Guangzhou Tongcao painting in late China Qing Dynasty 1840 — 1912 AD : technology revealed by Analytical Approaches


 Guangzhou Tongcao painting of China late Qing Dynasty witnessed the exchange of economy, culture, art and technology between China and foreign countries, which was an important exported product in the ancient China. In this paper, by using stereo microscope, Scanning Electron Microscope attached Energy Dispersive Spectroscopy and micro Raman Spectroscope, a Qing Dynasty Tongcao painting with the subject of family life of an official's female relative was analyzed. Our study revealed that: 1) the hexagonal cell morphology could be easily observed in the Tongcao pith which could contain much more pigment and increase the stereo sense of painting, and the cell of the ancient Tongcao pith had started to degrade, which indicated an urgent requirement on conservation of the tongcao painting; 2) alum was applied as the fixing agent in the preparation of pigments and the painting process; 3) both the China domestic pigments including red lead, calcite, lead white, carbon black and gamboge, and the synthetic pigments imported from Europe ( artificial ultramarine, Prussian blue and emerald green) was applied in the painting, which reflected the bi-directional communication of culture and technology between China and the western world.


Introduction
Tongcao ( ) painting, which was painted directly on Tongcao pith (the stem of Tetrapanax papyriferus without any chemical treatment), was a kind of exported watercolor painting. It was produced mainly in late Qing Dynasty of China (1840 in Guangzhou, and yet specially sold to European. As Tongcao painting imitated western painting style to some extent and vividly recorded nearly all aspects of local customs of Guangzhou at that time, it was favored by foreigners once it was created. Almost every foreigner would buy and bring it to homeland to share with his friends and families what they saw in Guangzhou before the invention of camera, so it also had another name of Guangzhou Postcard. As a representative exported painting from Guangzhou in the 19th century, Tongcao painting was a vivid re ection on the integration of trade, culture and art between China and the West along the Maritime Silk Road. Therefore, for a long time it was focused by experts in different elds, including history, archaeology, art and so on. As early as 1970s-1990s, American scholar Carl L. Crossman studied the creation and trade transmission of Tongcao painting [1 -2] . Since that time, more and more attention had been paid to this special kind of painting. In 2007, professor Jiang of Sun Yat-sen University tried to study Tongcao painting in the context of port history [3] , which aroused a tremendous interest on such painting among Chinese scholars. Later in 2008, professor Cheng focused on the manufacture technique of Tongcao pith and development of Tongcao painting in different periods [4] , whose research became the rst monographic study of Tongcao painting in China. Meanwhile, some western scholars carried a series of researches to explore the reparation and conservation of Tongcao painting from the usage of the new reparation material, treatment methods & speci c operation, and the control of the storing environment [5][6] . In 2014, basing on the images of Tongcao painting, Patrick Conner fully studied the style and features of Thirteen Hongs commercial area along the Pearl River in Guangzhou [ 7] . In the same year, Ifan Williams and Cheng Meibao cooperatively published a book, where all the Tongcao Paintings from 29 museums and galleries of the world were systemically sorted and classi ed according to their content and subjects [ 8] . Furthermore, Guangzhou Thirteen Hongs Museum (a museum collecting historial artifacts of Thirteen Hongs, which was the monopoly organization managing the foreign trade in Guangzhou appointed by Qing government) published a book to show abundant exquisite Tongcao paintings stored in its museum, which demonstrated a great deal of historical information of historical Guangzhou [9] .
Overall, the study of Tongcao painting is still in the early phase and principally relies on methods like Iconography and the analysis of historical documents, where scienti c studies are very rare. As Tongcao painting combines selection and cutting of Tongcao pith, treatment of alum, painting and other techniques, it partly re ects the technological complexity of folk painting in Guangzhou at that time. Given these considerations, the attempt to comprehensively analyze representative Tongcao painting samples from a technological perspective is required to disclose the materials and drawing procedure of such painting. Not only will these studies bring new insights into techniques that produce Tongcao painting, but they are also helpful to understand the technical decisions made by craftsmen basing on the background of economy, culture and art in late Qing Dynasty. Technical analyses on Tongcao painting have more practical impact that provides the scienti c foundation of preserving and restoring this kind of painting.
In this paper, we analyzed the historical Tongcao panting by using optical stereo microscope (OM), Polarized Light Microscope (PLM), micro confocal laser Raman spectroscope (μ-RS), and scanning electron microscope with energy dispersive X-ray spectrometer (SEM-EDS). Basing on the analyses, we attempted to reveal, from a technological perspective, what kinds of materials and how they were prepared in the whole manufacturing process of Tongcao painting. Meanwhile, we discussed the types of pigments (both China domestically produced and the imported) used in the painting, which could throw a light on exploring the cultural and technological communication between China and Europe. The historical Tongcao Painting was kindly provided by the Anthropology Museum of Sun Yat-sen University. It depicted a scene of a senior o cial lady of imperial court appreciating owers accompanied by a maid (Fig. 1), and the painting was applied with different kinds of colors including blue, green, yellow, red, white and black. Not only this painting was rich in colors and of exquisite image, meaning the complexity of manufacturing technique, but such kind of subject was universal in Tongcao painting and welcomed by foreigners at that time, which was a representative piece at that time. However, it was a pity that information on its production year appeared on the tag of the painting was absent, which brought di culty for us to judge its speci c production date of it. Therefore, it was generally identi ed to be an artwork of late Qing Dynasty (1840-1912AD).

Materials And Methods
The detection points of number 1 to 16 were selected for Raman spectrum analysis nondestructively ( Fig.  1), whose details could be seen in Table 1, including the areas and colors they referred to. The Tongcao pith produced in Guizhou province in 2015 was collected and cut into several pieces (respectively named as x 0 , x 1 , x 2 , x 3 ) for simulative experiments.
According to the records of The Painting Manual of the Mustard Seed Garden ( ) [10] , we prepared the solution of gelatin and alum in ratio of 7:3, which was then coated on x 0 for 0 times, x 1 for 4 times, x 2 for 8 times and x 3 for 12 times for the chemical analysis. Meanwhile, the unpainted piece (named as q 0 ) and coloring pieces (respectively named as q 1 and q 2 ) of the painting were served as compared sample.

Microscopy
A RH-2000 optical stereo microscope (Japan) with multiple range of 35-2500x was used to in situ and non-destructively observe the microscopic state of cells and pigments of samples. As con gured with Hirox software system, not only could this microscope capture the high-de nition image in real time, but it was able to synthesize the image in 3D.
A Polarized Light Microscope (ortholux11PPL-BK, Leitz, Germany) was used to check the purity, shape and size of blue particles which were separated from a fragment of Tongcao painting, helping to identify the composition of blue pigment.

Micro Confocal Laser Raman Spectroscopy
The Renishaw Invia laser Raman spectroscope (Gloucestershire, UK) was chie y employed to analyze phase structure of different pigments on the painting, which used argon ion laser as light source, with an excitation wavelength of 785nm, an objective lens of 50x, a spot size of 1μm and a spectral resolution of 1cm -1 . Raman spectra were recorded in wavenumber mainly between 100 and 3000 cm −1 , with spectral accuracy of about 1 cm −1 . An optical microscope was used to focus the laser on samples, at ×50, throughout the analysis. Calibration is carried out on the Raman spectrometer on a daily basis using the Raman signal of silicon at 520 cm −1 . Background spectra of water and carbon dioxide are obtained in ambient air. Raman spectra presented here were smoothed without baseline correction.

Scanning Electron Microscope with Energy Dispersive X-ray Spectrometer
Equipped with INCA X-ray spectrometer (EDX, Oxford, UK)), The Quanta-400F thermal eld environment scanning electron microscope (ESEM, Philips, the Netherlands) was used to analyze the micro structure and composition of samples, with the experimental voltage of 20kv and the mode of low vacuum, under which the pressure of sample chamber was 60Pa. By using this machine, the very small pieces fell down from the painting, as well as the Tongcao pith of 2015 were collected and analyzed.

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 Taiwan. In Qing Dynasty, Tongcao was recorded to be used in painting by craftsmen in Guangzhou.
As we know, Tongcao pith had not been treated by any chemical process, and its hexagonal cell structure could be clearly seen under the microscopy (Fig. 2), which differentiated from that of Xuan paper with numerous and various sizes of bers mixed together irregularity. Under the SEM-EDS, surface of Tongcao pith looked like the honey honeycomb, which was helpful to the stack of pigments and form an uneven surface for the distribution of pigments, so the Tongcao painting looked stereoscopic and colorful. Generally, the cell wall of plants mainly consisted of cellulose micro brils, lignin, hemicellulose and a few pectin as well as glycoprotein [11] . 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 historical 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.

The usage of alum in Tongcao painting
Alum glue, a kind of solution which mixes the alum and gelatin, is chie y used as an adhesive in the process of drawing, mounting and repairing of painting and calligraphy [12] , which on one hand can enhance the binding force among plant bers, and on the other hand, has the function of xing pigments, keeping painting bright-colored, 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 t to receive the colors. This wash is frequently repeated during the progress of the work, so that before it is nished, it has received seven or eight coats. It is di cult at rst to conceive the utility of the alum, but upon re ection it appears to me, that it is this mineral which gives such a degree of permanence to the coloring of the Chinese [13] .
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 historical painting.
The chemical formula of alum is KAl (SO 4 ) 2 ·12H 2 O. As shown in Table 2 and Fig. 3, the chemical difference among samples was on the elements Al and S. It was easy to nd 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 q 0 , q 1 and q 2 apparently exceeded than that of x 0 , relatively close to x 1 or x 2 , which proved alum was applied both in the unpigmented area and pigments area of the historical Tongcao painting.

the pigments
In China, the archaeological evidence revealed that as early as the Shan dingdong Man archaeological site (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 historical 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 arti cial synthesis pigments. In addition, pigment also witnessed historical trade and technological transmission. Our study disclosed the following pigments were applied in the historical Tongcao painting. basically consistent with carbon black [14] . As we know, Carbon black was black outside appearance with particle size of 30-40μm, and was a kind of amorphous graphite, which was usually applied as black pigment in historical China.

The white pigment
As shown in Fig. 4-d, the Raman peaks of 153, 280, 711 and 1085 cm -1 was consistent with calcite [15] , while that of 1048 cm -1 was the characteristic peak of lead white [16] . Therefore, two white pigments (calcite and lead white) were used both in these areas. 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 known, lead white had better whiteness and coverage, so when painting the area of white dragon pattern (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.

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 [17] . Fig. 4-f showed that the spectra of three detection points (Number 5,12 & 14) were basically the same.

The red pigment
Serial Raman peaks of 121, 151, 223, 314, 389 and 548 cm -1 referred to red lead [16] , whose main component was lead tetroxide (Pb 3 O 4 ). The used of red lead could date back to Eastern Han Dynasty of China, which was introduced from the West via the Silk Road [18] . Because there was extremely few red lead in natural minerals, red lead was generally produced by certain processes, no matter in historical 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 [19] .

The blue pigment
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 [20] . Nevertheless, there were two kinds of ultramarine blue: the natural form extracted from Lapis lazuli and the synthetic form rst synthesized by Jean Baptiste Guimet in 1828, a French industrial chemist [21] . In recent years, researchers had used SEM-EDS, FITR, μ-Raman and other methods to nd out there were some characteristic peaks of 156, 283, 713 and 1086cm -1 belong to the calcite component in blue particles of natural ultramarine, which were absent in synthetic ultramarine [22] . To further support this, we separated some blue pigments from a fragment of this painting, treated them with absolute alcohol and ltered out the scattered blue particles on glass slides. Under the polarized light microscope, the majority of particles were round and homogeneous with diameter of 2-4μm, and meanwhile, they were pure in deep blue color (Fig. 5), whose characteristic were closer to synthetic ultramarine [23] .
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 exported 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 late Qing Dynasty, and in 1927 Chinese chemist in Nanjing university successfully synthesized ultramarine blue [24] .

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 [20] . As a kind of arti cial pigment, emerald green consisted of ne particle with high purities, which was rst synthesized in 1814 [25] 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 sul de, and meanwhile, the arsenic inside emerald green (Cu(C 2 H 3 O 2 ) 2 ·3Cu(AsO 2 ) 2 ) would be easily separated out in the wet air ,which made it extremely poisonous, so it was stopped to produce in 1950 [26] .
Under microscopy, the darker green pigment was the mixture of blue pigment, a few green pigment and black pigment. Fig. 4-g showed the Raman peaks of 278, 536, 2093 and 2156 cm -1 of the blue particle were consistent with prussian blue (Fe 4 [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 3), 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. 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 [27] . According to documentary records, British east India Company had been exporting prussian blue from Britain to Guangzhou since 1775 [28] . 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.
Results of Raman analyses of the above pigments could be summarized in Table 4: 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 re ected 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.
Since late Qing Dynasty, imported pigments began to be introduced into China at a large scale. At present, it was reported that synthetic ultramarine had been found in Mogao Grottoes of Gansu [29] , Cizhong Catholic Church of Yunnan [22] , Wuhou Memorial Temple of Chengdu [30] as a blue pigment of architectural painting, murals and painted sculpture. Emerald green, however, was mainly served as a decorative color of architectural painting in the process of construction and restoration. Besides the Cizhong Catholic Church mentioned above, archaeologists also found the use of emerald green in Drum Tower of Xi'an [31] , Zhendu Men [32] and Wuying Hall [33] of the Imperial Palace in Beijing. Moreover, it was worth noting that a portrait painting of Taoism gure in late Qing Dynasty also used synthetic ultramarine and emerald green [34] . Compared with these pigments, the use case of prussian blue seemed to focus on painting in China. It was reported that prussian blue was used in Zhenhai Tower [28] , a Qing Dynasty oil painting for export, and glass paintings collected by Palace Museum [35] in Beijing.
These cases revealed that in late Qing Dynasty of China, craftsmen had consciously used imported synthetic pigments to partly replace traditional mineral pigments like azurite and malachite. As a kind of folk painting art at that time, pigments applied in Tongcao painting also accorded with such development trend. The analytical result of pigments in our studies revealed both China's domestic mineral pigments and plant dyes, and western synthetic pigments, were applied in the making process of this Tongcao paintings. We also discovered 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.

Conclusion
There are several signi cant points to be concluded: 1) The cells of Tongcao pith was hexagonal and hollow, which was connected together through cell walls to form a relatively stable honeycomb structure. This special structure was helpful to the stack of pigments and form an uneven surface for the distribution of pigments, which might be the direct reason for strong stereoscopic effect of Tongcao painting. Meanwhile, the cell of historical Tongcao pith has started to degrade, which indicated an urgent requirement on conservation of Tongcao painting.
2) This Tongcao painting was supposed to be treated by alum up to 8 layers in the painting process, which con rmed the truth of Downing's travel notes.
3) With the growing frequency of communication between China and the West in Qing Dynasty, both western synthetic pigments, and China's domestic mineral pigments and plant dyes, were applied in the making process of this Tongcao paintings. In addition, multi-pigments with different concentration were mixed together to paint different areas in pursuing a certain kind of visual effect. These not only demonstrated the complexity of manufacturing process of Tongcao painting, but also witnessed the arrival and application of western pigments in Guangzhou. Meanwhile, The existence of synthetic ultramarine, prussian blue and emerald green was helpful to further identify the production date of this painting to late Qing Dynasty, mainly the mid-late 19 th century.