Through OM, the samples were found to consist of a complex multilayered system, with each layer displaying distinct characteristics, color, composition, and thickness. Furthermore, the SEM–EDS analysis enabled the observation of the micromorphology of the inorganic components, along with the identification of the respective elemental composition (Table S1). In this way, the samples of the mural were analyzed, and as an example, the optical micrographs and the SEM–EDS analysis of sample 5 are presented (Fig. 2). The front black painting layer corresponds to the surface of the painting, which presents few detachments that reveal the underlying yellow layer (Fig. 2a). On the back, few residues of the first two painting layers were observed over the yellow layer: a barely notable white layer over a black layer. Despite the minimal amount of some pictorial layers available to be analyzed, it is important to consider the existence of the layers for the comprehensive analysis. Thus, the numbering of pictorial layers was assigned starting with the one closest to the mural panel: white 5-I, black 5-II, and yellow 5-III (Fig. 2b). In the cross-section, it was observed the stratigraphy of the sample: departing from the black layer 5-II, yellow 5-III, white 5-IV, black 5-V, and black 5-VI (Fig. 2c). The SEM micrograph of the same section illustrates the distinctive microstructures observed within each layer, particularly the extensive porosity of the yellow layer (Fig. 2d). The mapping of the elemental composition identified: C, O, Si, Al, Ti, S, Cr, Pb, and Cl (Fig. 2e–m). Based on elemental quantification, the predominance of C (50–60%) and O (25–37%) was related to the organic composition of the painting layers like the binders and additives which are the majority components of the formulations (Fig. 2e, f). The identification of Si (1.4–6.8%) was related to silicates used as fillers and traces of Al (0.1–0.3%). These fillers predominate in the yellow 5-III and are dispersed in the black 5-V and 5-VI layers (Fig. 2g, h). In addition, Ti (0.3–8.4%) was identified with the highest concentration in the white layer 5-IV and was related to titanium dioxide (TiO2), a common compound used as a white pigment. In the rest of the layers, it was also identified but less than 1% (Fig. 2i) [8]. S, Cr, and Pb were identified in the yellow 5-III (Fig. 2j–l), which correspond to the inorganic pigment chrome yellow, a mixture of lead chromate with lead sulfate (PbCrO4 + PbSO4) [10, 11]. Cl was detected primarily in the black layer 5-II, which may be associated with a component of the mural panel, such as an additive or a compound utilized in a previous fumigation (Fig. 2m). The elemental composition of each painting layer is presented in Table S2.

Micrographs and elemental mappings of the mural sample 5. Optical micrographs: a frontal layer black 5-Vl (20 ×), b back of the sample with residues of layers: white 5-I, black 5-II and yellow 5-III (20 ×), and c stratigraphic cross-Sect. (5 ×). d SEM micrograph (BSE, 20.0 kV, 75 ×) of the cross-section. Elemental mappings (EDS, 20.0 kV, 75 ×) of predominant elements: e C, f O, g Si, h Al, i Ti, j S, k Cr, l Pb, and m Cl

In the analysis of the pictorial layers of the other samples, it was observed that most of them contained a similar elemental composition. In some layers, it was also possible to identify particles composed of Na, Mg, K, Ca, and Fe, and in some cases, particular microstructures such as asbestos fibers were identified (Fig. S1), along with diatom micro skeletons and several amorphous fillers, which had been previously identified in the study of other Siqueiros paintings [12].

Based on the binders and fillers identified in the analysis of the contemporary mural, the ATR-FTIR spectra of the materials were acquired to use them as reference proposals (Fig. S2). According to the analysis of sample 5, the front and back layers of the other samples were studied (Fig. 3). The ATR-FTIR spectra of the black 5-VI frontal layer and the black 5-II back layers were acquired and analyzed directly. In the spectrum of the black 5-VI frontal layer, the most prominent bands were identified and were found to be related to functional groups of polyvinyl acetate (PVAc) polymer resin. These included the following: 2974, 2927, 2860, 1729, 1440, 1370, 1227, 1018, 945, 793, and 603 cm−1, once these bands were assigned, the signal at 1640 cm−1 was observed along with the broad band at 3500–3200 cm−1 and were considered as the O–H bending and stretching, of water from ambient humidity (Fig. 3a). This composition was identified in all the frontal pictorial layers: black 3-VI (Fig. 3b), white 1-IV (Fig. 3c) and 6-IV (Fig. 3d), beige 2-VIII (Fig. 3e), and red 4-VII (Fig. 3f). In the layer red 7-III (Fig. 3g), signals were identified at 3675, 1010, and 666 cm−1 and were related to talc (Mg3Si4O10(OH)2).

ATR-FTIR spectra from layers. Front: a black 5-VI, b black 3-VI, c white 1-IV, d white 6-IV, e beige 2-VIII, f red 4-VII, and g red 7-III. Back: h black 5-II, i yellow 4-V, j yellow 7-II, k white 1-I, and l white 3-I. Signals of PVAc, nitrocellulose, DEHP, EA-MMA acrylic binder, talc, quartz, and CaCO3 are indicated

Regarding the study of back painting layers, contrasting compositions were identified. In the spectrum of the black 5-II back layer, absorption bands related to nitrocellulose were identified at 3500–3000, 1640, 1449, 1379, 1278, 1064, and 837 cm−1. Due to the OH group vibrations of water can overlap with the signals of nitrocellulose, in addition to the identification of characteristic signals such as N–O group, the relative intensity between the signals was considered. In addition, the bands of diethylhexyl phthalate (DEHP) were identified at 2876, 2862, 1724, 1600, 1580, 1449, 1379, and 743 cm−1. DEHP is a common plasticizer used in nitrocellulose lacquer formulations [13]. Additionally, the bands associated with silicates, possibly like quartz, due to characteristic bands at 1064, 791, and 467 cm−1, were identified (Fig. 3h). The same components were also identified in the spectrum from yellow back layer 4-V (Fig. 3i) and in the spectrum from yellow 7-II, where the DEHP and nitrocellulose signals are attenuated (Fig. 3j). On the other hand, in the spectra from white layer 1-I, the bands identified at 2925–2872, 1724, 1445, 1235, and 1145 cm−1 were related to an ethyl acrylate-methyl methacrylate (EA/MMA) acrylic binder, along with the bands related to talc, and bands at 1398 and 870 cm−1 that were relate to calcium carbonate (CaCO3) since Ca was identified in the elemental analysis by EDS of this layer (Fig. 3k). The CaCO3 fillers were only identified in this pictorial layer. In the white layer 3-I (Fig. 3l), bands related to talc and low intensity bands related to the acrylic binder were observed. Regarding the back side of samples 2 and 6, ATR-FTIR spectra were not acquired since it had residues of support panel. The signals of the identified compounds are presented in Table 1.

Due to the stratigraphic complexity observed in the studied samples, the identification of the materials along the painting layers was complemented through micro-FTIR analysis. The analysis of the upper layers from sample 5 is presented with the respective chemical mappings (Fig. 4). The different layers of the stratigraphic cross-section were examined, and the results were correlated between the optical micrograph (Fig. 4a) and the 1.1 µm2 IR reflectance spectra, which form the chemical mapping. The chemical mappings of the surface were observed as areas of increasing intensity from blue to red (Fig. 4b, c). Based on the compounds identified in the ATR-FTIR spectra, the characteristic bands from the binders were related to the respective bands in the reflectance spectra from the chemical mappings and from characterized commercial samples. The analysis of the commercial samples was conducted to complement the general identification of PVAc (Fig. S3a) and nitrocellulose (Fig. S3b).

Micro-FTIR analysis from the mural sample. a Optical micrograph of the stratigraphic cross-section. Chemical mapping with the intensity distribution of the reflectance peaks of b signal at 1749 cm−1 related to the carbonyl group from acrylic and PVAc resins from black 5-V and 5-VI layers and c signal at 1678 cm−1 related to the nitro group from nitrocellulose in white 5-IV and yellow 5-III layers

On the upper side of the optical micrograph from the microsample, a portion of the inclusion acrylic resin is observed (Fig. 4a). In the corresponding area of the chemical mapping, a signal was identified at 1749 cm−1 in the absorbance spectrum and was related to the stretching of the carbonyl group (C = O) from the acrylic resin. The signal was detected with a higher intensity (in red) and with less intensity (yellow and green areas) in the other painting layers, indicating that part of the sample is covered by the resin. At the same time, the band was identified and related to the PVAc resin from black layers 5-V and 5-VI, due to the chemical structural similarity and because the polymer was identified through ATR-FTIR spectroscopy (Fig. 4b). Conversely, in the region corresponding to the white layer 5-IV and yellow 5-III, a signal was identified with great intensity at 1678 cm−1. This could be related to the vibration of the nitro group (N–O), which would indicate the presence of nitrocellulose as a binder in these pictorial layers (Fig. 4c). Due to the discontinuity of the white 5-I and black 5-II layers, analysis by this technique was not possible.

The results of micro-FTIR and the identified binders in all the samples are summarized in Table S3.

Fractions of the multilayered microsamples were taken and solubilized in CDCl3 to be analyzed by NMR. In the 1H-NMR spectra, the corresponding compound signals of each mixture were observed, and through the 2D-NMR experiments, the structures of the major organic compounds were elucidated.

In the 1H-NMR spectrum in CDCl3 from sample 5, several signals with different intensities were observed (Fig. 5). Some of these signals exhibited multiplicity and are narrow, indicating the presence of low-molecular-weight compounds, while broad signals were indicative of larger compounds. In the 2D-NMR spectra, the signals related to PVAc binder (Fig. S4a) and DEHP plasticizer (Fig. S4b) were confirmed since they were identified in the surface layers analysis through ATR-FTIR [14,15,16]. Additionally, the signals related to EA, MMA, and n-butyl methacrylate (nBMA) acrylic monomers were identified, which was not found in the superficial layers but was related to the intermediate layers of the sample. In the 1H-NMR spectra from the other samples, the signals of the same components were identified along the spectra with different intensities; in some cases, it was not possible to identify them (Fig. S4c). The chemical compounds and the 1H and 13C NMR data are listed in Table 2.

1H-NMR spectra (700 MHz, CDCl3, 300.0 K) of the mural sample 5. The signals of PVAc, DEHP, EA, MMA, and nBMA are indicated

Subsequently, the same sample was analyzed using DMSO-d6. 1H-NMR was focused on broad and low-intensity signals between 3 and 6 ppm that are related to nitrocellulose, based on the references where the characterization of the nitrocellulose monomers is addressed [17, 18]. Thus, in the 1H-NMR spectrum, the signals identified at δ1H = 5.05, 5.14, 5.84, 4.08, 4.01, 4.74, and 4.50 pm were related to the reported chemical shifts of the 1H from the assigned as D1–D6 from 2,3,6-trinitrocellulose (TNC) monomer, while the signals at δ1H = 4.61, 3.20, 5.25, 3.72, 4.01, 4.74, and 4.50 ppm were related to 1H from the assigned as D1a–D6a from 3,6-dinitrocellulose (3,6-DNC) monomer (Fig. 6). Additionally, a commercial sample of nitrocellulose lacquer was analyzed to confirm the proposed chemical composition. The sample was analyzed through 1H-NMR and edited-HSQC spectra, where the identified signals were related to two main monomers: TNC and 2,6-dinitrocellulose (2,6-DNC) (Fig. S4d). The 1H-NMR data of the identified nitrocellulose monomers in the mural sample are listed in Table 2. By proposing that the TNC monomer was present in both samples while the second monomer differs, it is possible to differentiate the 2 formulations used.

1H-NMR spectra (700 MHz, DMSO-d6, 300.0 K) of mural sample 5. The signals of TNC and 3,6-DNC nitrocellulose monomers are indicated