For the identification of lining, interlining and fabrics in the original artifacts and also in the restoration materials, the techniques employed were mainly infrared spectroscopy with ATR accessory and thermal analysis.
The lining and the interlining of both the cope and the chasuble were made of cellulosic fibers, very possibly cotton (Fig. 3a (1)–(5)). The vibrations at 3334 cm−1 and 3290 cm−1 were assigned, respectively, to intramolecular and intermolecular hydrogen bonding (OH-stretching) within cellulose  and those at 2921 and 2856 cm−1, respectively, to the CH2 asymmetrical and symmetrical stretching vibrations [14, 15]. The signals at 1640 cm−1 were attributed to C = O stretching vibrations from the glycosides  and also to adsorbed water . The band at 1429 cm−1 was associated with CH2 scissoring in cellulose , and the signals at 1367 and 1318 cm−1 to, respectively, C–H bending [14, 16] and C–O stretching of the aromatic rings of the polysaccharides . The 1028 cm−1 vibration corresponded to C–O stretching vibration of polysaccharides [14, 16] and that at 903 cm−1 to the beta-linkage of cellulose .
The thermal curves corresponding to the lining and interlining of the cope and the chasuble showed the presence of cotton in both vestments (Fig. S1). The mass losses occur between 270 and 450 °C with maximum rates at ca. 360 °C that are related to the cellulose decomposition.
The fabrics and the core of the metal threads were composed of silk, which is a polyamide-based fiber (Fig. 3b and c). This was very common in liturgical vestments studied in other places [5, 8]. Signals at 3279 and 3072 cm−1 corresponded to N–H stretching vibrations of amide A and amide B groups [17, 18]. The intensity of the bands at 2926 and 2856 cm−1, assigned to CH2, was very high in the case of the yellow fiber and also in the infrared spectra collected on the metal threads from the cope (Figs. 3b (2),(3)) and chasuble (Fig. 3c (2)) (the core is a yellow fiber). This fact could be associated with the presence of any component employed, such as a characteristic dye or mordant (binder) employed in the dyeing process. In this case, the signals were assigned to the presence of wax, as will be detailed in the following sections. In addition, only in these three samples, a signal at 1738 cm−1 was observed and assigned to the presence of C = O stretching of carboxylic groups involved in an ester linkage. This vibration could be attributed to the monoesters present in beeswax . Bands at 1625 and 1518 cm−1 were associated with amides C = O stretching and C–N–H bending vibrations . The signal at 1445 cm−1 was assigned to CH2 scissoring and that at 1230 cm−1 to N–H bending, C–N stretching and C–C stretching vibrations [17, 20].
The three thermal curves corresponding to the gallon cope (Fig. 4a (1)), capillo fringes (Fig. 4a (2)) and gallon chasuble (Fig. 4a (3)) showed an important mass loss at 325 °C, that corresponded to the decomposition of amino acid groups and the breakdown of the peptide bonds of the silk fibers [16, 21]. This temperature was lower than that collected from silk fabrics used as a reference material (Fig. 4a (4)), situated at 340 °C. The maximum of DTG was slightly lower due to the deterioration happening over time, which can diminish the temperature of decomposition .
Regarding the restoration materials, the new lining (1) and the core of the new gallon (2) were made with polyester (Fig. 3d (1)–(3)). The bands at 1713 cm−1 and at 1245 cm−1 were assigned to the asymmetric C = O stretching [22, 23] and to the C–O stretching vibrations . The signal at 1097 cm−1 corresponded to C-O–H bending , and those at 1020 cm−1 to O = C–O–C or secondary alcohols and at 973 cm−1 to C = C stretching . Other parts such as the green fabrics of the gallon (4) or the core of the new metal thread (5) were fabricated with cotton (Fig. 3d (4)–(6)).
The thermal curves fully matched with those of polyester (Fig. 4b (3)) and cotton (Fig. 4b (4)) for both the new gallon (including the core and the green fabrics) (Fig. 4b (1)) and the new metal thread (also with some part of lining) (Fig. 4b (2)). Obviously, the degradation processes did not happen in this case and the temperatures matched with the standard samples. The stage in which the most weight loss occurred was between 390 and 470 °C. This loss was due to the degradation to breaking of the bonds within polyester .
In both garments, cope and chasuble, a similar and homogeneous disposal of the strips in a S-twisted form around the yellow silk core was observed except in some areas of the chasuble gallon.
In the cope, the widths of the threads were ca. 250 μm in the gallon (Figs. 5a and 6a, Table 1) and higher (ca. 275 μm) in the case of the metal threads (Figs. 5b and 6b, Table 1). The EDX analyses of the gallon showed metal strips with compositions of 91% of silver and 9% of gold (Fig. 7a, Table 1) Silver was detected thanks to its peak at 2.98 keV, assigned to the Lα line. Peaks at 2.12 and 9.71 keV corresponded to the Mα and Lα emission lines of the gold. Over the metal appeared small particles (Fig. 6c), constituted by silver and chloride. The elemental chemical mapping (Fig. 7b) showed the distribution of silver in the inner layer of the sample, stretch lineal marks of gold over silver and the particles of silver chloride laid externally on the metal. The EDX analysis of the metal thread of the cope showed 74% of silver and 26% of gold, and also chlorine and sulfur. The gold percentage was higher than in other metal threads studied (Table 1). Similarly, to the gallon, the surface of the metal thread showed lineal scratches responsible for the heterogeneous distribution of silver and gold, which was shown in the mapping analysis (Fig. 7c). The disposal of the external and internal layers and components was clearly determined by SEM images (Figs. 6c and 7b). Other authors have employed cross sections to characterize the interior as well as the surface layers of the metal threads , some using focused ion beam . Also, EDX experiments could be carried out at different accelerating voltages in order to determine the in-depth composition .
Figure 8a and b shows the XRF spectra of the gallon cope and the metal thread cope. Silver, gold and copper were observed (the copper thanks to the peak at 8.05 keV corresponding to Kα signal). Chlorine was mainly detected in the gallon (as was also observed by SEM–EDX), where more silver was observed (Fig. 8a). A higher corrosion degree was detected in this zone of the cope due to the possible presence of silver chloride . Gold was detected mainly in the metal thread compared to the gallon (Fig. 8a and b). The results by XRF matched in a semiquantitative way with those by SEM–EDX.
In the case of the fringes of the capillo, the diameters of the metal threads (ca. 250 µm) (Figs. 5c and 6d, Table 1) and the composition (92.5% of silver and 7.5% of gold) were similar to those of the gallon of the cope. By XRF, the composition of the capillo (Fig. 8c) was very similar to that of the cope gallon. The surface of the fringes showed scratches and some brilliant spots (Fig. 6e). The chemical analysis carried out at the bottom of the scratches (inner layer) showed the presence of silver (Fig. 7d), whereas the brilliant spots were constituted by a high percentage of gold (Fig. 7e). Chlorine was also detected together with silver (Fig. 7d). In other parts of the fringes of the capillo, the metal showed scratches and some edges gnawed (Fig. 6f). The deeper zones analyzed showed a higher percentage of silver, increasing the percentage of gold in the upper zones (Table 1). Some variations in the compositions were observed in the EDX analyses performed in different areas of the capillo fringes (Table 1).
Two different types of the metallic strips were observed for the chasuble: some flat metal sheet with a width of ca. 600 µm (Figs. 5d and 6g), and strips wound around the yellow fiber of silk with a diameter minor than those found in the cope; the values were around 200 µm (Figs. 5d and 6h). The use of two different types of ornamentation clearly showed the higher quality of this piece and the best performance of the artists in its fabrication. In the case of the chasuble gallon, the main strips used were the flat ones (sheets), which were constituted by 42% of silver and 58% of gold (Fig. 7f, Table 1), whilst the strips wound around the silk were constituted by 88% of silver and 12% of gold (Fig. 7g, Table 1). Regarding the chasuble metal threads, their morphology and composition were very similar to those described for the metal threads cope, although in some parts, only silver was detected (Table 1). Chlorine and sulfur were punctually observed in the chasuble and dust on the surface (Table 1).
In the case of the studies of chasuble by XRF, the gallon (Fig. 8d) presented the highest amount of gold and the thread the lowest (Fig. 8e). As aforementioned, these results could suggest a best knowledge and performance in the fabrication of the chasuble, in which gold was only used for the gallon and not in the threads forming part of the fabrics. Again, the results obtained by XRF and EDX coincided.
The results obtained by SEM–EDX and XRF were also ratified by XRD. Gold and silver have face-centered cubic structure with very similar cell parameters: 4.079 Å in gold (PDF 04–0784) and 4.072 Å in silver (PDF 02–1098). Gold forms continuous solid solutions with silver and copper; both have the same crystallographic structure as the gold . The signal of the (200) planes of silver was located at 2θ = 44.1° and that of gold at 2θ = 44.4° (Fig. 9). Mainly gold was detected for the gallon chasuble. In the metal thread of the chasuble, silver was clearly detected and a shoulder that was assigned to gold. For the other pieces (gallon cope and capillo fringes), signals of gold and silver were detected (Fig. 9). Although the formation of solid solutions could not be discarded, based on SEM mappings and EDX punctual analyses it seemed that two different layers were detected: the internal formation of silver and the external of silver and gold, although this latter disappeared in some areas due to the scratches observed.
The diameters of the metal threads in the form of strips employed for restorations were ca. 200 µm (Figs. 5e and 6i), similarly to those of the cope. The core was formed by cotton as was also found by FTIR and thermal analysis. In the case of the gallon restoration, the width was ca. 200 and the core was composed of cotton and polyester fabrics (Figs. 5f and 6j). Cellulosic fibers, very possibly cotton, joined the different threads. By XRF, mainly silver (Fig. 8f) but also titanium was detected in the spectra collected (Kα signal at 4.5 keV). The EDX analysis showed the presence of mainly silver (by both techniques, EDX and XRF), aluminum and nitrogen by EDX, and silver and zinc (Kα signal at 8.6 keV) by XRF in the metal thread (Fig. 8g).
Both garments were decorated, as usual happened in the liturgical vestments [2,3,4,5, 8, 10].
The lining (1) was greenish in the cope (value of a* of -3.03) and brownish in the chasuble (values of a* of 4.70 and b* of 24.02). The lightness values were higher for the chasuble (L* = 68.42) (Fig. 2a and 2c, Table 2). The new lining (1) (Fig. 2b) used for restoration had values of a* closer to zero and of b* minor than that of the chasuble, so provided a yellowish tone in the final appearance; the lightness values of the new lining were higher (L* = 87.36) than the originals (Table 2).
Although the interlining (2) and the fabrics (3) were visually very similar for both pieces (Fig. 2a and 2c), the chasuble interlining presented higher values of a* (4.02 vs 2.90) and b* (24.96 vs 21.94) and the cope fabrics higher values of b* (22.75 vs 18.41) (Table 2), so the tone of the cope was more yellowish and of the chasuble was more brownish.
The metal threads (4) and gallons (5) (Fig. 2a and 2c) were characterized by a core of fibers and metal strips wound around them as usual in this type of embroidery [5, 8, 24,25,26,27,28]. The internal yarns of the two pieces (Fig. 5a-d) were visually yellow. The values of b* in metal threads, gallons and fringes were high. Again, the values of b* were higher for the cope; the higher values of a* and minor of b* corresponded to the chasuble (Table 2), very possibly for the influence of the fabrics underlayer. The new metal thread (4) for restoration (Fig. 2b) provided a very similar tone but with higher lightness (76 vs 58 or 65). In the case of the new gallons (5) for restoration (Fig. 2b), which was placed in the cope, the hue was clearly less reddish, with values of a* close to zero. Greenish yarns were observed with the naked eye and with optical microscopy (Figs. 2b and 5f).
The cope was more richly decorated with green, blue, yellow and pink yarns (Fig. 1). Fibers of different colors (blue, green, yellow and pink) from the cope were studied (Table 2). Blue color was the darkest (minor value of L* ca. 56) together with the pink (56). Values of a* were negative for blue and green fibers (− 1.61 and − 4.97, respectively). The chromatic values for the yellow fibers were similar to those of the gallons and the metal threads of the cope in order to simulate the gilding tone, although the value of b* was higher (43) (Table 2).
SEM applied to the blue fiber of the cope showed the presence of silk fibrous (Fig. 10a), and the micro-Raman study of this blue color showed scattering peaks at 1700, 1574, 1363, 1309, 1249, 1014, 757, 597 and 251 cm−1, which matched with the Raman spectrum of indigo [29, 30]. A comparative figure with the spectrum obtained in our study in blue fibers and the pattern sample of indigo is shown in Fig. 11. The signals at 1666 and 1224 cm-1 could be assigned to the amide vibrations of silk structure .
The SEM study of green fibers showed a morphology similar to the blue ones (Fig. 10b). Small particles deposited over the fiber have also been observed (Fig. 10c). The EDX analysis of these particles showed the presence of Na, Al, K, Cl, S and Ca together with C, O and N, which could be due to dust or the rest of a mordant, very possibly alum (KAl(SO4)2·12H2O). The Raman spectroscopy showed the presence of indigo, similarly to the blue color. However, the green color may be attributed to a mix of indigo and a yellow dye that did not yield a strong Raman spectral component.
The SEM observation in the yellow fibers showed similarity with those two samples previously studied (Fig. 10d). The Raman scattering of the yellow fibers yielded a poor-quality, unidentifiable Raman spectra. However, the fibers were covered by a layer of other components (Fig. 10e). This layer may be attributed to an incomplete degumming process typically from pre-industrialized processes or to the wax used to protect and strengthen the embroidery thread, frequently used during the embroidery manufacture, and possibly responsible for the yellow color in these fibers. Differential scanning calorimetry has been applied to the wax analysis in artworks in order to detect the absorption (endothermic) or liberation (exothermic) of heat [31, 32]. The yellow fiber sample was heated between 30 and 80 °C and after cooling in this temperature range. The treatment was repeated four times. The heating between 30 and 80 °C showed an endothermic effect with maximum at 67.14 °C, and the cooling between 80 and 30 °C showed an exothermic peak with maximum at 57.64 °C (Fig. 12). These effects were attributed to melting and crystallization processes, respectively, of the wax present in the sample. These effects were reported in the several heating–cooling cycles carried out. The FTIR analysis also confirmed the presence of wax in the yellow fibers (Fig. 3b (3)).
Regarding the pink fibers, the morphology was very similar to the previous fibers studied. Raman scattering on this fiber was weak, yielding a poor-quality Raman spectrum, and was not possible to detect the compounds responsible for the color. On the surface of the fibers, appeared particles constituted by a high content of C and also O, N, Al, S, Cl, K, Ca and Ba were present, which was attributed to the red dye used to obtain this color and/or to the treatment of the textile fibers with a solution of mordant during the dyeing process, again suspected alum.
SEM images of the colored fibers of the restoration material (new green fabrics of the gallon of the cope) showed morphology of fibers compatible with polyester (Fig. 10f).