Is There an International Klein Pink?

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Introduction
Yves Klein (1928Klein ( -1962 was an artist who sought to free painting from historic constraints of subject matter, media, and methods of paint application. In the early 1950s, he began experimenting with monochromatic works in green, yellow, blue, pink, red, and orange. These works were first publicly presented in his 1954 books Yves Peintures and Haguenault Peintures [1,2]. As he continued to create monochromes, Klein became frustrated that the color of pure pigments dulled upon mixing with a binder, saying: "I did not like colors ground with oil. They seemed to be dead. What pleased me above all were pure pigments in powder like the ones I often saw at the wholesale color dealers. They had a burst of natural and extraordinarily autonomous life . . . What upset me was to see this incandescent powder lose all its value and become dulled and lowered in tone once it was mixed with glue or whatever medium was intended to fix it to its support" [3,4]. To overcome this difficulty he collaborated with Édouard Adam, a Parisian color-man, to create a paint using synthetic ultramarine blue (Na 8-10 Al 6 Si 6 O 24 S 2-4 ; Pigment Blue 29) bound in poly(vinyl acetate) (Rhodpas M 60 A manufactured by Rhône Poulnec, [PVAc]) dispersed in ethanol and ethyl acetate [5], and in 1960 they submitted the recipe of their new paint, International Klein Blue (IKB) to the French Institute National de la Propriété Industrielle and received a Soleau envelope number 63,471 [6]. Although not an official patent, this attested to the importance of their invention and served as proof of priority for this new paint [6,7]. Subsequent studies of paint samples taken from Klein's objects painted with IKB, including paintings, sculptures, and paint rollers by researchers at the Centre National de la Recherche Scientifique in Paris, France confirmed the identity of the medium as PVAc and the pigment as ultramarine [6].
While Klein is best known for his IKB paintings and eventually came to also favor a monochromatic trinity created through the use of blue, pink, and gold [4], he never their bottom edges hovering over a tray of pigments. These works summon pigments from the void and also reflect his contemporaneous interest in dissociating himself from the act of creation. In his 1961 Chelsea Hotel Manifesto, he wrote "I felt the urge to register the signs of atmospheric behavior by recording the instantaneous traces of spring showers on a canvas, of south winds, and of lightning . . . " [10]; with his sculptural rains, Klein conjures such an atmospheric imprint.
sulfate dihydrate and lithopone with pigments to create a specific tonality or to modify the handling properties of the paint. It is also unclear if Klein knew he was choosing to use organic pigments that might prove more prone to light-induced fading than inorganic pigments.
At the time of writing, no published work identifying Klein's red and pink pigments could be found in the literature, and Klein also provided little information about his materials. In his self-published newspaper from 1960, Dimanche: Le Journal d'un Seul Jour, he described the use of carmine and madder pinks and indicated that the term "IKP" or "International Klein Pinck" [sic] referred to a pink madder lake (literally "lacque de garance rose") ( Figure 1) [8]. However, this has not been confirmed through scientific analysis of artworks. It is also unclear if he used a single pigment across all his pink works, similar to his use of a single blue pigment, and if the colorants in his red and pink works are the same, with tonality differences arising from different pigment to filler ratios.  Dissociation from the act of creation also lies behind Klein's anthropometry series, of which Untitled Shroud Anthropometry (ANT SU 4) is a prime example. In these works, Klein created paintings through the use of 'living brushes,' often nude women, who smeared themselves or rolled in paint and then imprinted their figures onto canvas as verbally directly by Klein. Through this non-traditional use of the traditional artist's model, Klein sought to accomplish two objectives. Firstly, to recreate a practice he had learned about during his stay in Japan where artists used their bodies as brushes [10]; and secondly, to use the presence of these women in his studio to prevent himself from "secluding himself in the overly spiritual spheres of creation" and avoid what he termed the "terrifying mirror" created when an artist faces their canvas alone [10].
The final work in this study is a pink monochrome screenprint that was included along with blue and gold leaf monochromes in a 1961 exhibition catalog that accompanied Monochrome und Feuer at Museum Haus Lange [9]. Although Klein reportedly created the gold monochrome print himself, they were actually prepared by the museum staff, and it is unclear who printed the pink and blue prints [4]. Incidentally, this was Klein's only museum retrospective during his lifetime, and featured many of the artist's signature works, such as architectural drawings, his Mur de Feu, as well as blue, pink, and gold monochromes. Interestingly, two works from that exhibition include "IKP" in their titles as described in the accompanying catalog: a painted sponge sculpture titled Lecteur I.K.P.  These two paintings follow Klein's classic approach to the creation of two-dimensional monochromes [6]. Both pictures utilize a stiff board as a support, and in the case of the larger work Klein applied an additional fabric layer to the board; he typically used fine cotton gauze. Klein then applied his paint using a lambskin roller to achieve the richly textured surface he preferred.
In a 1957 show, Pigments purs, at the Galerie Colette Allendy, Klein exhibited a tray of ultramarine blue pigment on the floor, something he considered to be a painting on the ground, with the pigment fixed in place by gravity [1]. Over this tray, he installed a Overall, these five works offer a chance to interrogate Klein's pink and red color choices through a variety of artistic creations and media that encompass much of his oeuvre. Colorants and fillers were identified by Raman and surface-enhanced Raman (SERS) spectroscopy, micro-Fourier transform infrared spectroscopy (µ-FTIR), portable X-ray fluorescence spectroscopy (p-XRF), and scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM-EDX). Binders were also investigated with the use of µ-FTIR. The results presented here show that Klein used a variety of different pigments, and in at least two cases altered the colors in his works from pink to red or made deliberate use of layering to manipulate the visible surface colors.

Analysis at the Menil Collection
Cross-section samples were embedded in Epofix epoxy resin (Electron Microscopy Sciences, Hatfield, PA, USA), coarse ground using Micro Mesh MX sheets (120 and 150 grit), and fine polished using Micro Mesh sheets (1500-12,000 grit) (Scientific Instrument Services, Palmer, MA, USA). Images of the samples under both normal oblique and UV illumination using B-2A or V-2B filter cubes were obtained using a Zeiss AxioCam MRc5 camera controlled by Zeiss Axiovision AC software release 4.5 (White Plains, NY, USA) and mounted onto a Nikon Labophot-Pol optical microscope (Melville, NY, USA) equipped with 10×, 20×, and 40× objectives. Scale bars were created in Adobe Photoshop (Adobe Inc., San Jose, CA, USA) using images of a micrometer scale taken using the same objective.
Dispersive Raman spectra were collected on a Renishaw InVia Raman microscope (West Dundee, IL, USA) running WiRE software version 5.5 using a 785 nm excitation laser operating at powers of 75 to 782 µW at the sample as measured using a PM100D laser power meter (Thorlabs, Newton, NJ, USA) equipped with an S120C photodiode power sensor. A 50× objective was used to focus the excitation beam on the sample supported on a glass microscope slide. The resulting Raman spectra are the average of 1 to 15 scans of 10 s duration. The spectral resolution was 3-5 cm −1 across the spectral range analyzed. Sample identification was achieved by comparison of the unknown spectrum to the spectra of reference materials, the KIK/IRPA Raman reference library [11], and those published in the literature.
µ-FTIR spectra were collected in transmission mode using a Bruker Lumos µ-FTIR microscope running Opus software version 8.2 (Billerica, MA, USA). Samples were prepared by flattening them in a diamond compression cell (S.T. Japan, Ft. Myers, FL, USA), removing the top diamond window, and analyzing the thin film in transmission mode on the bottom diamond window. The spectra are the average of 64 or 128 scans at 4 cm −1 spectral resolution.
Backscatter electron images of the uncoated cross-section samples were taken with a JEOL JSM-IT100 SEM (Peabody, MA, USA) with 20 kV voltage, running under low vacuum mode with a pressure of 50-55 Pa and a probe current of 40-50 (unitless). EDX analysis using the integrated detector was performed under the same voltage and pressure conditions, but with higher probe currents (65-75) to increase the counts.
p-XRF spectra were collected using a Bruker Tracer III-SD handheld energy-dispersive X-ray spectrometer (Madison, WI, USA) equipped with a Peltier-cooled XFlash silicon drift detector (SDD) with a resolution of 145 eV and a 5 mm diameter approximate spot size. The excitation source was a Rh target X-ray tube, operated at 40 kV and 10 µA current, and spectra were collected over 180 s (live time). Spectra were obtained with Bruker S1pXRF software version 3.8.30 and spectral interpretation was performed using Bruker Artax Spectra 7.4.0.0 software.

Analysis at the Museum of Modern Art
Optical Microscopy was conducted using a Leica DM IRM microscope using a 10× objective. Cross-sections were embedded in BioPlastic ® (Aldon Corp., Avon, NY, USA), a blend of polyester and methacrylate monomers in a styrene solvent, trimmed with a jeweler's saw, and dry polished with Micro-Mesh ® (Micro-surface Finishing Products, Wilton, IA, USA) silicon carbide or aluminum oxide abrasives.
µ-FTIR spectra were collected in transmission mode using a Nicolet iS50 µ-FTIR coupled with a Thermo Nicolet Continuum infrared microscope equipped with an MCT detector (Waltham, MA USA). Spectra were collected in the 4000-600 cm −1 range with a 4 cm −1 resolution and 128 scans using the Thermo Scientific OMNIC 9.0 software package. Spectra were examined using the Spectral Search and Multicomponent Search tools available in Thermo Scientific OMNIC Specta 2.0 software.
Silver nanoparticles (AgNPs) for surface-enhanced Raman spectroscopy (SERS) were prepared according to a method developed by Lux et al. [12]. A solution of 12.5 mL of 0.5 mM silver sulfate (Ag 2 SO 4 ) (≥99.99%; MilliporeSigma, Burlington, MA, USA), 0.5 mL of 1% sodium citrate dihydrate (C 6 H 9 Na 3 O 9 ·2H 2 O) (≥99%; Thermo Fisher Scientific Inc., Waltham, MA, USA), and 1 mL of 1% D-glucose (≥99.5%; MilliporeSigma, Burlington, MA, USA) were mixed in a Hydrothermal Synthesis Autoclave Reactor PTFE Tank (Baoshishan, Zhengzhou, China) previously cleaned with 30% v/v nitric acid in distilled water. The starting solutions were all made with 18 MΩ deionized water. Once closed, the vessel was positioned in the center of a Panasonic model NN-SD372S inverter microwave and heated at 810 W for 2 min. Then 1.5 mL aliquots of this stock solution of citrate-capped AgNPs were centrifuged at 12,000 rpm for 15 min. The supernatant containing the citrate solution in excess was removed and replaced by 18 MΩ deionized water to avoid sodium citrate interference in the SERS spectra.
Dispersive Raman spectra were collected on a Renishaw InVia Raman system (West Dundee, IL, USA) equipped with a 785 nm diode laser operated between 0.3 to 3 mW, a 1200 lines/mm grating, and a Leica confocal microscope with a 50× LWD or 100× objective. Final spectra represent an average of five acquisitions of 10 s. Spectra were examined using the Spectral Search and Multicomponent Search tools available in Thermo Scientific OMNIC Specta 2.0 software. SERS was carried out using the same Renishaw InVia Raman system, employing here a 532 nm diode laser operating at 0.25 mW with an 1800 lines/mm grating. Spectra were evaluated using the Spectral Search and Multicomponent Search tools available in Thermo Scientific OMNIC Specta 2.0 software. Sample identification was achieved by comparison of the unknown spectrum to spectra of reference materials, the KIK/IRPA Raman reference library [11], and those published in the literature.
p-XRF spectra of Untitled Red Monochrome (M 63) were collected using a Bruker Tracer 5g handheld energy-dispersive X-ray spectrometer (Madison, WI, USA) equipped with a 20 mm 2 graphene window (SDD), detector area with a resolution of 145 eV and an 8 mm diameter approximate spot size. The excitation source was a Rh target X-ray tube, operated at 40 kV and 4 µA current, and spectra were collected over 120 s (live time). Spectra were obtained and evaluated with Bruker Artax Spectra 8.0.0.476 software.

Red and Pinks Revealed
The results obtained from this multi-analytical campaign show that Klein used a variety of pink and red pigments to achieve his monochromatic surfaces. µ-FTIR analysis ( Figure 3) revealed that all four paintings and sculptures contain PVAc as the binder, while the screenprint was made with an oil-based ink ( Table 1 lists the materials identified, and Table 2 provides the characteristic FTIR and Raman peaks for these materials, the Raman spectra are shown in Figure 4). µ-FTIR spectra of samples taken from the paintings and sculpture also showed peaks indicative of a hydrated sulfate-based filler or lake-base ( Figure 3), with peaks at 3407 and 1642 cm −1 likely corresponding to the stretching and bending vibrations of water, and that at 1120 cm −1 deriving from S−O stretching in the sulfate ion [13,14]. p-XRF and SEM-EDX analysis of the Menil Collection and Louisiana Museum works, in addition to p-XRF of the Guggenheim work, indicated aluminum was the counterion, likely present as hydrated aluminum sulfate (Al 2 (SO 4 ) 3 ). Calcium carbonate (CaCO 3 ) was identified as a filler in a pink sample taken from the screenprint associated with Monochrome und Feuer. Both calcite and aragonite, two crystal forms of CaCO 3 , were observed in the µ-FTIR spectrum, where the latter was identified by the sharp ν 2 (CO 3 2− ) peak at 855 cm −1 .
A cross-section sample taken from the earliest of these works, Untitled Red Monochrome (M 63), displays two distinct layers (Figure 5a,b). The first, more orange-hued red layer contains the lake pigment eosin Y (C 20 H 6 Br 4 Na 2 O 5 ; Pigment Red 90:1) (Figure 5), which was identified by both Raman and µ-FTIR spectroscopies. The presence of this pigment was further confirmed by the large amount of bromine (Br) detected with p-XRF. Raman analysis of the second, surface layer produced overwhelming fluorescence that necessitated the use of SERS, through which rhodamine B (C 28 H 31 ClN 2 O 3; Pigment Red 173) was identified. Both of these pigments fluoresce orange under ultraviolet illumination [15], which explains the lack of distinction between the two layers in the cross-section when examined with microscopy.      However, closer inspection of the edge of Untitled Red Monochrome (M 63) under UV illumination (Figure 5c) reveals more information about the layering of these paints and suggests that there may be three layers in total. Over the bright pink rhodamine B layer, which is revealed through spalling of the surface layers, and which appears to be the lowest paint layer applied, are two additional visibly distinct layers: a middle layer that fluoresces bright orange and the surface layer that fluoresces a muted deep red. Raman analysis of scrapings taken from the middle and surface layers detected only eosin Y. Thus, Klein appears to have applied the final color in two coats, and the surface coat now appears darker under UV possibly due to aging, dirt, or photodegradation in the presence of fillers or white pigments [16].
Analysis of a sample from Grand Monopink (MP 16) indicated the presence of an alizarin lake (C14H8O4; Pigment Red 83). The absence of any definitive signatures for purpurin or pseudopurpurin likely excludes the use of a natural alizarin lake pigment [17], as multiple anthraquinones are usually present in pigments extracted from the madder root [18]. Early in the 20th century, synthetic lakes were beginning to replace natural ones largely due to their superior lightfastness [19][20][21], so it is likely that Klein would have had easier access to synthetic lakes. His writings do not indicate whether he preferred natural or synthetic pigments. Although Klein used synthetic ultramarine, and synthetic pigments were found in his green and orange monochromes [5], he may simply have chosen what was offered by suppliers at the time.
It is worth noting that the SERS spectra obtained for alizarin in this study suggest a particular bonding mechanism that explains the Raman shifts reported here. Cañamares  (Figure 5c) reveals more information about the layering of these paints and suggests that there may be three layers in total. Over the bright pink rhodamine B layer, which is revealed through spalling of the surface layers, and which appears to be the lowest paint layer applied, are two additional visibly distinct layers: a middle layer that fluoresces bright orange and the surface layer that fluoresces a muted deep red. Raman analysis of scrapings taken from the middle and surface layers detected only eosin Y. Thus, Klein appears to have applied the final color in two coats, and the surface coat now appears darker under UV possibly due to aging, dirt, or photodegradation in the presence of fillers or white pigments [16].
Analysis of a sample from Grand Monopink (MP 16) indicated the presence of an alizarin lake (C 14 H 8 O 4 ; Pigment Red 83). The absence of any definitive signatures for purpurin or pseudopurpurin likely excludes the use of a natural alizarin lake pigment [17], as multiple anthraquinones are usually present in pigments extracted from the madder root [18]. Early in the 20th century, synthetic lakes were beginning to replace natural ones largely due to their superior lightfastness [19][20][21], so it is likely that Klein would have had easier access to synthetic lakes. His writings do not indicate whether he preferred natural or synthetic pigments. Although Klein used synthetic ultramarine, and synthetic pigments were found in his green and orange monochromes [5], he may simply have chosen what was offered by suppliers at the time.
It is worth noting that the SERS spectra obtained for alizarin in this study suggest a particular bonding mechanism that explains the Raman shifts reported here. Cañamares et al. determined that the pKs of alizarin are 5.25 and 11.5, which give rise to the monoanionic and dianionic species, respectively [22]. Considering that SERS analysis was done under acidic pH with the introduction of HNO 3 for hydrolysis, the resulting SERS spectrum indicates the bonding of negatively charged, monoanionic anthraquinone to the positively charged silver nanoparticle clusters (Ag 3 ) + [22,23]. This species was observed for all the SERS spectra obtained with alizarin in this study.
Visible inspection of samples taken from Grand Monopink (MP 16) suggested the presence of two layers, a deeper pink top layer and a lighter pink bottom layer ( Figure 6). However, visible light microscopy and SEM-EDX analysis of an embedded sample failed to identify the presence of multiple layers (Figure 6), as did SEM-EDX analysis of unembedded flakes. While both pink and red areas contain calcium, identified as calcium sulfate (CaSO 4 ) by Raman, SEM-EDX shows the amounts to be consistent across the samples indicating that the color difference does not arise from different amounts of calcium-based filler. Klein is known to have applied multiple layers of paint to his IKB monochromes and subtle differences in formulation sometimes result in perceptible color differences in his IKB monochromes [7]. If multiple layers are present in Grand Monopink (MP 16), they are similar enough to be indistinguishable by the analytical techniques used in this study. Visible light microscopy and SEM-EDX analysis of the samples also showed the presence of small, dispersed particles of ultramarine. The apparent heterogeneity in their distribution suggests that the pigment is present as a contaminant, perhaps from powdered pigments in Klein studio, and was not deliberately added to the pink paint.
Colorants 2023, 2, FOR PEER REVIEW et al. determined that the pKs of alizarin are 5.25 and 11.5, which give rise to the mon ionic and dianionic species, respectively [22]. Considering that SERS analysis was under acidic pH with the introduction of HNO3 for hydrolysis, the resulting SERS s trum indicates the bonding of negatively charged, monoanionic anthraquinone to the itively charged silver nanoparticle clusters (Ag3) + [22,23]. This species was observed f the SERS spectra obtained with alizarin in this study.
Visible inspection of samples taken from Grand Monopink (MP 16) suggested presence of two layers, a deeper pink top layer and a lighter pink bottom layer (Figu However, visible light microscopy and SEM-EDX analysis of an embedded sample f to identify the presence of multiple layers (Figure 6), as did SEM-EDX analysis of un bedded flakes. While both pink and red areas contain calcium, identified as calcium fate (CaSO4) by Raman, SEM-EDX shows the amounts to be consistent across the sam indicating that the color difference does not arise from different amounts of calcium-b filler. Klein is known to have applied multiple layers of paint to his IKB monochro and subtle differences in formulation sometimes result in perceptible color differenc his IKB monochromes [7]. If multiple layers are present in Grand Monopink (MP 16), are similar enough to be indistinguishable by the analytical techniques used in this st Visible light microscopy and SEM-EDX analysis of the samples also showed the pres of small, dispersed particles of ultramarine. The apparent heterogeneity in their dist tion suggests that the pigment is present as a contaminant, perhaps from powdered ments in Klein studio, and was not deliberately added to the pink paint. Spalling of the red surface paint of Red Rain (S37) (1961) revealed a pink under suggesting that multiple paint layers were also present in this work. Cross-section ana using visible and ultraviolet illumination and SEM-EDX confirmed this stratigraphy ure 7) and revealed a thick lower layer containing large particles rich in aluminum Spalling of the red surface paint of Red Rain (S37) (1961) revealed a pink underlayer suggesting that multiple paint layers were also present in this work. Cross-section analy-sis using visible and ultraviolet illumination and SEM-EDX confirmed this stratigraphy (Figure 7) and revealed a thick lower layer containing large particles rich in aluminum and sulfur, likely the hydrated Al 2 (SO 4 ) 3 seen by µ-FTIR. The thin surface layer also contains Al 2 (SO 4 ) 3 , but SEM-EDX showed that small amounts of silicates and barium are also present, the latter likely present as barium sulfate; these are the small, bright white particles visible in the backscattered electron image (Figure 7c). As in Grand Monopink (MP 16), scattered ultramarine particles were observed in both paint layers and on the surface below the over-paint present in other samples analyzed.
Colorants 2023, 2, FOR PEER REVIEW 11 sulfur, likely the hydrated Al2(SO4)3 seen by µ-FTIR. The thin surface layer also contains Al2(SO4)3, but SEM-EDX showed that small amounts of silicates and barium are also present, the latter likely present as barium sulfate; these are the small, bright white particles visible in the backscattered electron image (Figure 7c). As in Grand Monopink (MP 16), scattered ultramarine particles were observed in both paint layers and on the surface below the over-paint present in other samples analyzed. Analysis of the lower pink layer with SERS ( Figure 4) revealed the presence of two lake pigments, rhodamine 6G (C28H31N2O3Cl; Pigment Red 81) and a synthetic alizarin [23]. The reason for the addition of rhodamine 6G to synthetic alizarin is unknown, although lake pigments were often adulterated with additional synthetic pigments [24]. The large particle sizes revealed by SEM, relatively low amounts of binder, and the hygroscopic nature of aluminum sulfate may account for the observed paint spalling.
The thinner, surface layer of red paint in Red Rain (S37) contained a different red pigment, Para Red (C16H11N3O3; Pigment Red 1), which was identified by both Raman and µ-FTIR spectroscopies. A peak at 1290 cm −1 suggests the presence of synthetic alizarin in the layer as well. There is no evidence to suggest that the thin surface layer is not artistapplied.
The pink paint on Untitled Shroud Anthropometry (ANT SU 4) (1961) also contains Para Red and synthetic alizarin on an aluminum sulfate base, although the binder-to-pigment ratio appears to be higher than on Red Rain as the pigments were only detected by Raman spectroscopy; Para Red was below the detection limit of µ-FTIR.
The latest of these works, the pink screenprint in the collection at MoMA, was created in 1961. Although Klein's involvement in the choice of printing ink for this work is unknown, the analysis offers an opportunity to compare colorants that Klein personally selected for his artworks with one that may have been chosen or recommended by the printer. Rhodamine 6G was again detected by SERS ( Figure 4) and perhaps suggests an evolution towards consistency in the choice of pigment for an artist largely preoccupied with color. This pigment was also detected by µ-FTIR (Figure 3).

Technique
Two of the works studied here, Red Rain (S37) and Untitled Red Monochrome (M 63), have lower layers of pink paint and surface coats of red paint. In a 1956 article by Bernadette Allain about Klein's Galerie Colette Allendy show, she describes a 20 × 30 cm painting where through the purple-violet surface one can "feel the powerful life of red" (literally "sen la vie puissante du rouge") suggesting that Klein may have deliberately layered colors in this painting [25].
However, in addition to deliberate layering, artists, including Klein, rework their paintings. Stitch notes instances of Klein repainting a pink work to make it yellow, a red Analysis of the lower pink layer with SERS ( Figure 4) revealed the presence of two lake pigments, rhodamine 6G (C 28 H 31 N 2 O 3 Cl; Pigment Red 81) and a synthetic alizarin [23]. The reason for the addition of rhodamine 6G to synthetic alizarin is unknown, although lake pigments were often adulterated with additional synthetic pigments [24]. The large particle sizes revealed by SEM, relatively low amounts of binder, and the hygroscopic nature of aluminum sulfate may account for the observed paint spalling.
The thinner, surface layer of red paint in Red Rain (S37) contained a different red pigment, Para Red (C 16 H 11 N 3 O 3 ; Pigment Red 1), which was identified by both Raman and µ-FTIR spectroscopies. A peak at 1290 cm −1 suggests the presence of synthetic alizarin in the layer as well. There is no evidence to suggest that the thin surface layer is not artist-applied.
The pink paint on Untitled Shroud Anthropometry (ANT SU 4) (1961) also contains Para Red and synthetic alizarin on an aluminum sulfate base, although the binder-to-pigment ratio appears to be higher than on Red Rain as the pigments were only detected by Raman spectroscopy; Para Red was below the detection limit of µ-FTIR.
The latest of these works, the pink screenprint in the collection at MoMA, was created in 1961. Although Klein's involvement in the choice of printing ink for this work is unknown, the analysis offers an opportunity to compare colorants that Klein personally selected for his artworks with one that may have been chosen or recommended by the printer. Rhodamine 6G was again detected by SERS ( Figure 4) and perhaps suggests an evolution towards consistency in the choice of pigment for an artist largely preoccupied with color. This pigment was also detected by µ-FTIR (Figure 3).

Technique
Two of the works studied here, Red Rain (S37) and Untitled Red Monochrome (M 63), have lower layers of pink paint and surface coats of red paint. In a 1956 article by Bernadette Allain about Klein's Galerie Colette Allendy show, she describes a 20 × 30 cm painting where through the purple-violet surface one can "feel the powerful life of red" (literally "sen la vie puissante du rouge") suggesting that Klein may have deliberately layered colors in this painting [25].
However, in addition to deliberate layering, artists, including Klein, rework their paintings. Stitch notes instances of Klein repainting a pink work to make it yellow, a red painting from 1956 that he covered in blue, and a 1957 painting where he covered blue with bronze [4]. She also describes a red monochrome from 1957 that has an underlayer of pink; this subtle color change makes it difficult to decide if this is a case of deliberate revision or of Klein deliberately layering his colors. The works described by Allendy and Stitch were made from 1955-57, earlier than the works studied here, and if the layering of pink and red in Untitled Red Monochrome (M 63) and Red Rain (S37) is not an alteration but a deliberate choice, it suggests that Klein continued to explore the possibilities of optical translucency. Stitch also notes that in the case of the red monochrome, the fading of the red surface paint allows the pink underlayer too much prominence and says the monochromatic effect is lost [4]. If such a change happened in Klein's lifetime, the wide variety of pigments found in this study could document his search for more stable colorants.

Implications for Conservation
Klein's works have always presented problems for conservators because of their unique surfaces and intense monochromatic color [7,26,27]. The presence of organic pigments in these objects adds additional complexity due to potential issues surrounding lightfastness and solubility. The lightfastness of pigments is impacted by many factors including the strength of the tint, the thickness of the paint film, particle size, the identity of the binding medium, and the presence of other pigments [28][29][30]. However, the identification of a variety of pigments indicates that these pink and monochromes can be expected to be light sensitive in a way that IKB monochromes are not.
Rhodamine B and eosin Y pigments are considered fugitive, equivalent to Blue Wool Scale (BWS) 2-3, meaning that they may show visible fading after 4-10 years of exposure to light that contains an ultraviolet component (unfiltered daylight) and 7-20 years if the UV component is filtered out, as is in museum environments [31,32]. Rhodamine 6G and Para Red are somewhat more lightfast, with BWS ratings of 4-6, and can be expected to exhibit noticeable color change after 23-30 years on display if ultraviolet light is not filtered out, and 67-670 years if it is [30][31][32]. Synthetic alizarin is more stable than that extracted from plants but is still considered to have only fair lightfastness [33]. Therefore, these works are considerably more fugitive in their colorfastness than IKB works and care should be taken when designing their display and storage conditions.
In addition to issues with lightfastness, organic pigments may also be solvent sensitive and show solubility, limiting potential treatments. For instance, rhodamine 6G and synthetic alizarin are sensitive to polar solvents including ketones and esters, and Para Red is sensitive to water, which may impact treatment choices [30].

Conclusions
Analysis of five pink and red works by Yves Klein paints a more complex picture of his working practice with color. His 1960 statement in Le Dimanche indicated that madder lake was 'IKP". However, in the works studied here, alizarin, likely synthetic, was found in only three objects, Grand Monopink (MP16), Red Rain (S37), and Untitled Shroud Anthropometry (ANT SU 4), and is the primary colorant only in the first of these. The other artworks contained different organic colorants, including rhodamine 6G, rhodamine B, and eosin Y, suggesting that he had not yet settled on a singular "IKP" pigment. Perhaps had Klein lived longer, this exploration of the color pink could have led to the same standardized practice he arrived at with IKB, or maybe for reds and pinks he had no defined vision of what the ideal color was, unlike his strong vision of a single, perfect blue. Table 2. µ-FTIR, Raman, or SERS peaks for pigments and binding media identified in the selection of works by Yves Klein studied here.

Material
Characteristic Peaks