3.2.2. Lead Pyrochlore (Naples Yellow)

A second important outcome of this study was the detection of at least two types of Naples yellow lead pyrochlore pigments in the yellow and green painted enamels of the porcelains. Naples yellow refers to a variety of synthetic lead antimonate pigments with a pyrochlore structure, prepared by mixing lead oxide with various compounds, including mainly antimony and tin, in the presence of various fluxing agents [1,3,12,13,18,47–54]. This pigment, in the form of Sb- or Sn-rich types, has been used as a glass and glaze pigment since antiquity [55–57] and as a painting pigment after the Quattrocento/Renaissance [53,54]. Possible variations of the pigment might also include the addition of zinc, iron and silicon elements [14,18,49,50]. The adjustment of the proportions of these compounds at varying firing temperatures allows for the production of modified forms of the pigment displaying different hues. The composition of Naples yellow type pigments is therefore nonstoichiometric with the incorporation of various cations into the lead pyrochlore structure (Pb2−xM'xM2−YM"YO7−<sup>δ</sup> with M,M" = Sb, Sn, Fe, Si, Zn; M' = RE). This phase is built with two sublattices, one of (big) Pb2<sup>+</sup> cations, the second of (small, covalent-bonded) cations forming tetrahedral and octahedral entities sharing some oxygen atoms. During the firing process of the enamels, composition of the pigment used may be further affected with other species present in the enamel matrix (Fe, Si, Sn, etc.) [14,18,50,58,59]. Some of these incorporating elements may also have different speciations (e.g., Sn2<sup>+</sup> or Sn4+, Sb3<sup>+</sup> or Sb5+, Fe2<sup>+</sup> or Fe3+), and furthermore, the oxygen nonstoichiometry very much depends on the conditions of the firing process (reducing/oxidizing conditions).

In practice, the lead pyrochlore pigment can be prepared as a powder in advance and then mixed with the glass/glaze powder before deposition, the so-called "*anime*" and "*corpo*" recipes [57,60,61]. Alternatively, all the ingredients can be mixed together to obtain the yellow glass/glaze powder. In the first method, the grains of lead pyrochlore are rather big (>>µm), while in the other, the homogeneous nucleation on cooling gives rise to small precipitates (<a few µm). With a high-magnification microscope objective, a single pigment grain could be analyzed, and the Raman spectrum obtained was strong without a background, and almost no signature of the glassy matrix was recorded (see e.g., Figure 7A—TH457 yellow; see also in ref. [34]). In the case of nucleation, both the signature of lead pyrochlore precipitates and glass were recorded together with a significant background (e.g., Figure 6D(c,c')).

In the last decades, several Raman spectrometry studies performed on various pottery glazes, glasses and enamels [1–3,11–18,47–52,57–59] as well as oil paintings [53–56] allowed for the identification of Naples yellow lead pyrochlore pigments. In general, these pigments have a very characteristic Raman signature, which basically includes the strong stretching mode peaks of Pb ions at the low-wavenumber region (~115–145 cm−<sup>1</sup> ) and M/M"–O stretching modes from ~300 to 600 cm−<sup>1</sup> . Furthermore, Sb-rich pyrochlore pigment is mainly distinguished by strong peaks at ~110–130 and ~505 cm−<sup>1</sup> , while Sn-rich-type pigment exhibits stronger peaks at ~130–135, ~335 and 450 cm−<sup>1</sup> (the Sn–O stretching mode) with the disappearance of the ~505 cm−<sup>1</sup> peak [10,13,18,47–52]. Zn-containing mixed lead pyrochlores exhibit a medium 450–480 cm−<sup>1</sup> component [18]. Although Sn-rich lead pyrochlore was observed in most of the yellow and green enameled areas (Figure 6B(c) (R1048), Figure 6D(c,c') (R1135), Figure 9A(a) (R1006), Figure 9B(a,a') (SN284), Figure 10A(b,b') (TH487), Figure 11A(a) (R1056) and Figure 13B(c,c') (F1341C)), Sb-rich compositions were clearly observed in the yellow enamels of the TH457 bottle from the Thiers Collection (Figures 3 and 7A) and the R1175 plate with an English family's coat of arms (Figures 2 and 8 A(a,a')). Mixed lead pyrochlores were also observed in some cases (Figure 7B(a,a') (F1371C) and Figure 12 B(b,b',b") (F1429C)). As a second piece of proof, the XRF spectrum recorded on the yellow enamel of TH457 showed both Sn and Sb signals (Figures 14 and 15).

signals (Figures 14 and 15).

3.2.3. Red to Violet Colors

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mixed lead pyrochlores exhibit a medium 450–480 cm−1 component [18]. Although Sn-rich lead pyrochlore was observed in most of the yellow and green enameled areas (Figure 6B(c) (R1048), Figure 6D(c,c') (R1135), Figure 9A(a) (R1006), Figure 9B(a,a') (SN284), Figure 10A(b,b') (TH487), Figure 11A(a) (R1056) and Figure 13B(c,c') (F1341C)), Sb-rich compositions were clearly observed in the yellow enamels of the TH457 bottle from the Thiers Collection (Figures 3 and 7A) and the R1175 plate with an English family's coat of arms (Figures 2 and 8 A(a,a')). Mixed lead pyrochlores were

**Figure 14.** Representative portable XRF (pXRF) spectra recorded on the TH457 bottle for the glaze and different painted enamels. **Figure 14.** Representative portable XRF (pXRF) spectra recorded on the TH457 bottle for the glaze and different painted enamels. *Heritage* **2020**, *3* FOR PEER REVIEW 16 of 27

**Figure 15.** Representative pXRF spectra recorded on the TH457 bottle for pink and black enameled areas; bottom: zoom in on the energy window characteristics of Sn and Sb elements for different areas. **Figure 15.** Representative pXRF spectra recorded on the TH457 bottle for pink and black enameled areas; bottom: zoom in on the energy window characteristics of Sn and Sb elements for different areas.

8A(c,d), orange and red; and Figure 8B(c), red), the use of both methods was observed. Hematite has a well-defined Raman signature, with a strong (ca. 1305–1310 cm−1) mode (resonance signal of magnon mode under green laser excitation) and narrow peaks between 200 and 600 cm−1 [62,63] (see e.g., Figures 6D(b,b',b''), 7A, 8A(c), 8B(c) and 12C(b,b')). The magnon mode is very sensitive to the particle size of hematite [63] and the oxygen stoichiometry [62], with small particles giving an orangered color [29]. Broadening of hematite peaks indicates a partial substitution of iron with other

A specific Raman feature referred to the use of gold nanoparticles dispersed in the glassy matrix of the enamel (this type of enamel is at the origin of the label "*Famille rose*" [15,19], with a characteristic broad fluorescence background peaking at ~500–600 cm−1 under a green laser, corresponding to ~500 nm on the absolute scale [14,18]; see Figure 6C(c) (R1041), Figure 7A (TH457), Figure 8A(d) (R1175), Figure 9B(c) (SN284), Figure 10A(a,a'),B(d) (TH487), Figure 12C(a) (F1429C) and Figure 13C(a) (F1341C)). This method was first experimented with in 17th-century France by the glassmaker Bernard Perrot and then some years later by Johann Kunckel in Germany and certainly by others in Italy before [16,18,64–67]. The presence of gold was also confirmed by pXRF analysis (see arrow on Figure 15—pink, in which the small Au Lα peak is obvious), with a hardly visible peak well-identified by data fitting. In addition, it is important to note that a very small band characteristic of the As–O stretching band at ca. 820 cm−1 could be detected along with the fluorescence background characteristic of the use of colloidal gold (seen clearly in Figure 10A(a,a') (TH487) and less intensely in Figure 9B(c) (SN284) and Figure 13C(a) (F1341C)). This band corresponds to the use of Perrot's preparation route for obtaining the colloidal gold precipitate using an arsenic salt, which is different

elements, possibly Ti and Al coming from the iron sources [29].

The last important characteristic of the painted enameled porcelains analyzed was the presence
