Micro-structural and fiber characterization
Cross-sectional photomicrographs of the Lajian paper were observed under white and UV light, as shown in Fig. 2. The paper exhibited the traditional multi-layer structure for Lajian papers according to traditional folklore [1,2,3]. The cross-section images exhibited a bright red color under white light (Fig. 2a) and stratigraphic color layers under UV illumination (Fig. 2b). The microphotograph, shown in Fig. 3b, revealed the following color layers from top to bottom (shows in the inner part): a thin varnish layer (A), an orange-red layer (B), a yellow-orange layer (C), a white layer (D), and a light yellow-orange layer (E) by fluorescence under UV illumination. The cross-section analysis revealed a varnish layer (A), a dyed orange-red layer (B), a colorless fiber layer (D), and two yellow-orange fiber layers (C and E). The varnish layer indicated that the paper surface was likely wax-coated. The orange-red fiber layers are likely the result of pigment and dye application [9]. The uncolored fiber layer (between the two yellow-orange layers) may have resulted from the inability of the organic dye to entirely penetrate the paper. The back did not exhibit an orange-red color, thereby indicating that each paper layer was treated differently. This result indicated that these colorants were added to the paper surface by brushing.
The SEM image (Fig. 3a) of the cross-section revealed that the thickness of the paper was approximately 100 μm. The thickness of the upper and lower layers was approximately 60 and 40 μm, respectively. Figure 3b shows some needle-like and flaked materials on the fiber surface. The EDS results indicated that the main elements for these needle-like materials were C and O (the atomic percentage was greater than 95%); these results may be ascribed to organic dyes. The flaked materials may be kaolinite (Al2O3·2SiO2·2H2O) as the main elements were Al, Si, and O. According to SEM–EDS results, kaolinite may have been used as powder or coating pigment while needle-like material may have been used in dyes.
Figure 4 shows the micro-structure of the upper surface. Figure 5a shows that some cracks and holes were present on the surface. The holes indicated that some materials were peeled off from the upper surface; this peeling means that the coating materials did not bond well with the subsequent materials. This phenomenon was very similar to that shown in our previous findings on the pink single-layered Lajian paper [6]. Figure 4b shows irregular particles and a disrupted varnish layer on the upper surface. Almost no particles were found to be mixed with varnish, thereby indicating that the particles and varnish were applied to the upper surface separately. Needle-like materials were also found in these irregular particles in Fig. 4c under higher magnification. EDS analyses indicated that the major elements for the irregular particles included Pb, C, O, Al, and Si. The presence of Pb indicated that minium (Pb3O4) or lead white [(PbCO3)2·Pb(OH)2] may have been used. The concurrent existence of Al and Si indicates the presence of kaolinite (Al2O3·2SiO2·2H2O). EDS analyses of the varnish indicated that the main elements were C and O (atomic percentage exceeded 98%). The X-ray excitation depth was only 2–3 µm under an accelerating voltage of 5 kV. As such, the varnish was likely composed of wax. SEM results implied that a mixture of kaolinite, inorganic pigment, and organic dye was used as coloran to obtain an orange-red color, and that wax was coated on the whole upper surface separately. This result highlighted the need for further studies of mixed organic and inorganic components to comprehensively understand Lajian paper.
Traditional Chinese paper was made from plant fibers. Optical and microscope identification are effective tools for identifying fiber morphology [10,11,12,13,14,15]. Figure 5 shows the fiber structure under SEM observation. SEM images showed long tubular fibers with uneven surfaces and holes or crevices. Bast fibers typically exhibit these characteristics [10, 11]. Other fibers with many shallow grooves may be bamboo fibers [10, 11]. According to SEM test, the raw materials used in the Lajian paper are likely bast fiber and bamboo fiber, which were the characteristic components of traditional Chinese paper with significant consistency and strength.
In the second layer, no orange-red colorants were clearly observed under UV illumination, and no Pb and Al elements were detected by SEM–EDS. The light yellow-orange paper fibers observed under white light may be ascribed to organic dye. Both layers exhibited the same fiber morphology observed using SEM and OM. Figure 6 shows the wax coated back paper surface. Some fibers were mixed with wax, thus indicating that the second layer was not filled before being dyed. Thus, the production process for the second layer of Lajian paper involved treating the layer with organic dye and coating the layer with wax.
Spectral characterization
XPS, a notably sensitive analytic method, was performed on the Lajian paper to obtain more information on the chemical composition of its surface. Figure 7 shows that the main elements of the first layer included C, O, N, Pb, Al, and Si. The absence of Ag and Au may be due to the uneven distribution of flakes on the upper surface. The appearance of N implied that animal glue or gelatin may have been used as a binder. The appearance of Pb, Al, and Si was consistent with the SEM–EDS results and is likely ascribed to the presence of minium and kaolinite.
Raman spectroscopy identified pigments highly efficiently and non-destructively. The Raman spectral analysis of the first layer (Fig. 8) exhibited peaks at 122 vs, 152 m, 224 w, 314 w, 392 w, 480 vw, and 550 vs cm−1; these peaks levels were attributed to the vibrational modes of Pb–O bonds [16, 17] and are characteristic of minium (Pb3O4). However, there was no evident signal for PbCO3, PbO, or PbO2. Minium commonly occurs in small amounts as a bright red or orange powder [18], and it was widely used as a red and orange pigment in ancient China. This result indicated that minium was intentionally sprayed on the upper surface and remained stable over time in the Lajian paper sample.
ATR-FTIR analyses were used to identify unknown organic materials on the upper surface (Fig. 9). The broad absorption at 3369 cm−1 was attributed to the stretching vibration of hydroxyl groups (O–H) and N–H groups. Significant bands at 2869 and 2949 cm−1 were identified as methyl and methylene groups. The vibrations at 1150 cm−1 and 1058 cm−1 could be assigned to C–O–C and C–O stretching, respectively [19]. Bands at 1600 cm−1 (stretching amide I), 1540 cm−1 (stretching amide II absorption, combination of C–N stretching and NH bending), and 1455 cm−1 (amide III absorption, C–N stretching/bending) may be attributed to animal glue (gelatin) [19,20,21]. Traditionally, in ancient China, glue was added to a dyed solution to obtain a uniform color [22, 23]. Therefore, animal glue may have been used as glue during the dyeing process. It has been reported [24] that introducing pigment to animal glue resulted in splitting the amide II band into two components. ATR-FTIR analyses revealed that the band at 1540 cm−1 was split into 1542 and 1523 cm−1; this split was attributed to a change in the protein structure because of the effects of minium or kaolinite coating pigments. The bands at 921 cm−1 and 885 cm−1 were characteristic for the stretching mode of Al-O. Combining the strong band at 1058 cm−1 and the SEM–EDX results may confirm the use of kaolinite during the papermaking process. The notably strong band at 1722 cm−1 is characteristic of the carbonyl group vibration (C=O), thus indicating that the organic colorant or glue may have carbonyl groups.
SERS was conducted and spectra were obtained from the paper fibers. Figure 10 reveals peaks at 1577, 1532, 1468, 1230, 1194, 1130, 1101, 1052, 1011, 455, and 414 cm−1; these peak levels were consistent with the literature [25] and could be ascribed to curcumin, the main component of turmeric. These results coincided with those obtained from the ATR-FTIR of turmeric (approximately at 1621, 1599, 1237, 1150 cm −1) [19, 26]. The bands at 1722 cm−1 could be ascribed to Chinese insect wax, according to the ATR-FTIR results. It was reported that Chinese insect wax, which had carboxyl groups, was used widely to produce Lajian paper to form bright white coating in ancient China [3, 4].
Natural organic dyes and their corresponding lake pigments were important materials in the artist’s palette from ancient times until the second half of the nineteenth century, when the industrial production of synthetic dyes began. Turmeric—which is made from the rhizome of Curcuma longa L. (family Zingiberaceae)-is a main ingredient of curry powders. Turmeric is well known for its color, flavor, and digestive properties. Some researchers have reported that turmeric was used as a dye in ancient China [27,28,29]. Curcumin is the main colorant in turmeric, which is used to produce yellow-orange colorants. Previous research demonstrated that turmeric was typically stable, pH sensitive, and exhibited a yellow, red, or dark orange color under acidic, alkaline, or neutral conditions, respectively [30]. The paper acidity test showed that the pH value of the paper surface was 7.1–7.4. This is why the organic dyestuff exhibited a dark orange color under UV illumination (Fig. 2). Likewise, Suo [30] reported that turmeric dye has a needle-like micro-structure, as shown in Figs. 3 and 4. According to these results, turmeric may be confirmed as an organic dye in Lajian paper.
