitrogen laser (337 nm) and operated in linear mode. typical operating conditions in positive (negative) ion mode were accelerating voltage, 23000 (−23000); grid voltage, 60.0%; guide wire voltage, 0.05%. typically, 100 spectra were co-added to obtain spectra with adequate signal-to-noise ratio. resolution was such that monoisotopic molecular weights could be measured in the mass range of interest. Mass was calibrated with csi in both positive and negative modes. Samples were prepared by placing small paint particles on a stainless steel sample plate with a needle point, and then using a drop of acetone to self-adhere the sample in place. neither sample dimensions nor weight were measured, but typical samples were on the order of 10–20 μm along an edge, and, generally, the smallest sample that could be handled with a sharp needle point was sufficient to produce spectra.

Fourier transform infrared spectroscopy (FTIR)

Ftir spectrometric analyses were carried out using a nicolet 510 instrument coupled to a Spectra-tech ir-plan infrared microscope with a 32× objective. the sample was compressed onto a diamond cell (2 × 2 mm) with a stainless steel roller and the sample area defined by double apertures contained in the microscope. an absorbance spectrum (4000–500 cm−1) was measured (resolution setting 8 cm−1) and subtracted against a blank background. the spectrum was compared with a database of artist’s materials at the Straus center for conservation and irug.

Pyrolysis–gas chromatography–mass spectrometry (py-GCMS)

the sample was inserted into a quartz pyrolysis tube (cdS analytical inc, 465 Limestone road, oxford, pa 19363). the sample and tube was placed inside a platinum heating coil which was then placed into the pyrolysis injector (cdS pyroprobe 2000) and pyrolyzed at 750°c

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for 10 s. the sample then passed to a dB5-MS column (30 m × 0.25 mm, 1 μm phase coating) through a split-splitless inlet (ratio 23.4:1, split flow 21 mL per minute) heated to 300°c. the agilent 6890n gc oven heated the column from an initial temperature of 40°c (1 minute) to 300ºc at a ramp rate of 10°c per minute and maintained the final temperature for 20 minutes. the mass spectrum of the separated components was collected using an agilent 5973 mass selective detector. Mass spectra were compared to niSt and Wiley library standards.

Carbon-14 dating

the samples were given routine sample treatment methods for cellulosic materials. they were first carried through solvent extractions with hexane, ethanol, methanol and water using soxhlet apparatus, in order to remove more recent added solvent-soluble coatings that may have been used during conservation treatments. the samples were then treated with a sequence of mineral acids and bases, 0.1n Hcl, 0.25n naoH, 0.010n Hcl, to remove acid and base contaminants. the recovered material was washed extensively in distilled deionized water, dried and then combusted in vacuo in the presence of cuo. carbon dioxide was cryogenically purified from the combustion gases. the carbon stable isotope ratio for each sample was measured on a vg isotech stable mass spectrometer. pure carbon was generated from the carbon dioxide gas using the graphitization method of Slota et al. [33]. the 14/13 ratio of the graphite was measured by accelerator mass spectrometry on a national instruments tandetron aMS. the calibrations were carried out using oxcal 3.10 [34], and calibration data from intcal04 [35].

REFERENCES

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AUTHORS

NarayaN KhaNdeKar received his BSc (hons) first class and phd from the department of organic chemistry, university of Melbourne, australia. He then completed a postgraduate diploma in the conservation

of easel paintings at the courtauld institute of art, university of London, uK. He has been a teacher of theory and research scientist at the Hamilton Kerr institute, Fitzwilliam Museum, university of cambridge; conservator of paintings at Melbourne university gallery; and a scientist at the Museum research Laboratory, getty conservation institute. Since 2001 he has been a senior conservation scientist at the Straus center for conservation, Harvard art Museums, uSa, and a senior lecturer on the history of art and architecture at Harvard university. Address: Straus Center for Conservation, Harvard Art Museums, 32 Quincy Street, Cambridge, MA 02138, USA. Email: [email protected]

Carol MaNCusi-uNgaro holds a joint appointment as associate director for conservation and research at the Whitney Museum of american art, uSa and founding director of a center for the technical Study of Modern art at Harvard art Museums. She has written on the techniques of Jasper Johns, Brice Marden, Mark rothko, Jackson pollock and Barnett newman and continues to engage in research documenting the materials and techniques of living artists as well as other issues pertaining to the conservation of modern art. Address: Center for the Technical Study of Modern Art, Harvard Art Museums, 32 Quincy Street, Cambridge MA 02138, USA, Email: [email protected]

harry Cooper has been curator of modern and contemporary art at the national gallery of art since February 2008. For the previous ten years he was modern art curator at the Harvard art Museums, where he organized exhibitions of the work of piet Mondrian, Frank Stella, Medardo rosso and others. He earned his phd from Harvard university in 1997 with a dissertation on Mondrian. He has taught in art history departments at Harvard, Hopkins, and columbia, and has written on such artists as Stuart davis, philip guston, ellsworth Kelly, Jasper Johns and Brice Marden. Address: National Gallery of Art, 2000B South Club Drive, Landover, MD 20785, USA. Email: [email protected]

ChristiNa roseNberger received her Ba from Harvard college and an Ma in modern art from columbia university, uSa. She is currently pursuing her doctorate at the institute of Fine arts at new York university, writing a dissertation on the early work of agnes Martin. She is the research coordinator for the center for the technical Study of Modern art at the Harvard art Museums. in this capacity, she investigates the materials

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and issues associated with the making and conservation of works of the modern era by collecting, preserving and presenting relevant materials and research. Address: as Mancusi-Ungaro. Email: [email protected]

KatheriNe ereMiN graduated from the university of cambridge, uK with a Ba in natural sciences and a phd in metamorphic petrology. From 1994 to 2004 she worked as a research scientist at the national Museums of Scotland. Since 2005 she has been a conservation scientist at the Straus center for conservation and technical Study at Harvard art Museums. She specializes in inor-ganic materials, primarily glass, ceramics and metals. Address: as Khandekar. Email: [email protected]

Kate sMith is an assistant paintings conservator at the Museum of Fine arts, Boston, uSa focusing on the technical examination of the seventeenth-century dutch and Flemish collection. She graduated from Smith college in 1994 with a Ba in art history, completed her masters in paintings conservation from Buffalo State college, uSa in 2001 followed by a postgraduate internship at the Straus center for conservation and technical Studies at the Harvard art Museums. Kate has also worked as an assistant conservator at both the Harvard art Museums and the isabella Stewart gardner Museum, uSa. Address: Paintings Conservation, Museum

of Fine Arts, 465 Huntington Avenue, Boston MA 02116, USA. Email: [email protected]

JeNs steNger studied physics at the universities of Konstanz, göttingen, SunY Stony Brook, and the Humboldt university of Berlin, all in germany where he received a phd in 2002. after a two-year research stay at the university of california, Berkeley, uSa he joined the Straus center for conservation at the Harvard art Museums in 2004 as the andrew W. Mellon Fellow in conservation Science. Since 2007 he has worked as an associate conservation scientist at the Straus center. Address: as Khandekar. Email: [email protected]

daN Kirby received his phd from northeastern uni-versity, uSa where he studied bioanalytical appli-cations of mass spectrometry. after careers as an ana-lytical chemist in both semiconductor electronics and pharmaceutical industries, in 2004 he turned to conservation science and began investigating the use of laser desorption mass spectrometry for the analysis of artists’ materials at the Straus center of the Harvard art Museums and was appointed an andrew W. Mellon associate in conservation Science in 2009. current interests include extending the use of Ldi-MS to the analysis of proteinaceous materials in works of art. Address: as Khandekar. Email: [email protected]

Résumé — Trois peintures d”un ensemble de trente-deux, attribuées à Jackson Pollock (1912–1956) ont été examinéees au Harvard Art Museum. On a étudié des échantillons par spectroscopie Raman, MEB avec sonde à dispersion d’énergie (EDS), désorption ionisation laser, spectrométrie de masse – à temps de vol –, IRTF, chromatographie par pyrolyse avec spectrométrie de masse, et datation au carbone 14. Toutes ces peintures contenaient des pigments et liants qu’on ne pouvait se procurer qu’après la mort de Pollock, notamment du PR 254, PY 151et une émulsion de copolymère de butyl-méthacrylate et isobutyl-méthacrylate. La datation au carbone 14 n’a rien détecté de surprenant.

Zusammenfassung — Drei Jackson Pollock (1912–1956) zugeschriebene Gemälde aus einer Gruppe von 35 wurden am Harvard Art Museum untersucht. Dazu wurden Ramanspektroskopie, Rasterelektronenmikroskopie mit energiedispersiver Röntgenmikroanalyse (REM/EDX), Laserdesorptionsionisierungs–time of flight–Massenspektrometrie (LDI-MS), Pyrolyse-Gaschromatographie / Massenspetrometrie (py-GCMS) und Kohlenstoff 14 – Altersbestimmung (C14) verwendet. Alle drei Gemälde enthielten Pigmente, die zu Pollocks Lebzeiten verfügbar waren, inklusive PR 254 und PY 151, und ein Copolymer aus n-Butyl-methacrylat and iso-Butyl-methacrylat. Die C14- Analyse erwies keinen “Bomben-”Kohlenstoff aus oberirdischen Atomtests.

Resumen — Tres obras de un grupo de treinta y dos pinturas atribuidas a Jackson Pollock (1912–1956) fueron examinadas en el Harvard Art Museum. Las muestras de pintura se analizaron utilizando espectroscopía Raman, microscopía electrónica de barrido-espectrometría dispersiva de rayos X (SEM-EDX), espectrometría de masas de ionización inducida por láser (PDI-MS), espectroscopía por transformada de Fourier (FTIR), cromatografía de gases por pirólisis-espectroscopía de masas (py-GCMS) y datación por carbono 14 (C-14). Los tres cuadros contenían pigmentos y aglutinantes disponibles comercialmente solo tras la muerte de Pollock, incluyendo PR 254, PY 151, y un copolímero en emulsión de n-butilmetacrilato e iso-butilmetacrilato. El carbono 14 no detectó ninguna “bomba” de carbono a partir de pruebas atómicas atmosféricas.