3.2.1. Arsenic-Based Phases

In the painted enamels analyzed, the detection of two types of arsenic-based phases was significant. The first type refers to lead alkali arsenate apatite, recognized with the narrow (ca. 810–813 cm−<sup>1</sup> ) peak and a well-defined shoulder at ~770–775 cm−<sup>1</sup> [12–14,17,18,32–34], characteristic of the As–O stretching mode of apatite, for which a composition close to Na1−x−yKxCayPb4(AsO4)<sup>3</sup> (x = 0.1 and y = 0.5) has been deduced in a French soft-paste porcelain [34]. This type of opacifying phase was found in some of the enamels analyzed, mostly in white and blue as well as light pink and yellow ones [11–14,17,18,34], as observed in Figure 6B(a) (R1048), Figure 6D(a,a') (R1135), Figure 7B(a,a'),(b,b') (F1371C), Figure 8A(b), Figure 8B(a) (R1175), Figure 10A(a), Figure 11A(a,a'), Figure 12A(a,b), Figure 12B(b,b',b") and Figure 12 C(a') (F1429C). In the case of blue enamels, lead arsenate apatite typically forms as a result of a reaction between arsenic coming from the cobalt source (smalt) and the lead-based enamel/glaze (see further) during the firing process. Its composition also depends on various fluxing species, such as Na(K) and Ca, present in the enamel/glaze matrix [3]. Raman spectra of these enamels displayed the characteristic signature of lead arsenate apatite along with that of the glassy silicate matrix, indicating that the blue was obtained by dissolution of Co2<sup>+</sup> ions in the glassy matrix [29]. The bands expected due to the precipitation of cobalt aluminate or cobalt silicate (see further) were also not detected. Cobalt ores with an arsenic-rich composition are characteristic of European sources used between ~1500 and 1800 [17,18,35,36]. In particular, raw cobalt arsenide mined from the Erzgebirge (the Ore Mountains) in Saxony (Germany) were mixed with potassium glass to produce blue smalt, which was then exported as a coloring agent for glasses/enamels and paint for easel paintings during the 16th–17th centuries [3,36,37]. The detection of lead arsenate apatite could also be due to deliberate addition of an arsenic compound for opacification, especially in the case of the white enamels analyzed [34]. This phase has also been identified in the previous analytical studies of 18th–19th-century Chinese porcelains produced during the Qing dynasty [11–14] as well as in the blue decor of 17th- and 18th-century French enameled glass [16], enameled metalware [10,11,18] and soft-paste porcelains [17,34].

The second type of arsenate phase was characterized by a much broader (ca. 815–820 cm−<sup>1</sup> ) peak (Figure 6A(c,c') (R1045 blue), Figure 6B(b) (R1048 blue), Figure 6C(a) (R1041 blue), Figure 7A (TH457 white-pink), Figure 10A(c) (TH487 yellow) and Figure 13A(b') (F1341C blue)). An intermediate shape between those of the two arsenate phases was observed for Figure 9B(d) (SN284 pink). This broad arsenate band has already been observed in Kangxi Chinese porcelains for the blue areas [12–14] as well as certain French enameled productions [11,16–18]. The broadening of the As–O stretching modes may have arisen from the poor crystallinity or small size of the arsenate apatite crystals or from structural distortions differentiating the AsO<sup>4</sup> tetrahedra (only one strong peak with A<sup>1</sup> symmetry is expected for the stretching mode of an XO<sup>4</sup> tetrahedron). However, arsenates with different composition and structures from apatites (e.g., feldspar) have also been identified in Italian and Spanish majolica enamels [38–41]. Microdestructive analyses to be performed on porcelain shards, such as µ-diffraction, µXRF and/or transmission electron microscopy analyses, are needed to identify more precisely the arsenic-based phases given the ca. 815 cm−<sup>1</sup> broad band.

The two signatures observed in the blue enameled areas indicated the use of cobalt ores rich in arsenic, i.e., cobalt ores imported from Europe [11,15–18] or a mixture of Asian (Mn- and Fe-rich) [3,13,36,42–46] and European (As-rich) cobalt ingredients. However, the rareness—and high cost—of European ingredients led Japanese potters, and likely Chinese ones, to mix with Asian ingredients. This point will be discussed further using the XRF spectra obtained.
