Analyses of digital microscope and SEM-EDS
From SEM micro morphology of the red surface layer (Fig. 2a, 600× magnification), it has been seen that the particles are fine, but there is a large number of cracks, which indicates that the coloured decoration surface has undergone a certain degree of aging. The cross-sections from SEM (Fig. 2b, 100× magnification) and the digital microscope analyses (Fig. 2c, 200× magnification) show that there are four layers. The first, second, and third layers are ground layers. This means there are three putty layers whose colour is earthy brown with different shades of darkness. The fourth layer is the red surface layer.
In terms of the graininess of each layer, the first and second layers are coarser, the third layer is finer, and the fourth layer (surface layer with red colour) is the finest. This shows that the ground layer was applied three times using materials of increasingly finer particle sizes from the ground to the surface layer, and the red surface layer contained red pigment of extremely fine particulate matters. In terms of the thickness of each layer, the first layer is relatively thick, approximately 330µm; the second layer is approximately 150µm; the third layer is approximately 140 µm; and the fourth layer is approximately 40µm. This indicates that the thickness of the applied materials gets increasingly thinner. From the cross-sectional view, the sample is curved in the horizontal direction, confirming that its decorative layer of the plaque has warped.
The EDS result for the red surface layer is shown in Fig. 3. The contents of C and O are relatively high (weight %, 32.33% and 14.42%, respectively), and the total content of C and O is 46.75%. By deduction, it may contain organic substances. The content of S is 8.75%, and the content of Hg is very high (43.36%). The total contents of S and Hg is 52.11%. By deduction, the red pigment may be cinnabar. In addition, the content of Ca is 1.24%.
Xrd Analysis
To identify the inorganic substances in the plaque, XRD analysis was conducted on its surface and ground layer. The X-ray diffraction diagram (Fig. 4) of the red surface layer shows that the diffraction peaks at 2θ of 27º(vs), 29º(s), 32º(vs), 39º(w), 45º(s), 46º(w), 47º(m), 53º(m), 54º(m), 56º(m), 59º(w), 60º(w), 66º(w), 71º(w), 73º(w), and 76º(w) belong to cinnabar (HgS, Fig. 4, JCPD:99 − 0031). The diffraction peaks of the samples match well those of cinnabar, which shows that the cinnabar used has a good crystal form and high purity.
As shown in Fig. 5, the diffraction peaks of the ground layer almost match the diffraction peaks of brick powder [7] at 2θ of 21º(s), 26º(vs), 28º(s), 36º(w), 39º(w), 41º(w), 42º(w), 45º(m), 50º(m), 55º(w), 60º(w), and 68º(w). This shows that the ground layer contains brick powder. The diffraction peaks of the ground layer at 2θ of 29º and 48º are not found in the diagram of brick powder. They may be from calcite (CaCO3, Fig. 5, JCPD:05-0586), and the weak diffraction peaks represent its low content.
µ-ftir Analysis
The µ-FTIR analyses of the red surface and ground layer of the sample were carried out to identify the organic substances. The µ-FTIR result of the red surface layer compared with those of raw tung oil [7], boiled tung oil, and the current Chinese lacquer [8] (Fig. 6) shows the following: the absorption peak of the sample at 3296 cm− 1 belongs to the stretching vibration of OH [9]. The absorption peaks at 2921 and 2850 cm− 1 belong to the asymmetric stretching vibration and symmetric stretching vibration of methylene (-CH2) [10]. The absorption peak at 1723 cm− 1 belongs to the stretching vibration of C = O [11]. The strong absorption peak at 1651 cm− 1 belongs to the C = C stretching vibration of benzene rings [12]. The absorption peak at 1542 cm− 1 belongs to the stretching vibration of C = O. The absorption peak at 1459 cm− 1 belongs to the asymmetric deformation vibration of CH3. The absorption peak at 1253 cm− 1 belongs to the stretching vibration of C-O. The absorption peak at 1075 cm− 1 belongs to the stretching vibration of C-O and C-C. The absorption peaks at 990 cm− 1 and 965 cm− 1 belong to the wagging vibration of C-H conjugated double bonds. The weak absorption peak at 727 cm− 1 belongs to the rocking vibration of CH2.
The µ-FTIR analysis of the red surface layer shows that the absorption peaks exist at wavenumbers of 3296, 2921, 2850, 1651, 1459, 1278 and 1075 cm− 1, deducing that there is Chinese lacquer. The absorption peaks also exist at 2921, 2850, 1723, 1542, 1459, 1253, 990, 965, and 727 cm− 1, deducing that there is tung oil which is a traditional material in Bashu region of china. The weak absorption peak at 1542 cm− 1 is the peak that differentiates raw from boiled tung oil. This may be caused by the stretching vibration of C = O under C = C conjugation after the oxidation of tung oil. The absorption peak of the sample at 1723 cm− 1 being stronger than that at 1651 cm− 1 is a typical characteristic of tung oil being added to Chinese lacquer [13]. The absorption peak of tung oil at 1741 cm− 1 and the absorption peak of Chinese lacquer at 1625 cm− 1 affect each other, moving the absorption peaks of the sample to 1723 cm− 1 and 1651 cm− 1. The absorption peak of Chinese lacquer at 3428 cm− 1 moves to 3296 cm− 1 under the influence of the absorption peaks of tung oil at 2921 cm− 1 and 2850 cm− 1. Therefore, the red surface layer is a lacquer film layer containing Chinese lacquer and boiled tung oil.
Generally, the intensity of infrared bands obtained using the KBr tablet method is stronger than that obtained using ATR-FTIR [14]. At high wavenumbers in the range of 3600–3000 cm− 1, ATR-FTIR absorption peaks decline greatly [15]. The infrared spectrum for the current Chinese lacquer was obtained using the KBr tablet method, whereas the spectrum of the “Tian Di Chang Chun” horizontal plaque was obtained using µ-ATR-FTIR. Therefore, the absorption peak of the sample around 3296 cm− 1 is significantly weaker.
The infrared spectra of the ground layer of the sample, youman, and a blood product [7] are shown in Fig. 7. The main component of blood is protein. The wide absorption peaks at 3343 − 3147 cm− 1 belong to the stretching vibration of the N-H in protein. The absorption peak at 1630 cm− 1 belongs to the stretching vibration of C = O in amide bonds. The absorption peak at 1514 cm− 1 belongs to the deformation vibration of C-N. Comparing the infrared spectra of the sample, youman, and blood, it is deduced that the absorption peaks of the infrared spectra of the ground layer at 2921, 2850, 1741, 1542, 1459, 1157, and 990 cm− 1 belong to youman, which consists of flour, lime water, and boiled tung oil [7]. In the youman component, lime water absorbs CO2 from the air to form calcite, which is consistent with XRD results. The absorption peaks at 3296, 1630, and 1514 cm− 1 belong to the blood product, which is usually added to the ground layers of coloured drawing in ancient Chinese buildings to increase intensity. In summary, the ground layer may contain the blood product and youman.
Analyses Of The Coloured Decoration Materials And Techniques
From the analysis results above, the decoration layers of the horizontal plaque inscribed by General Feng Yü-hsiang include a Chinese lacquer film layer and a ground layer. The Chinese lacquer film layer is on the surface of the plaque and it is red. Its cementitious material is a mixture of Chinese lacquer and tung oil. Its pigment is high-purity cinnabar, whose particles are fine and crystal forms are good. The ground layer is made of the blood-product putty made of blood, youman, and brick powder. The putty particles used during the first two times are coarser. Fine-particle putty was applied at the last time. The particles are fine and were applied evenly and thinly to flatten the surface in preparation for the application of the red lacquer film.
The lacquer film layer of this plaque contains Chinese lacquer, tung oil and cinnabar. Therefore, the technique is a form of a coloured-lacquering technique. Since ancient times, the lacquering technique, including coloured-lacquering technique, has developed into a self-contained material technology. (1) Chinese lacquer is a good natural resin adhesive as expressed in the idiom of “like glue and lacquer”. Chinese lacquer has strong fusion capability and permeability. Using this property, Chinese lacquer is mixed with pigment to be applied onto objects. During the solidification into film process of Chinese lacquer, its urushiol, laccase, polysaccharide, glycoprotein, water, and other components undergo complicatedly biocatalytic oxidation polymerisation [16]. The formed Chinese lacquer film has good resistance to water, heat, acid, alkali, and corrosion, and has other properties such as antibacterial properties. It is both gorgeous and protective. (2) Tung oil is an excellent dry vegetable oil with fast-drying, lustre, heat, and corrosion resistance properties. Regardless of how Chinese lacquer is made, it cannot be as transparent as water. It also forms a film slowly and takes a long time to dry and change colour. Therefore, the addition of tung oil is beneficial to the transparency of Chinese lacquer. In addition, the Chinese lacquer modified by tung oil has a good air-drying property and flexibility, and makes the painting process smooth [17]. It also enhances the lustrous effect of the Chinese lacquer film on the surface, giving a better painting effect. For example, objects painted with red Chinese lacquer combined with tung oil look more festive. Once Chinese lacquer and tung oil are mixed, the mechanisms of polymerisation and oxidation reactions of the mixture have been changed. The oxidation degree of tung oil decreases while its net structure increases [18]. However, the firmness of the Chinese lacquer-tung oil mixture is not as good as Chinese lacquer alone. The paint falls off easily if only tung oil is used to make the paint [19]. (3) Cinnabar is a common red pigment with a bright red colour. In a long period of Chinese history, cinnabar was the first choice for red pigment. Ancient people believed it was spiritual and could ward off evil. According to traditional Chinese medicine records throughout Chinese dynasties, cinnabar is believed to have the functions of clearing the heart, calming, improving eyesight, and acting as an anti-insect and an antiseptic [20].
The ground layer of this plaque contains youman, the blood product, and brick powder. The substances containing the blood product in ground layers in coloured drawing in ancient Chinese buildings are usually referred to as the blood putty. Its techniques include a ground layer with linen or cotton and a ground layer without fabric materials. As there is no linen or cotton in the ground layer of this plaque and the ground layer was applied three times, the ground layer technique belongings to the SDH(San Dao Hui, that is the ground layer applied three times) technique without fabric materials, one of the typical the blood putty [21]. They are a inorganic-organic composite that is similar to sticky rice-lime mortar used in ancient Chinese city walls, water facilities, tombs, etc.[22]. (1) The blood is usually made of pig blood, cow blood, sheep blood, etc. Among these blood products, the blood products fermented from pig blood have the best quality. The blood products made of cow and sheep blood, which are slightly less viscous, have comparatively lower quality. Generally, cow blood is used for halal ground layers. The blood products have high fibrin content [23]. (2) Brick powder is the powder for bricks. Ancient Chinese used local clay to make bricks. Bricks are made of an inorganic silicate material with stable chemical properties. They are a common material for construction for building houses, city walls, water facilities, towers, bridges, dams, wells, mausoleums, etc.[24]. (3) The scientific principle for using the blood putty is that during the fermentation of pig blood, the fibrin reacts with the calcium ions in brick powder. Carboxylation in biochemistry happens. The gel that is formed exhibits an amphiphilic property where its hydrophobicity causes it to combine well with tung oil, while its hydrophilicity causes it to combine well with pigment [25]. The use of the blood putty on the ground layer not only protects the wood from corrosion, but it is also good for keeping the coloured surface layer flat and beautiful.
The Chinese lacquer, the blood product, tung oil, cinnabar, and bricks used for this plaque have a long history of application in China. Natural Chinese lacquer is one of the earliest polymer materials used by humankind. It can be traced back to the Neolithic Age. At the Kuahuqiao site in Xiaoshan, Zhejiang, 8,000 years ago, early humans used natural Chinese lacquer as a coating and an adhesive for boats and pottery [26]. The use of blood and tung oil on ancient lacquerware can be traced back to the Warring States period. In the coloured surface layer of the ear cup lacquerware unearthed from the Chu Tomb in Jiuliandun, Zaoyang, Hubei, a mixture of tung oil, linseed oil, and perilla seed oil was found, and blood was found in the bottom Chinese lacquer layer [27]. Cinnabar first appeared in the coloured potteries in the Neolithic period. It was found in many places at the Dadiwan site in Qin’an, Gansu, dating to about 7,000 years ago. A Chinese lacquer bowl was found in the third cultural layer of the Hemudu site of the Neolithic Age in Yuyao, Zhejiang, 6,600 ± 300 years ago. A layer of red paint on the outside of the bowl was identified to contain cinnabar and Chinese lacquer [28, 29]. Brick is the fundamental material for ancient Chinese buildings. Archaeological excavations confirmed that the earliest bricks discovered so far were unearthed at the Yangshao cultural Xinjie site, Lantian, Shaanxi [30].
From historical documents, the earliest handicraft book “Zhou Li·Kaogong Ji” (in the Spring and Autumn Period and the Warring States Period) recorded: the metal spoon that was used to pour (scoop) wine in ancient worship ceremonies for mountains and rivers was decorated with turquoise on the outside and red Chinese lacquer on the inside [31]. The “Han Fei Zi•Shi Guo” by Han Fei, a thinker at the end of the Warring States Period, recorded, “Yu made sacrificial artifacts with black Chinese lacquer on the outside and red Chinese lacquer on the inside” [32]. Together with the red wooden Chinese lacquer bowl unearthed from the Hemudu site, it can be seen that red is one of the most traditional colours of Chinese lacquering technique. The only surviving ancient Chinese lacquer technique book in China, “Xiu Shi Lu” (by Huang Cheng in the Ming Dynasty, and annotated by Yang Ming in the Ming Dynasty), clearly recorded the use of pig blood, brick powder, manshui (also known as youman) as the Chinese lacquer ground layer materials [33]. It can be deduced that the lacquering process of lacquerware has been fixed latest in the Ming Dynasty. Experiments have confirmed that the oil decorations on the pillars of the Bogda Khan Palace Museum in Mongolia in the Qing Dynasty used the one-linen-five-putty ground layer technique, where the red pigment was cinnabar, and linen fibre was used in the ground layer together with flour, blood, and brick powder [34].
In 1936, China was in war. The economy was depressed, and people’s living conditions were dire. The lacquerware techniques declined. However, this horizontal plaque inscribed by General Feng Yü-hsiang still used the already fixed traditional technique of the ground layers, the SDH layer technique. It was made of Chinese lacquer, the blood product, tung oil, and other materials. This shows its preciousness and reflects people’s respect and love for General Feng Yü-hsiang.