The Qianlong Emperor’s order: scientific analysis helps find French painted enamel among Palace Museum collections – Heritage Science

Metal matrix

The XRF analysis results of the metal matrix in the four pots are shown in Table 1. Here, the element analysis was performed using the XRF method. This method is only a semiquantitative analytical progress and has two limitations. The first is that light elements such as Mg, Na, B or even Al cannot be measured accurately. The second limitation is that XRF measurements are not precise about the volume analyzed, which is very different depending on the element measured. The penetration depth of the light elements (Mg, Al, Si, K, and Ca) is close to the surface, while it is more than several hundreds of µm for heavier elements such as lead (Pb) and gold (Au) and several mm for tin (Sn) and antimony (Sb) [31]. In this paper, only the apparent compositions were discussed.

The element analysis results show that pot A features obvious differences when compared to the other pots. The metal parts of Pot A contain 91% gold, 4.3% copper and 4.1% silver on average, and it is basically a golden painted enamel pot. The detection of Hg elements supports that while the main metal matrix for the other pots is copper, the surface of the copper is gilded with gold; in ancient China, it is common to use gold amalgam to gild metal with gold. In this case, craftsmen usually cover the metal with a mixture of Hg and Au; after heating, the Hg evaporates, and gold remains on the surface.

Structure

To comprehensively investigate the structure of each painted enamel pot, X-CT was performed (Fig. 2A, A-1, C, C-1). CT scanning was necessary because it could be seen from the outside that there were differences in the structures of the four pots, for example there was a small screw on the handle of the pot A, this difference indicates the probability of the existence of different character inner strcture; however, more characters were covered by enamel. X-ray perspective images gave interesting results about the differences between pots (Fig. 2A-2, A-3, A-4, C-2, C-3, C-4). As with the comparison result of the metal matrix, the comparison of the structure of the four pots set pot A apart from the other three pots. The results are shown in Fig. 2. There were two main differences. First, it can be clearly seen in the X-ray photos that the handle of pot A was fixed on a predesigned protrusion of the pot body with screws (Fig. 2A-3), while in the other three pots (represented by pot C), the handles were directly welded onto the pot body. From a usage point of view, the fixation on pot A looks more durable. The second difference is found in the lids of the pots. As seen in Fig. 2A-4, the lid in pot A has a delicate design, leaving an exposed metal part on the top as decoration. Although it is not clear whether decoration was the only purpose of this design, it is a stunningly exquisite design. The lids in pot C, B and D were created using integrated casting, as shown in Fig. 2C-4. During the process of CT scanning the lid parts of Pot A, hidden information was discovered, and these results are discussed later in the article.

CT scanning results for the four pots. (A) shows pot A and its corresponding overall CT (A-1). Pots B–D share similar structural characteristics, so in (C), pot C is chosen as the representative. (C-1) is the overall CT result for pot C. (A-2) is a detailed picture of the handle in pot A. (A-3) is the CT scanning picture of the handle in pot A. (C-2) is a detailed picture of the handle in pot C. (C-3) is the CT scanning picture of the handle in pot C. (A-4) CT scanning of the lid in pot A, while (C-4) CT scanning of the lid in pot C

The result for Pot B is chosen as the representative in B, while the Raman result for pot A is shown in A

A The pot in which the hallmark was found. This pot was most likely made in Paris, France. B X-CT image shows the hallmark located in the lid inner edge. C, D and E The three hallmarks found in that area. The meaning of these hallmarks has been clearly recorded in the archives of the Paris Goldsmiths Guild (F) and (G) show corresponding images found in the reference

Glaze and pigments

The element analysis results for the glaze and pigments are shown in Table 2. It is clear that the glaze on pot A is a Pb-rich type glass (21% on average), with a small amount of K and Ca (1.5% and 0.8%, on average). In contrast, the glazes on the other three pots are potash-lead glass, with contents of Pb and K of approximately 30% and 10%, respectively (with less than 1% Ca).

To determine the coloring agents in the enamels, Raman spectra were used to detect the crystal phase in the glaze that presented the color. The results are shown in Fig. 3. Pots B, C, and D share almost the same Raman results, so the result for Pot B is chosen as the representative for all three pots in Fig. 3B, while the Raman result for pot A is shown in Fig. 3A. In the table, the italic number emphasized the coloring agents in each enamels.

The greatest differences that could be found between pot A and the other three were in the white and yellow pigments. It can be seen from the Raman spectrum results (Fig. 3) that the white color on pot A was obtained from the opacification effect of tin dioxide (SnO₂) crystals in the glass, with symbolic peaks of 633 cm⁻¹ and 775 cm⁻¹, and that the white color on pots B, C and D was obtained by the opacification effect of lead arsenate crystals, which could be confirmed by the presence of a peak between 810–821 cm⁻¹. Chinese and foreign painted enamels obviously privilege the use of these two kinds of white pigments. Until now, in Chinese painted enamels, only lead arsenate was found to produce the color white, while European craftsmen preferred to use tin dioxide to create white color in painted enamels. For example, according to a publication, the use of cassiterite opacification was typical of Coteau’s work [32], but the conclusion is not that firm because some rare objects made in China (but under Jesuit guidance) were opacified with SnO₂ [14, 20]. Chinese craftsmen have used lead arsenate to paint white color on cloisonné since the late Ming and early Qing Dynasties in the seventeenth century [15, 16]. In terms of European white pigment technology, the opacification of cassiterite glass predates the Islamic period; it was used under the Roman Empire early in the fifth century and even earlier before that in the Celtic world, from the second to the first centuries B.C. [33, 34]. Compared with the common application of white tin in painted enamel and ceramic ware, arsenatewhite arsenic was used later in Europe and was mainly applied to glass products. From the 17th to nineteenth centuries, Venetian and Murano glass craftsmen used arsenic white crystals to make opaque glass [18]. It seems that the use of arsenic white in painted enamel was not found in Limoges until the nineteenth century [3, 30].

The Raman results (Fig. 3) show that the yellow pigment used in pot A is Pb–Sn-Sb yellow, which is characterized by the elements lead, tin and antimony. It can be seen from the Raman spectra in Fig. 3A that peak 134 cm-1 corresponds to the Pb–O lattice mode, and the 508 cm-1 peaks were characteristic of Sb-rich Naples yellow. The yellow pigments used in pots B, C and D were lead tin yellow. This result is also consistent with elemental analysis, and the Raman results of strong peaks at 333 cm-1 and 450 cm-1 also provided evidence of Sn-rich compositions. The two kinds of yellow pigments show obvious differences in color, as yellow containing antimony appears more orange. Yellow pigment rich in antimony was not introduced from abroad until the Kangxi period in the late seventeenth century [19]. However, before the sixteenth century and after the seventeenth century, the most commonly used yellow pigment in enamel was lead tin yellow, both in Chinese cloisonné and painted enamels.

The dark blue in the four pots is colored by cobalt ions, but it is worth noting that the dark blue in pot A contains a trace of arsenic. Based on the belief that arsenic white was not used, the arsenic here most likely comes from the cobalt material itself, which produces the blue color; the character of the existence of arsenic in cobalt raw material is very consistent with the characteristics of European cobalt ores [20]. Regarding pots B, C, and D, Table 2 shows that the As content in the blue enamels is always higher than those in all the other colors, except, of course, for the arsenic-opacified white. This means that an As-bearing chromophore may have been used for the blue color. At the same time, the Mn content seems to always be below the detection limit of the XRF equipment; both of these characteristics are compatible with the chemical composition of European smalt and completely inconsistent with the chemical composition of Asian cobalt ores sourced locally [27, 28].

The composition of the pink pigments used in the four pots is not clear and is unsupported by the data due to the detection limit of the XRF equipment used for the study. However, in pot A, the pink must be made from gold because elemental analysis shows that pot A contains 0.2% gold, and the gold content in the pink on the other three pots is so low that it is below the minimum detection limit of the equipment. To directly provide evidence of the existence of pink golden red in pots B, C and D, electron microscope observations need to be carried out in further research.

The purple colors are different across the four pots. The purple on pot A is formed by the color combination of golden red and manganese ions, where the gold content is 0.1% and the manganese content is 0.2%; the purple on pot B and pot D is made of cobalt blue and golden red; the cobalt content in pot B is 1.1%, and the cobalt content in pot D is 0.3%. Although gold is still not directly detected, it is speculated that the red is most likely golden red. The purple on pot C is colored by manganese and cobalt ions, with a manganese content of 3.1% and a cobalt content of 0.3%.

The green colors originate from the mixture of yellow and blue colors. Combining the results of Raman and XRF results, it could be observed that the yellow pigments used in the green color were consistent with the yellow used in each pot. All the blue pigments in green pigment in the four pots were Cu-ion blue.

The light blue color could only be found in pots B and D on the part of the neck below the lid. It is interesting that the light blue color was different from the dark blue color on the same pots. The light blue color originated from Cu ions, while the dark blue color was made from Co ions. The light blue color produced by Cu ions also rely on the glass environment, the color of the glass can vary from green to blue with same amount of Cu ions in different glaze recipes. According to XRF analysis result the glaze of light blue enamel is potash-lead glass, the copper ions in this kind of glass usually show intense spectral transmission peak at 450 nm, which produce blue color in this kind of glass.

All the above analysis results for the four pots indicate that pot A is quite different from the other three in terms of metal, structure, glaze and pigments. From this information, it can be inferred that it likely that pot A was not produced in China, as the materials and craftsmanship are more similar to those found in Europe. This deduction was confirmed by hidden information found in pot A.

Hidden hallmarks provide chronological information

Three shallow hallmarks were found on the inner side of the lid in pot A. These were enlarged by the microscope, as shown in Fig. 4 C, D and E. These three hallmarks appear to be the stamps that the Paris Goldsmiths’ Guild printed on French gold products. Compared with other countries, France has a set of quite complex systems for its gold and silver products, and there are roughly three types of product verification hallmarks: the craftsman’s hallmark, tax declaration hallmark and purity hallmark [21]. These marks can be used to verify product information such as manufacturer, and place and year of manufacture. Figure 4E should be the craftsman’s seal, usually composed of two or three capital letters: the two-letter seal is arranged on the left and the right; the left represents the first name abbreviated, and the right represents the last name abbreviated. If there are three letters, those on the left and right are initials, and the initials for last names are placed at the bottom in an inverted triangle arrangement. There are many crown symbols at the top of the logo, and lilies or other symbols are aligned with the middle point of the letter arrangement. The craftsman in the figure appears to be represented by “J · D · O”. At present, no corresponding craftsman has been found; Fig. 4D appears to be the tax hallmark. According to the relevant laws regulating the trademark and the guild, a goldsmith sends the semifinished products with the craftsman hallmark for declaration to the tax unit, where the tax collector prints the tax stamp. The design of this tax hallmark varies from region to region. For example, in Paris, the main body of the tax hallmark for large gold and silver products features the crown with letters, and different letters correspond to different time periods. A comparison of different historical documents [22] reveals that the hallmark found in pot A corresponds with the hallmark in Fig. 4G, so that it can be said that pot A must have been produced between 1782 and 1789. Figure 4C features the purity hallmark. In Paris, once tax stamped, semifinished products must be sent to the goldsmith guild to verify whether the purity meets the guild’s specifications. After verification, the purity hallmark is printed. The purity hallmark adopted by the Paris Goldsmiths’ guild is coded in alphabetical order, with one letter being changed each year. Modest provincial guilds can modify an alphabet stamp only every two to ten years. The hallmarks shown in Fig. 4C are a close match to Fig. 4F, indicating that the pot must have been produced in 1783 [22]. This finding is consistent with the information provided by the tax hallmark. In conclusion, pot A is a pot produced in Paris, France in 1783 (the 48th year of the Qianlong Emperor).