3.1. The Tongcao pith
As the carrier of Tongcao Painting, Tongcao pith, with the yellowish white appearance, was cut from stem of Tetrapanax papyriferus which was usually planted in southern China like Guizhou, Fujian, and other areas.
As we know, Tongcao pith had not been treated chemically, and its hexagonal cell structure could be clearly seen under the microscopy, which differentiated from that of Xuan paper with numerous and various sizes of fibers mixed together unregularly(Fig. 2). Under the SEM-EDS, surface of Tongcao pith looked like the honey honeycomb, which provided an ideal space for the stay of much more pigments with the small size of 5–10 µm, so the Tongcao painting looked more colorful and had the similar stereoscopic sense as oil painting. Generally, the cell wall of plants mainly consisted of cellulose microfibrils, lignin, hemicellulose, pectin and glycoprotein []. There were special cross-linked substance among these components, which enabled the cell wall to own enough mechanical strength in all directions, providing source for the structural support of Tongcao pith. However, compared with microstructure of Tongcao pith produced in 2015, cells of the ancient painting had changed in their shape, which suggested the start of degradation of secondary cell wall. Meanwhile, the connection among cell walls seemed very loose in the painting, which directly led to the strength reduction of its mechanical support and eventually resulted in the rupture and smash of Tongcao pith. Therefore, the related conservation work of Tongcao painting needed to be carried out emergently.
3.2. The usage of alum in Tongcao painting
Alum glue, a kind of solution which mixes the alum and gelatin, is chiefly used as an adhesive in the process of drawing, mounting and repairing of painting and calligraphy [], which on one hand can enhance the binding force among plant fibers, and on the other hand, has the function of fixing color, enhancing water resistance of paper and inhibiting growth of microorganism. In the mid-19th century, British traveler Downing once recorded in his travel notes the making and drawing procedure of Tongcao painting in detail as followed:
The paper being ready, it is washed over with a weak solution of alum, as they consider it is thus rendered more fit to receive the colours. This wash is frequently repeated during the progress of the work, so that before it is finished, it has received seven or eight coats. It is difficult at first to conceive the utility of the alum, but upon reflection it appears to me, that it is this mineral which gives such a degree of permanence to the coloring of the Chinese [].
Since the drawing technique of Tongcao painting had been lost, whether alum glue had been applied in the manufacture process of Tongcao painting was a controversial question. In order to solve this question, we compared the chemical difference among Tongcao pith with and without alum glue treated, and both pigments area and unpigmented area of the ancient painting.
Table.1 Semi quantitative analysis results of SEM-EDS(wt%)
| Samples | x0 | x1 | x2 | x3 | q0 | q1 | q2 |
| Al | 0.06 | 0.22 | 0.44 | 0.38 | 0.17 | 0.12 | 1.43 |
| S | 0.07 | 0.32 | 1.14 | 1.13 | 0.10 | 0.26 | 0.85 |
| K | 0.15 | 0.19 | 0.14 | 0.16 | 0.17 | 0.11 | 0.18 |
| Pb | / | / | / | / | / | / | 32.1 |
| Ca | 1.05 | 0.64 | 0.69 | 0.57 | 2.32 | 1.14 | 2.84 |
The chemical formula of alum is KAl (SO4)2·12H2O. As shown in Table.1 and Fig. 3, the chemical difference among samples was on the elements Al and S. It was easy to find that with the increase of coating times, content of Al and S was on an increasing tendency, indicating the higher concentration of alum. However, it basically stopped increasing when the number of coating times was over 8, revealing the adsorption capacity of cells tended to be saturated. On the other hand, content of Al and S remained in q0, q1 and q2 apparently exceeded than that of x0, relatively close to x1 or x2, which proved alum was applied both in the unpigmented area and pigments area of the ancient Tongcao painting.
3.3. the pigments
In China, the archaeological evidence revealed that as early as the Shandingdong Man period (30,000–40,000 years ago), hematite was ever used as the red pigment in burial area of this archaeological site. Subsequently, more and more types of pigments were used to paint the life of ancient human being. The pigment itself not only recorded ancestor’s knowledge of natural pigments including all kinds of mineral powder and plant dyes, but also memorized the new invention of artificial synthesis pigments. In addition, pigment also witnessed ancient trade and technological transmission. Our study disclosed the following pigments were applied in the ancient Tongcao painting.
3.3.1. The black pigment
Figure 4-a was the Raman spectrum of black pigment, where Raman peaks of 1324 and 1596 cm− 1 were basically consistent with carbon black []. As we know, Carbon black was black outside appearance with particle size of 30–40 µm, and was a kind of amorphous graphite, which was usually applied as black pigment in ancient China.
3.3.2. The white pigment
As shown in Fig. 4-d, the Raman peaks of 153, 280, 711 and 1085 cm− 1 was consistent with calcite [], while that of 1048 cm− 1 was the characteristic peak of lead white []. Therefore, white pigment was the mixture of calcite and lead white. Surprisingly, judging from the intensity of Raman shifts at 1048 cm− 1 and 1085 cm− 1, it was easily to conclude that the proportion of calcite and lead white used as the white pigments in different areas were different. As we know, lead white had better whiteness and coverage, so when painting the area of white dragon (Number 6), the artist specially selected the higher proportion of lead carbonate as the white pigment to protrude the dragon and meanwhile, more lead carbonate could reduce the interference effect of blue pigment under it.
3.3.3. The yellow pigment
Serial Raman peaks of 1431, 1595 and 1627 cm− 1 (Fig. 4-b) demonstrated the existence of gamboge [16] in different tested areas of Number 7, 8, 9&16. Gamboge was a kind of gelatinous resin secreted by plant of gamboge, mainly planted in India, Vietnam and Thailand. As a kind of yellow pigment, gamboge was used to paint the wooden objects unearthed from the tomb of Astana in Xinjiang province, which could date back to Tang Dynasty of China(618-907A.D) [].
3.3.4. The red pigment
Figure 4-f showed that the spectra of three detection points (Number 5, 12 & 14) were basically the same. Serial Raman peaks of 121, 151, 223, 314, 389 and 548 cm− 1 referred to red lead [16], whose main component was lead tetroxide (Pb3O4). The used of red lead could date back to Eastern Han Dynasty of China(25-220A.D), which was introduced from the West via the Silk Road []. Because there was extremely few red lead in natural minerals, red lead was generally produced by certain processes, no matter in ancient times or modern times. It was a kind of pigment often used in Chinese works of art, the color of which varied from orange to red depending on its purity [].
3.3.5. The blue pigment
Under microscopy, particles of blue pigment were bright and blue with the size of 5–10 µm. It was clearly seen in Fig. 4-c that Raman spectra of four detection points numbered 1, 2, 13 and 15 were basically the same, where Raman peaks of 259, 548, 808, 1096, 1355 and 1645 cm− 1 referred to ultramarine blue []. Nevertheless, there were two kinds of ultramarine blue: the natural form extracted from Lapis lazuli and the synthetic form firstly synthesized by Jean Baptiste Guimet in 1828, a French industrial chemist[]. In recent years, researchers had used SEM-EDS, FITR, µ-Raman and other methods to find out there were some characteristic peaks of 156, 283, 713 and 1086 cm− 1 belonging to that of the calcite component in blue particles of natural ultramarine, which were absent in synthetic ultramarine[]. Therefore, blue pigment of this painting was synthetic ultramarine.
Compared with natural ultramarine, the price of synthetic ultramarine was cheaper, so it was not surprising that the craftsmen would select such pigment in Tongcao painting, as they belonged to the export paintings of assembly line production. Besides, as its color was more gorgeous than azurite(a traditional Chinese precious mineral pigment), craftsmen in Guangzhou hence added it to their palette without hesitation. It originated in France and might be introduced to China by sea in later Qing dynasty, and in 1927 Chinese chemist in Nanjing university successfully synthesized ultramarine blue [].
3.3.6. The green pigment
The painting showed two kinds of green pigment: one was the bright green chair, and the other was the dark green leaves.
In the spectrum of green chair ( Number 3, Fig. 4-e), serial Raman peaks of 106, 119, 152, 172, 216, 240, 291, 323, 370, 431, 492, 538, 681, 755, 842, 950, 1438, and 1563 cm− 1 were consistent with emerald green[20]. As a kind of artificial pigment, emerald green consisted of fine particle with high purities, which was first synthesized in 1814[] and was brighter and durable than copper carbonate used at that time. However, it tended to fade and blacken when exposed to an atmosphere containing hydrogen sulfide, and meanwhile, the arsenic inside emerald green (Cu(C2H3O2)2·3Cu(AsO2)2) would be easily separated out in the wet air ,which made it extremely poisonous, so it was stopped to produce in 1950[].
Under microscopy, the darker green pigment was the mixture of blue pigment, a few green pigment and black pigment. Figure 4-g showed the Raman peaks of 278, 536, 2093 and 2156 cm− 1 of the blue particle were consistent with Prussian blue (Fe4 [Fe (CN)6]3) [20], which could also explained why the existence of high content Fe (2.39%) in darker green area by SED-EDS (Table.2), while two broad Raman peaks of 1336 and 1595 cm− 1 referred to carbon black. In addition, the SEM-EDS analysis revealed that this area contained a small amount of Cu (0.49%) and As (0.26%), so Raman peaks of 176, 217, 369 and 952 cm− 1 of the green particle would be related to emerald green.
Table.2 Semi quantitative analysis results of Number 11 by SEM-EDS
| Weight% | C | O | Al | Si | S | Cl | K | Ca | Fe | Cu | As | Pb |
| Area | 50.6 | 41.89 | 0.17 | 0.32 | 0.13 | 0.31 | 0.49 | 1.82 | 2.39 | 0.49 | 0.26 | 1.14 |
Prussian blue was a kind of synthetic blue pigment which had similar color with azurite. As early as 1704, Ghislain Diesbach revealed the manufacturing process of Prussian blue, which was later introduced to painters as a blue pigment. Until the mid-18th century, it had been widely used in European oil painting []. According to documentary records, British east India Company had been exporting Prussian blue from Britain to Guangzhou since 1775 []. Apparently, the clever craftsmen that time mixed the Prussian blue, a few emerald green and a little carbon black to paint green leaves, which looked somewhat dark green and formed a different visual effect from green chair.
As early as Nanyue Kingdom period (204–112 BCE), the connection had been established between Guangzhou and the world through the sea road, which could be reflected in the silver box of Persian style and the blue glasses from Western Asia that were excavated from the Western Han Nanyue kingdom mausoleum. Since Tang dynasty, Guangzhou had been regarded as one of the biggest harbor cities, which played an important role in promoting the cultural communication between China and western world. When it came to Ming & Qing Dynasty, more and more European arrived and carried out all kinds of business activity in Guangzhou, which could be recorded by lots of historical documents. In this process, Chinese goods including ceramic, tea and silk were numerously shipped and sold to European Countries, and on the other hand, many goods such as oil painting, glass and pigments were imported into Guangzhou and other cities in China.
The analytical result of pigments discovered both China’s domestic mineral pigments and plant dyes, and western synthetic pigments, were applied in the making process of this Tongcao paintings. Our study also revealed that multi- pigments with different concentration were mixed together to paint different areas in pursuing a certain kind of visual effect, which not only demonstrated the complexity of manufacturing process of Tongcao painting, but also witnessed the arrival and application of western pigments in Guangzhou.