The color of black, red, white, green, and blue of the architectural drawing are measured by Raman and ED-XRF spectroscopy. The spectra of the pigments are successfully obtained. The detailed results are shown in the subsequent sections.
Black pigment
In the black areas, all the measured points give alike Raman signals. Two typical spectra collected from P14 and P3 are shown in Fig. 4a and 4b. There are two broad Raman bands at around ~ 1333 and ~ 1588 cm− 1. These two bands are assigned to carbon black [8, 9]. The XRF spectra of the black area presented in Fig. 4d indicates that Ca, Fe and Pb elements are observed which is in accordance with the elements of the ground layer (Fig. 4c). The component of the black pigments should contain light elements (such as C element) which are beyond the detection limit of the instruments. According to the results of Raman and ED-XRF analysis, it is surely that carbon black was used as a black pigment in the architectural drawings. The Carbon black used usually comes from the incomplete combustion products of organic material such as plants, oil, and bones. In ancient China, carbon black is the main black pigment in the artworks. It has been found that carbon black was used as black pigments in the pottery figurines of the Han Dynasty, the murals of Mogao Grottoes, the architectural drawings in Forbidden City [1, 10]. Even now, it is also used as a black pigment in calligraphy, advertising, and drawings. The extensive and long-term application of carbon black is mainly because of its stable physical and chemical properties, and a rather low cost. Therefore, it is thought carbon black is used as the black pigment in the architectural drawings.
White pigment
i
Figure 5 The Raman spectra and ED-XRF of the white area a) P6, b) P9, c) ED-XRF of ground layer, d) ED-XRF of the white layer
The typical spectra obtained from the white area is shown in Fig. 5a and 5b. They both give only one Raman band at 1049 cm− 1 which is assignable to ν(CO32−) modes in lead white (PbCO3) [11–13]. It is deduced that white lead is used as the white pigment. For further confirmation, the ED-XRF spectra of the corresponding white area and the ground layer have been collected and are presented in Fig. 5d. Ca, Fe and Pb elements are found. By comparing with the spectra of the ground layer (Fig. 5c), it can be seen that the amount of Pb element in the white area is much higher than the amount of Pb element in the ground layer. It means that the components of the white pigments contain Pb element. The white pigment which contains the Pb element is lead white. These results are in keeping with the result of the Raman analysis. Meanwhile, S and Ca element is also observed at the white color region from ED-XRF spectra. and the amount of Ca element is higher than the amount in the ground layer, so it is deduced that there is calcium sulfate in the white area. However, the Raman spectra in the white color region haven’t given the corresponding bands of calcium sulfate, it is evident that there is a very small amount of calcium sulfate in the white area. White lead as a white pigment is an ancient pigment which was widely used in murals and architectural drawings, such as the 17th -century panel painting, medieval manuscripts [14, 15].
Blue pigments
As presented in Fig. 6, spectra collected from blue areas (P7, P10, and P13) can be divided into two series. In Fig. 6a, the Raman bands at 246, 540, 594, 670, 1574 cm− 1 are found and can be ascribed to indigo (C16H10N2O2) which is derived from the plant Indigofera tinctoria and related species [16, 17]. In Fig. 6b and 6c, only one peak at 1574 cm− 1 are presented and can be also attributed to indigo. The XRF spectra of the blue area presented in Fig. 5e indicates that no new elements are found, except for Ca, Fe, and Pb elements which are also found in the ground layer. It demonstrates that the pigment used in the blue areas is plantal pigment which contains light elements (such as C, H, O, and N elements) beyond the detection limits of the instruments. Combining with the results of Raman analysis, it is surely that indigo is used as a blue pigment in the architectural drawings. Indigo, as an ancient pigment, found in many arts, such as Mediaeval Latin manuscripts, the Maya wall paintings. In China, indigo was usually used to dye the clothes, like the Western Han silk found in Changsha Mawangdui.[18] Even now, it is still be used to dye the jeans. Indigo was also found in some architectural drawings. In Forbidden City, Yong Lei has found that indigo was used as the primary blue pigment in the architectural drawings of Jianfu Palace which was constructed at A.D. 1742 and restored in A.D. 1802 [19]. After restoration, it is found that smalt was adopted to replace indigo as a blue pigment in the architectural drawings. Therefore, it is deduced that indigo was often used as blue pigments in the architectural drawings of royal architecture before smalt was widely used in China after A.D. 1802. The Altar of Agriculture, also as a royal architecture, was very likely to adopt indigo as blue pigment and these architectural drawings were painted at least before A.D. 1802.
Red pigments
In the red areas, the measurements were carried out in four typical colored areas (P1, P7, P10, P13). The P1 and P7 present red color while P10 and P13 present red color. The Raman spectra of these four points are presented in Fig. 7. Raman bands at 122, 142, 392, and 549 cm− 1 are observed in P1 and P7 (Fig. 7a and 7b). Bands at 122, 392, and 549 are ascribed to red lead [20]. The band at 142 cm− 1 belongs to massicot (PbO) which is a yellow pigment [21]. In P10, bands at 142 and 288 cm− 1 are ascribed to massicot (Fig. 6c) while bands at 122, 392, and 549 cm− 1 are attributed to red lead. The band at 142 cm− 1 is stronger than the band at 122 cm− 1. It is thought that the amount of massicot in P10 is higher than in other measured areas (P1 and P7). In P13, Raman bands at 122, ~ 145, 314, 392, and 549 cm− 1 are observed (Fig. 4d). Bands at 122, 314, 392, and 549 are attributed to red lead. The bands at ~ 145 cm− 1 can be divided into two weak peaks, as shown in the insert Figure, 142 and 149 cm− 1. The band at 149 cm− 1 is also attributed to red lead while the band at 142 cm− 1 belongs to massicot (PbO). Raman analysis demonstrates that red lead and massicot are the pigments used in the red areas of the architectural drawings.
From the analysis of XRF spectra in the red areas and the ground layer (Fig. 7f and 7e), the relative amount of Pb element in the red area is much higher than the relative amount in the ground layer. It reveals that the red pigment is lead-based pigments. This result is in keeping with the result of Raman analysis.
Which pigment is the primary pigment in the red areas? Red lead was often found in the architectural drawings, such as the Drum Tower (Xi’an, China) which was built in 1380 A.D. [1], the Jingfu palace, and Yangxin Hall in the Forbidden City [22, 23]. Red lead, also called minium, its usage was dating back to the fifth century BC in China. It was a widely used pigment in ancient China. For instance, red lead has been found in Mogao Grottos in Dunhuang, Gansu province in China, which is the famous World Heritage Site [12]. Meanwhile, red lead was also used to print the stamps in the later Qing Dynasty [24]. Up to now, massicot hasn’t been found in the red areas of ancient Chinese architecture. So, it is thought that massicot is the impurity in the red areas. In the ancient productive process of red lead, the reaction temperature was very difficult to control accurately. When the reaction temperature exceeded ~ 520℃, red lead would transform into massicot. In summary, red lead is the red pigment while massicot is the impurity in the red areas.
Green pigment
As shown in 2, the Raman spectrum on the green area has not been measured successfully due to very strong fluorescence. The ED-XRF spectra of the ground layer and the green area are presented in Fig. 7a and Fig. 7b respectively. Except for the Ca, Fe, and Pb elements which are also observed in the ground layer, Cu and Cl elements are found. Therefore, it is thought that the green pigments should contain Cu and Cl elements. Generally, three typical green pigments contain Cu elements used in the ancient arts, namely malachite (CuCO3·Cu(OH)2), atacamite (Cu2(OH)3Cl), and green emerald (Cu(C2H3O2)2·3Cu(AsO2)2) [25]. According to the elemental composition of these three pigments, only atacamite contains Cl element. It is deduced that the green pigment used in the architectural drawings is atacamite. Atacamite has been widely used in the wall drawings of Mogao Grottos and the architectural drawings of Dagaoxuan Temple in the Imperial Palace. However, the architectural drawings of the late Qing dynasty had adopted the artificial pigment (emerald green) as the green pigments due to the excellent properties and low cost. Emerald green has been found in the Renshou Hall (restored in A.D. 1850) and the drawings from Jiayu Pass and other northern Chinese ancient architectures [25, 26]. So, it is supposed that emerald has been widely used in China since the 1850s which atacamite wouldn’t be used as green pigments. Therefore, according to the find of atacamite, it could be deduced that these architectural drawings in the Sacrificial Storehouse had been painted at least before 1850 which is coincident with the results for the blue pigment.
In summary, with the help of portable Raman spectroscopy and Hand-held ED-XRF, the pigments used in the architectural drawings are successfully identified. The detailed results are listed in Table 1. According to the history of pigments application in architectural drawings with the historical record about the Altar of Agriculture, it is much more likely that these architectural drawings were drawn during the renovation in the eighteenth year of the Qianlong period of Qing Dynasty (A.D. 1753).
Table 1
Samples analyzed, their featured Raman bands, chemical names/formula
Colors | Raman bands (cm− 1) | Chemical names/formula |
Red | 121, 542, 388 | Red lead, Pb3O4 |
145, 285 | Massicot/litharge, PbO |
Blue | 247, 540, 594, 670, 1308, 1460, 1573 | Indigo, C16H10N2O2 |
White | 1047 | White lead, PbCO3∙Pb(OH)2 |
Black | ~ 1333, ~ 1580 | Carbon black |
Green | No | Atacamite, (Cu2(OH)3Cl) |