Sem-edx
Cross sections of ground samples taken from Pedernal and A Man from the Desert (1941), which were painted before Stieglitz’s death in 1946 and O’Keeffe’s subsequent permanent move to New Mexico in 1949, are presented in Fig. 2a–d. The optical (Fig. 2a and c) and backscattered electron (BSE) micrographs (Fig. 2b and d) show that the samples are composed of two layers which are labeled here as ground (1) for the lower layer that consists mainly of calcium-rich particles with some lead-based agglomerates, and a preparation layerFootnote 2 (2), primarily composed of lead particles with a few calcium particles (indicated by the lower Z-contrast), that is in contact with the visible paint layers immediately above. Energy dispersive X-ray microanalysis (EDX) analysis reveals the spatial co-occurrence of Ba and S, indicating the presence of the mineral barite (BaSO4) evenly distributed through both layers (Additional file 1). Apart from the difference in chemical composition of each respective layer, there are some morphological differences between the ground and preparation layers. For example, the pigment distribution in the preparation layer shows an abundance of small individual lead-based pigment particles that surround larger agglomerates of this material (Fig. 3) creating low-density regions and micro-pores between the particles. In a semipermeable system like a painting, the presence of these agglomerated particles results in a breccia-likeFootnote 3 [46] appearance and provides the layer with porosity. The cross-section prepared from the commercial pre-primed canvas roll, in Fig. 2e and f, shares the same identical elemental compositions, layer structures, and pigment size/distributions as Pedernal and A Man from the Desert indicating that these paintings were prepared on the commercially pre-primed canvas, a fact confirmed by the continuous priming layer under the original tacks along the tacking edges of both paintings. These characteristics are thus “fingerprints” for O’Keeffe’s use of this commercial material on these works of art.
In contrast, the ground samples collected from Storm Cloud, Lake George (1923) and Pink and Green (1960), made both before and after the annually gallery exhibitions of new work of O’Keeffe’s oil paintings from 1923 to 1946, exhibit a single ground layer (Fig. 2g–j). The BSE images (Fig. 2h and j) of these sections reveal differences in morphology and pigment size/distribution of the lead pigment particles in comparison to the gallery exhibition-era paintings with lead particle sizes more homogeneous, evenly distributed, and compact. These findings support the archival information about the artist’s use of commercial pre-primed canvas during the years of annual exhibitions of new work from 1923 to 1946.
As is observable to the naked eye, both Pedernal and A Man from the Desert exhibit protrusions with a “waxy” appearance or pinpoint losses where there was once a protrusion (Fig. 1b). Cross-sections of these protrusions from Pedernal and A Man from the Desert are presented in Fig. 4. The optical image of a cross section from the sky of Pedernal and the corresponding BSE image is presented in Fig. 4a, b. From these images three different layers are visible: in addition to the ground and preparation layers there is a top layer composed of a mixture of Pb and Co associated with a cobalt aluminate pigment (labelled 3 in Fig. 4a, b). The soap aggregate in Fig. 4b has a low Z-contrast due to the fact that the region is rich in organic material studded with occasional lead white pigment particles. Another observation is that the soap protrusion partially ripped during the process of microtoming (indicated by the arrows in Fig. 4b) which provides a qualitative indication of the softness of the soap-rich region. A similar structure is observed in the cross section of A Man from the Desert (Fig. 4c. d) where the same three layers can be identified. The soap protrusion area in the right upper corner of Fig. 4d abuts the preparation layer. It is notable that the soap protrusion extends through the top layer into the preparation layer densely packed with lead-based pigment agglomerates (indicated with an asterisk in Fig. 4d) and in direct contact with the soap protrusion aggregate in the upper right corner of the sample (square in Fig. 4d). Similar composition was observed in the soap protrusion collected from the un-painted commercial pre-primed canvas roll rich in organic material indicated by the low Z-contrast (Fig. 5). In this cross section nearly all the lead white pigment has dissolved leaving only a few intact pigment particles within the protrusion (Fig. 5b). EDS analysis confirms that the remaining particles are lead white pigment (Fig. 5c). The organic-rich area of both soaps does not exhibit a mineralized core (crystalline material), as has been observed previously for crystalline lead soap protrusions [16, 17], possibly due to the relatively young age of O’Keeffe’s paintings and the early stage of the development of the soaps.
Ft-ir
FT-IR spectra were collected for both layers of the commercial pre-primed canvas paintings grounds and the single layer ground samples of the artist primed canvases, as well as for soap protrusion samples from Pedernal and the commercial pre-primed canvas roll. The main objective was to compare the FT-IR spectra of the different grounds to gain insights into their influence into the soap protrusion formation (Fig. 6). Representative spectra obtained for the ground layer of Pedernal, A Man from the Desert and the commercial canvas roll were identical and dominated by bands at 2513, 1796 and 1400 (ν CO32−), 871 cm−1 (out of plane bending C-O) and 712 cm−1 (in-plane bending C-O) characteristic of calcium carbonate (CaCO3) (Fig. 6b). Also demarcated in Fig. 6 are weaker vibrational bands associated with the oil-based binding medium (2958 (νCH3), 2929 (νCH2), 2852 (νCH3), 1740 (νasC = O) cm−1. The spectrum obtained from the preparation layers of both paintings (Fig. 6a pink and green lines) exhibit very different features that may be ascribed to basic lead carbonate (hydrocerussite, (PbCO3)2·Pb(OH)2)) at 3536 (O–H), 1400 (νsC-O), 1045 (νsC-O) and 681 cm−1 (in-plane bending C-O) along with oil-based medium. The band at 838 cm−1 assigned to the presence of neutral lead carbonate (PbCO3, the mineral cerussite) is observed in the spectra obtained for the preparation layers [24]. Together these features indicate that there are just minor differences in the distribution and concentration of lead carbonate in the commercial canvas roll (Fig. 6a yellow line) and ground samples collected from Pedernal and A Man from the Desert (Fig. 6a pink and green lines). Other weak bands ascribed to barium sulfate (BaSO4, the mineral barite) around 1180 and 1080 cm−1 were identified in the spectra of the commercial pre-primed canvas and the preparation layers of Pedernal and A Man from the Desert. Variation in the intensity of these bands is likely due to the inhomogeneous distribution of the barite within the layer. These results are in accordance with a non-invasive study performed by Salvant et al. of similar works by O’Keeffe, where the co-occurrence of lead white, calcium carbonate, barium sulfate and lead soaps was reported [1]. In Fig. 6a, the spectra obtained for the artist prepared grounds from Pink and Green and Storm Cloud, Lake George (red and blue, respectively) are compared to the preparation layer of the commercial canvases. The spectra from Pink and Green and Storm Cloud, Lake George are dominated by bands characteristic of basic lead carbonate and the oil-based binding medium but possess none of the features indicate the formation of metal carboxylate soaps. In contrast, the most notable spectral feature from the preparation layers of A Man from the Desert, Pedernal, and the commercial pre-primed canvas is the band at 1520 cm−1 which indicates the presence of amorphous lead carboxylates (Fig. 6a) [18,19,20,21].
A broad shoulder at 2958 cm−1 (ν CH3) in the spectra of the preparation layers of A man from the Desert, Pedernal and the commercial pre-primed canvas is attributed to an increase of CH3 that could be caused by hydrolysis and fragmentation of the polymeric chain due to water from the ambient environment [32]. While it is important to consider exposure of the preparation layer to humidity as a mechanism that promotes the formation lead carboxylate soaps, we cannot discount the deliberate addition of fatty acid containing materials to the preparation layer formulation. This is supported in Fig. 6a where the spectra of the paintings with commercial pre-primed canvases have preparation layers that exhibits a band maximum around 1709 cm−1 characteristic of ν(C = O) in carboxylic acids associated with fatty acids not covalently bound to the polymerized oil network [22, 23]. When comparing this feature to same region of Pink and Green and Storm Cloud, Lake George (Fig. 6a) this band has its maximum at 1740 cm−1 and is associated with ν(C = O) in a glyceryl ester form attached to the linseed oil network.Footnote 4 Since the ground of Pink and Green and Storm Cloud, Lake George were prepared by artist using traditional methods, these spectra can be used as baseline for what would be expected from a typical ground medium prepared by the artist. The spectrum from the preparation layer of the commercial pre-primed canvas roll, in contrast, has a much higher free fatty acid content compared to the rest of the samples (yellow trace, Fig. 6a). Thus these spectral differences in the ν(C = O) IR band suggest a higher concentrations of free fatty acids in the commercially pre-primed canvases beyond what would be expected by hydrolysis alone.
To more thoroughly explore the free versus network bound carboxylate content of the commercial pre-primed canvas and the artist primed ground materials, the envelope region between 1800 and 1700 cm−1 was subjected to deconvolution and peak fitting analysis as shown in Fig. 6c. In this region, carbonyl stretching bands from both network bound glyceryl esters (RCOOR’) and free fatty acids (RCOOH) may be observed as overlapping bands. As seen in the spectra from A Man from the Desert and Pedernal, the total peak area contribution of C = O in the form of free fatty acid (bands at 1700 and 1710 cm−1), is higher than the contribution in glyceryl ester form [22, 24, 25]. Similar features were observed for the commercial pre-primed canvas roll in Fig. 4c where the contribution of C = O in ester form is relatively low in comparison with the artist primed canvases used in Pink and Green and Storm Cloud, Lake George. In the case of the commercial pre-primed canvas roll sample, a small shoulder at around 1726 cm−1 associated with an aldehyde C = O was identified, which contributes to the broadening of the carbonyl band. Acids and aldehydes are known degradation products of linseed oil [22, 24, 25, 44, 45]. In contrast, the C = O band with maximum at 1740 cm−1 in the spectra of the artist primed canvas grounds (Pink and Green as well as Storm Cloud, Lake George) indicate that the C = O contribution in network bound esters is significantly higher than the contribution of C = O in the form of fatty acids. These results are in accordance with the absence of soap protrusions in Pink and Green and Storm Cloud, Lake George. The presence of higher amounts of C = O in form of free fatty acid can be due to: hydrolysis of glycerol esters within the oil network, the presence of a not chemically treated oil, and/or to the presence of additives (e.g. metal stearates) in the commercial pre-primed canvas grounds.
As has been shown by previous studies [18, 26], a broad band with maximum centered around 1520 cm−1 is commonly associated with amorphous lead carboxylates. While these studies are based on pure materials and controlled experimental conditions that cannot be applied directly to real paint samples, they do inform the chemical reactions occurring in complex modern oil paintings. For example, [26] showed that the presence of saturated fatty acids (SFAs) and metal ions is a sufficient condition for crystalline metal soaps to form. Therefore, that a 1520 cm−1 feature is observed both in the protrusions aggregate of Pedernal and the commercial pre-primed canvas (Fig. 6d purple and red lines), suggest that the protrusions in Georgia O’Keeffe paintings may currently be mostly in an amorphous phase, but with potential crystallization of the soap material occuring over time. The spectra of the protrusions contain several unattributed bands (at 1632, 1308, 1102, 969, 840 and 770 cm−1 (labelled in Fig. 6d) in addition to the bands from lead soaps. While not conclusively identified, these bands could suggest the presence of oxalate and sulfate components [48, 49]. Soap protrusions aggregates can remain active and the process to reach a stable end stage depends on several factors: the composition of the paint, the availability of fatty acids, environmental pollutants as well as environmental conditions [6, 7, 17, 52]. The presence of these oxidation products suggests an inherent instability of the paint and therefore particular attention should be taken regarding the storage conditions and during possible cleaning actions.
Gc–ms
The relative amounts of free fatty acids in the oil network vs. fatty acids carboxylates (metal carboxylates) may be discerned through fatty acid profiles obtained by means of GC–MS analysis. Fragments from the samples were extracted and derivatized by two different trimethylsilyl derivatization reagents: 1,1,1,3,3,3-hexamethyldisilazane (HMDS) and N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) where HMDS induces silylation of free fatty acids present in the oil binding medium. BSTFA, on the other hand, will silylate all fatty acids present in the sample including free fatty acids and metal carboxylates [27,28,29,30]. The use of these two derivatization agents can qualitatively determine the metal carboxylate and the free fatty acid content in the ground samples. Figure 7 reports the results obtained from the HMDS and BSTFA derivatization methods, showing an estimated relative response of the trimethylsilyl fatty acids and metal carboxylates present in the ground samples compared to the total of the fatty acids (azelaic, palmitic and stearic acids) content.
What may be immediately discerned from Fig. 7 is that the artist-prepared ground used in Pink and Green has a much lower concentrations (based on peak height) of total palmitic and stearic acids than that of the commercial pre-primed canvases. This result could mean either that there is a lower amount of oil medium overall in Pink and Green (which is inconsistent with our visual observations of this work), or that a lower proportion of the medium is in the form of free acids or soaps. Based on the FTIR results, and the fact that the priming layers in all cases are based on lead white, it seems very likely that a lower proportion of the medium in the ground for Pink and Green is in the form of free acids or soaps. Comparison of the BSTFA and HDMS results in Figs. 7 and 8 indicates that there is a higher contribution of metal soaps as compared to free fatty acids in commercial pre-primed canvases vs. the artist-prepared ground. This is consistent with the FTIR results, which showed the presence of lead soaps in the commercial pre-primed canvases but not in the artist-prepared grounds. In addition, for the three commercial pre-primed ground samples, Fig. 7 shows that there is a higher proportion of stearic acid in the soap fraction as compared to the free fatty acid fraction.
Formation of metal soaps is commonly believed to be the consequence of the hydrolyzed oil, followed by reaction with the metal ions from the pigment [7, 33]. However, soap protrusions can be also be produced by hydrolysis of additives [34, 35]. Our FT-IR results showed that the relative amount of fatty acids saponified in the pre-primed commercial canvas grounds was much higher than the amount of free fatty acids presents in the artist-prepared canvas ground. A possible explanation, based on the high amounts of metal carboxylates identified by means of GC–MS, is that the metal carboxylates were part of the formulation of the commercial pre-primed grounds [12, 36]. One hypothesis that may explain the presence of the soap protrusions is that these metal carboxylates were added to the commercially primed canvas formulation which has the deleterious effect of making the artists material more reactive and susceptible to external conditions (e.g. environmental conditions and conservation treatments) [12,13,14,15]. The growth process of the lead soap protrusions requires sufficient free fatty acids to migrate through the multilayered paint system [33]. The paintings studied here are also varnished. Hence, absorption of environmental moisture probably occurs from the back. A combination of moisture absorption from the back of the canvas, combined with the excess of FFA and the porosity of the preparation layer (observed in the SEM images), facilitates migration of the metal ions and SFAs from the ground to the upper layers. Once the lead soaps are in the upper layer, that is likely rich in additives due to the formulation of this modern paint, they can crystallize and aggregate, creating the soap protrusions.