Resin
Identification of the resin was achieved by removing a sample of untinted material from the interior of the HSML gourd (S5), followed by analysis with HPLC-PDA and Py-GC/MS. These results were then compared with literature data and with those obtained from the investigation of two reference samples of raw and boiled Eleaegia pastoensis resin. The first of these reference samples (raw resin) was a gift from the barniz de Pasto workshop of Gilberto and Oscar Granja in Pasto, which one of the authors visited in 2011; it was broken from a brick of unprocessed resin, destined to be colored and used in the decoration of a modern object. The second (boiled resin), collected in 2012 and subjected to repeated boiling, was provided by Emily Kaplan, Objects Conservator at the Smithsonian National Museum of the American Indian (NMAI).
The HPLC-PDA chromatograms acquired from the two references of Elaeagia pastoensis resin displayed slightly different profiles in terms of their relative abundance of detected compounds, which is likely attributable to the different manufacturing process undergone by the two materials upon harvesting (none vs. boiling). The most characteristic molecules found in both reference samples include two widely occurring natural flavones, luteolin (16.11 min) and apigenin (17.88 min), as well as luteolin monomethylether or related derivative (17.66 min) (Fig. 3b). The series of compounds identified in the two reference resins, along with their observed distribution, are consistent with HPLC-PDA-MS results previously reported by Newman and coauthors for Elaeagia pastoensis species [6]. According to their study, the distinction between specimens from Elaeagia pastoensis and Elaeagia utilis may rely on the detection of a few additional components in chromatograms of the latter that are not present in the former, namely quercitin, kaempferol, and the monomethyl ether of apigenin. Data collected from sample S5 show an excellent correspondence to those obtained for the reference of Elaeagia pastoensis boiled resin (Fig. 3), both in terms of the series of characteristic components detected and the relative amounts observed, and are consistent with the results reported in Newman’s article for samples from this plant species.
Similarly to what was observed for HPLC-PDA, when analyzed with Py-GC/MS, the two references of Elaeagia pastoensis resin were found to contain the same series of distinctive compounds in different relative amounts, which, as stated above, might be due to the different manufacturing process undergone by the two materials upon harvesting (none vs. boiling). Upon derivatization, organic compounds containing acidic functional groups in the sample, such as carboxylic acids, alcohols, and phenolic compounds, are deprotonated by the alkaline TMAH and the resulting tetramethylammonium salts are thermally converted to the corresponding methyl esters in the hot injection port of the gas chromatograph. The most characteristic molecules found in the two references examined here include: glycerol derivatives (8–8.5 min), along with various fatty acids, mainly stearic (23.36 min), palmitic (21.52 min), and azelaic (17.48 min); several compounds containing a single aromatic group (15.5–18 min), including methyl ester of 3,4-dimethoxybenzoic acid, 1-(3,4-dimethoxyphenyl)ethanone, 1,2-dimethoxy-4-(1-propenyl)-benzene, 3,4-dimethoxybenzaldehyde, methyl ester of azelaaldehydic acid, 1,3,5-trimethoxybenzene, and 4-ethenyl-1,2-dimethoxybenzene; two cinnamic acid derivatives, i.e. methyl p-methoxycinnamate (19.01 min) and methyl 3,4-dimethoxycinnamate (21.14 min); a series of unidentified, but recurrent compounds (24–28 min); the dimethyl ether (29.71 min) and trimethyl ether (31.05 min) of the flavonoid apigenin, as well as the tetramethyl ether of the flavonoid luteolin (32.68 min); and numerous pentacyclic triterpenoids (30–40 min) (Fig. 4b). The series of compounds detected in the two reference resins and their observed distribution are consistent with Py-GC/MS data previously reported for Elaeagia pastoensis species by Newman and coauthors [6]. Their work shows that Py-GC/MS chromatograms of Elaeagia utilis specimens contain similar components as Elaeagia pastoensis, albeit in different relative amounts. Notably, two of the unidentified compounds eluting, in this case, between 24 and 28 min, are reported to have been consistently found in Elaeagia pastoensis samples, but only rarely in Elaeagia utilis species, and can therefore be used as markers. The detection of the latter molecules in the current analysis, along with the observed overall distribution of marker compounds, confirms that the two reference resins analyzed belong in fact to the Elaeagia pastoensis species. The chromatogram obtained from sample S5, dominated by an intense peak of methyl 3,4-dimethoxycinnamate, is consistent with that of the reference of Elaeagia pastoensis boiled resin (Fig. 4), both in terms of the series of distinctive compounds detected and their relative amounts, and concurs with data previously reported by Newman for samples from this plant species.
The combined results of HPLC-PDA and Py-GC/MS analysis conclusively confirmed the use of barniz de Pasto to decorate the gourd, further corroborating the previous classification based on visual examination and stylistic considerations. Despite many descriptions of mopa mopa in its raw state being greenish-yellow [6, 40, 41], the resin used on the interior of the gourd appears reddish-brown, which is a common feature to all the barniz de Pasto objects in the HSML collection. Chromatographic analyses have shown that no other colorants are present at the sampling location aside from a series of flavonoids that are inherent to the resin’s composition itself. Therefore, a possible explanation for the observed color may lie in the fact that the layer of resin applied to the interior of the gourd is approximately 1-mm thick, i.e. significantly thicker than the resin used on the outer decoration, and laid directly onto the brownish gourd substrate. The resin found on the exterior, on the other hand, is typically no more than a few microns thick and laid on top of a silver leaf, which may account for its transparency and light reflectivity.
Pigments and colorants
The gourd has a limited palette of translucent colors over metal leaf: green is found in the background; red, pink, yellow, gold, and blue dominate the decoration’s flora and fauna; while saturated white, cream, and black are used as outlines of the decorative elements. In his account, Humboldt described the colors employed in barniz de Pasto objects as dilute indigo for blue; pure indigo for black; achiote, a dye derived from Bixa orellana seeds, for red; the powdered extract of Escobedia scabrifolia, a saffron-like root, over silver leaf for gold; and lead oxide for white [14]. In the present study, characterization of the pigments and colorants was mostly expected to confirm the presence of those locally available described by Humboldt. It is also worth noting that a key feature of the mopa mopa resin is its waterproof nature, as well as the remarkable physical changes it undergoes from a pliable and elastic substance capable of being stretched into translucent sheets to the shiny, rigid, and impermeable finish observed on the surface of the gourd. The intrinsic hardness of the resinous matrix in its final form [6] might be one of the reasons why identification of the colorants embedded in it has posed a longstanding challenge, and the few attempts commissioned from external laboratories by the HSML prior to this study failed (Newman R, Unpublished report, 2015).
As expected, in-situ XRF measurements ubiquitously detected silver, used in the gourd’s metal leaf, but yielded no results on the colored areas, most of which, indeed, were believed to be made of organic dyes. In addition, XRF spectra displayed surprising mercury peaks, along with chlorine, at several locations; this observation could be explained by the findings in Burgio’s 2018 publication [33], in which the authors report on their discovery of mercury(I) chloride (Hg2Cl2), or calomel, deliberately employed as a white pigment on a 17th-century barniz de Pasto box in the collection of the Victoria and Albert Museum. Our initial analysis with a portable system was subsequently refined by using μXRF to avoid interference from adjacent regions and precisely locate mercury and chlorine in the incredibly detailed decorations of the gourd (Fig. 5a). The presence of calomel, inferred by XRF in all the object’s saturated white details, flora and fauna outlines, and in the silver-white crescent moons that frame the decoration quadrants, was unambiguously confirmed by Raman spectroscopy due to the detection of distinctive bands at 167 and 275 cm−1 (Fig. 5b) [33].
Calomel, naturally formed from the alteration of other mercury-containing mineral ores such as cinnabar (HgS) upon reaction with chlorine in the atmosphere or in chorine-rich soils [42,43,44,45,46,47,48], has been known as an ingredient in medications and cosmetics [49,50,51]. However, it can also be manufactured artificially. A method of production known for centuries and reported in a 19th-century study of Japanese pigments involves sublimation of a pulverized mixture of alum, mercury, and common salt in a furnace [52]. Albeit produced in China prior to the 10th century, only in the first half of the 16th century did this compound become known to the Western world [53], where it started to be manufactured, likely in England, in the mid-17th century [54]. Deposits of cinnabar, a natural precursor of calomel, are widespread in Central and South America, and their exploitation dates back to prehistoric times [55]. Intensive mercury mining in the New World first began ca. 1400 B.C.E., predating the emergence of complex Andean societies, and the earliest activity targeted cinnabar for the production of vermilion [56]. This practice strongly increased under the Spanish colonial government, when the use of the mercury amalgamation technology for the extraction of gold and silver reached its acme [57, 58]. Nowadays, the Nariño province, where Pasto is located, is still one of the most important regions for gold and silver mining. Although 45 cinnabar deposits have been recently inventoried in Colombia [59], little is known about which ones were exploited by pre-Hispanic cultures [55, 60]. Based on the available documentary sources, it cannot be ruled out that cinnabar exploitation to produce mercury for gold and silver mining in Colombia may have begun under the Spanish government. If this hypothesis is confirmed, the possibility that natural calomel may have been employed for the HSML gourd would be as viable as the use of an artificially produced material.
Whites commonly used in South American artifacts have ranged from calcium-containing minerals, such as calcite, gypsum, and apatite [61,62,63,64], to lead white [65]. Similarly, scientific analysis carried out to date on qero cups has highlighted a predominant use of lead white in the form of hydrocerussite (basic lead carbonate, 2PbCO3·Pb(OH)2) and cerussite (neutral lead carbonate, PbCO3) [5, 6], along with a naturally-occurring mixture of cristobalite (SiO2), anatase (TiO2), and α-quartz (SiO2) [30, 66]. In the collection of the HSML, lead white was detected in a barniz de Pasto box dating to 1684 (Newman R, Unpublished report, 2015), and this finding appeared to support Humboldt’s description of white in barniz de Pasto objects as “made imperfectly with white lead oxide” [14]. In the past year, however, calomel was also identified by the authors on two barniz de Pasto caskets dated to 1650 and 1700 in the HSML holdings (Pozzi F and Basso E, Unpublished report, 2019), and similar results were obtained by scientists at the Fitzwilliam Museum (Cambridge, UK) from the analysis of a 15th-century illuminated manuscript and a late 16th-century portrait miniature [67]. Before Burgio’s work, calomel was never reported as a pigment in cultural heritage objects. In addition to substantiating her initial finding of calomel as an intentionally used white pigment, such recent discoveries clearly indicate that this material was more commonly employed in works of art and cultural heritage objects than initially thought.
Red was an important color both to European and pre-invasion indigenous cultures [68], and, in the region of Pasto, several mineral and botanical sources could yield an intense red hue. Among the red pigments historically available in South America, cinnabar had been used extensively as face and body paint by the Inkas, for ceramics, as well as in Inka qeros [24, 26, 69]. Similar shades of red could be also produced from certain indigenous dyes, such as cochineal, obtained from female insects of the Dactylopius coccus Costa species, and achiote, extracted from the seeds of the Bixa orellana plant [70]. In his account, Humboldt described the red in barniz de Pasto objects as “derived from Urucu (Bixa orellana, mixed with rubber milk) powder” [14]. Initial XRF analysis of the HSML gourd’s red decoration revealed only trace amounts of mercury, leading the authors to rule out a widespread presence of cinnabar. Further testing with FORS, yielding spectra characterized by apparent absorbance maxima at 520 and 560 nm (Fig. 6a), immediately suggested the possible use of an organic red of animal origin [71]. While SERS of a microscopic sample removed from the object’s inner neck (S2) failed, likely due to the interference of the thick mopa mopa layer in which the dye is embedded, extraction with BF3 and methanol followed by HPLC-PDA analysis enabled the conclusive identification of carminic acid—the main coloring component of cochineal—and some of its methylated derivatives (Additional file 2: Figure S2). This dye, also found by other researchers in a selection of Andean qeros [30, 31], was particularly popular in the 16th and 17th centuries, becoming the third most profitable traded commodity from the New World for the Spanish after silver and gold, and displacing kermes as the source of luxurious crimson and scarlet textiles within 50 years of its introduction [65]. Called grana by the Spanish, it had been cultivated on Opuntia cacti from about 600 C.E. by the indigenous populations, who had developed a sophisticated dyeing process with it, to the point that the many shades of red derived from cochineal are each identified by name in the contemporary chronicles [69, 72, 73].
Humboldt’s account of the yellow and gold found on barniz de Pasto objects states: “Yellow is derived from the powdered root of the Escobedia Flor. Per. or from saffron from the earth. Gold is also made from Escobedia applied on top of a silver leaf” [14]. He believed the colorant to be obtained from Escobedia scabrifolia, a locally grown saffron-like root that is still used today in Colombian cuisine to color foods and drinks [74]. Yellow dyes are among the most difficult cultural heritage materials to characterize analytically, mostly due to the lack of adequate spectral databases to be used in identification studies and the fact that hundreds of plants can produce yellow extracts [75,76,77]. As observed in the case of red areas, XRF spectra acquired from yellow and golden portions of the decoration did not display any elements that could indicate the presence of inorganic pigments such as, for example, orpiment. The organic nature of these colorants was further supported by UVRFC imaging of the object (Fig. 7a), in which all yellow dyes typically show as red [78,79,80]. From an analytical standpoint, FORS is generally not suitable to characterize yellow colorants, as their spectra appear broad and featureless, while SERS, again, yielded no results due to the hard texture of the embedding resin, thus interfering with the analysis. At first glance, the HPLC-PDA chromatogram of a microscopic sample removed from a yellow inlay (S4) appeared to simply contain the resin’s characteristic flavonoids, while neither Escobedia scabrifolia nor other yellow dyes—references of which have been also examined for comparison—were detected. However, close inspection of the data revealed that the peak of luteolin, relatively to the other two flavonoids present, was more intense in S4 (yellow decoration) than in the Elaeagia reference resin and S5 (untinted resin), which might cautiously be interpreted as indicative of the use of a luteolin-based yellow dye (Additional file 3: Figure S3). However, it was not possible to confirm this hypothesis as, in this case, a decision was made to avoid removal of another sample to preserve the object’s integrity.
Identification of the gourd’s blues and blacks, found both in areas of the decoration as well as in all the dark outlines of the inlaid figures, was relatively straightforward. Humboldt described the blue as “made by dissolving indigo in a lot of water and heating the resin a little” and the black as produced “with a large quantity of indigo, heating it a lot” [14]. As per his account, an indigotin-based dye, likely indigo, was detected by FORS due to the observation of an apparent absorbance maximum at 660 nm in the blue inner rim and in some of the decorations’ outlines (Fig. 6b). This identification was further supported by the Raman spectra of a selection of blue and black elements accessible for analysis with the system used, in which the main band of indigotin arises at 1572 cm−1 [81].
While Humboldt did not specify the pigments supposedly employed for green areas in barniz de Pasto objects, he did mention artisans chewing a ball of blue (presumably indigo) and one of yellow (an organic yellow, likely from Escobedia scabrifolia) to produce green. In accordance with his description, IRRFC and UVRFC images of the gourd’s green background suggested the presence of indigotin, which showed as red in the former, and of organic yellows, which showed as red in the latter (Fig. 7) [78,79,80]. The use of indigotin was conclusively confirmed by FORS (Fig. 6c) and Raman spectroscopy, while the yellow component, in this case, was not analyzed micro-invasively, although it is safe to assume that it could be the same colorant found in the yellow and golden areas of the gourd’s decoration.
Decoration stratigraphy
While a wealth of information on the materials of the gourd was gathered using non-invasive techniques, removal of five cross section samples followed by examination of their stratigraphy with optical microscopy and SEM/EDS was crucial to shed light both on the object’s manufacturing techniques and on the exact composition of its metallic elements, primarily the silver leaf. Detailed results regarding the layer structure of an artifact created with the barniz de Pasto technique are reported in the following section for the first time.
Polarized light and UV photographs of a first sample (S1) (Fig. 8a), taken from the gourd’s green background at a location on the neck where a large portion of the decoration had detached, revealed a stratigraphy that includes, from the bottom up, three layers of resin of different colors, one orange, one yellow, and one green. SEM/EDS enabled detection of a silver leaf of 1-μm thickness between the yellow and green resin layers. This leaf is composed of nearly pure silver with traces of copper and mercury, as well as low amounts of chlorine, possibly indicating the presence of silver chlorides as degradation products.
Another sample (S2) (Fig. 8b) collected from the red inner border showed that, unlike the green resin layer, the red layer directly on top of the silver leaf in this area is extremely thin, measuring about 20 μm versus the over 100 μm of each of the three layers of tinted resin observed in the previous sample.
Interestingly, a third sample (S3) (Fig. 8c) removed from one of the gold and blue triangles on the gourd’s inner rim showed the presence of two distinct silver leaves: the first is located in the lowermost portion of the cross section, lying underneath a blue resin layer that ranges between 10 and 20 μm of thickness; the other, whose degradation is clearly indicated by the presence of silver chlorides, is sandwiched between a superficial, 3 to 15-μm golden-white layer, mostly composed of a mixture of calomel and lead white, and a dark yellow mopa mopa layer of about 100-μm thickness. It is worth noting that the absence of gold in the EDS spectra supports Humboldt’s description of golden colors in barniz de Pasto objects as tinted resin over silver leaf; however, the actual stratigraphy of the gourd’s golden areas as revealed by scientific analysis appears to be significantly more complex than he believed it to be.
A yellow sample (S4) (Fig. 8d) taken from an existing loss in one of the leaves near the unicorn figure was characterized by a relatively simple structure, with a layer of yellow-tinted resin of 120 to 150-μm thickness lying on top of the 1-μm thick silver leaf.
Finally, an additional sample (S6) (Fig. 8e), removed from one of the crescent moons on the red border that frame the quadrants of the gourd’s decoration, displays a more intricate stratigraphy that includes multiple layers. Polarized light and UV photographs, as well as BSE images (Fig. 9a) and EDS data (Fig. 9b–d), revealed, from the bottom up, the following structure: a yellow layer of mopa mopa resin of about 50-μm thickness; a 1-μm thick silver leaf, generally well preserved except for a blackened, exposed portion at left that contains degradation products mainly in the form of silver sulfides; a thin layer of red-tinted resin measuring approximately 20 μm, similar to that observed in sample S2; another 50-μm thick layer of mopa mopa that appears to have been tinted orange; a second silver leaf, with associated silver chlorides indicating, again, a possible degradation process; and the uppermost layer, with an abundance of 2-μm calomel particles dispersed in a resin layer of 10-μm average thickness. Silver(I) sulfide (Ag2S) is reported as one of the most common advanced degradation products of silver, typically found in the form of monoclinic crystals known to mineralogists as acanthite and arising from a corrosion phenomenon referred to as tarnishing [82]. The presence of silver sulfide in a number of white-, silver-, and golden-looking details of the gourd’s decoration was also confirmed with Raman analysis, due to the detection of broad bands at 92 and 241 cm−1 [82], respectively assigned to the Ag lattice vibrational modes and Ag–S stretching modes.
Radiocarbon dating
Preliminary hypotheses on dating the HSML gourd were based solely on stylistic considerations and on the visual examination of its decorative features. In particular, the finely detailed decoration, along with the inclusion of religious motifs and figurative elements from European early modern manuscripts as well as local flora and fauna, appear to be indicative of earlier artifacts dating to before ca. 1650. However, European artistic fashions continued to be popular in South America long after they ceased to be practiced in Europe; therefore, radiocarbon dating was performed, despite needing a 5-mg sample of gourd substrate (S7). Results showed date ranges of 1492–1644 C.E. with a 95% probability and 1522–1638 C.E. with 68% probability, which conclusively places the object under study in the early barniz de Pasto period.