3.1. RBS-PIXE Analysis
Figure 2 and
Figure 3 respectively report the RBS spectra of the unglazed back and external glazed surface of the analyzed AZR3, AZR5, AZR7, and AZR1 potsherds. Several steps can be seen in the RBS spectra, corresponding to the various detected elements.
Assuming the bulk matrix to be composed mainly by SiO
2 in agreement with the literature [
7], the RBS analysis refers to ~a 40–50 μm depth, being 100 μm the penetration depth of the used proton beam in a SiO
2 matrix. Therefore, in the case of measurements performed on the external surface, the RBS signal will reasonably reflect the contribution coming from both the glaze and the ceramic body. Instead, the RBS data collected from the back-side of the samples will be informative for the elemental composition of the bulk and can be used for comparison.
The relative atomic concentrations of detected elements as obtained from the RBS analysis are summarized in
Table 2 and
Table 3.
Worth of notice, the use of the SIMNRA code for the best-fit of the spectra allowed us to put in evidence three light elements, i.e., C, N, and O. The O-element is bound to the other detected elements, giving rise to oxides such as SiO2, Al2O3, CaO, Na2O, MgO, PbO2, and Fe2O3, BaO.
Regarding the PIXE analysis, the analyzed thicknesses are comparable to the RBS ones performed with the same ion beam probe. However, as already remarked, PIXE analysis has also allowed us to reveal elemental trace composition. Additionally, in this case, PIXE was applied to both the external glazed surface, in order to have information on the glaze/bulk composition, and to the unglazed back, in order to detect the bulk composition to be used for comparison.
Figure 4 and
Figure 5 report the PIXE spectra of the unglazed back and external glazed surface, respectively.
The relative weight concentrations of the major, minor and trace elements detected by PIXE in the unglazed back and external glazed surface of the analyzed AZR3, AZR5, AZR7, and AZR1 potsherds are reported in
Table 4 and
Table 5, respectively.
As a general result, a moderate homogeneity is observed in the elemental components of the potteries, which could suggest an elaborate elutriation, mixture, and processing in the pottery manufacture, corresponding to a high productivity level at that time.
In agreement with previous results [
7], the composition of the ceramic body of all the investigated specimens indicates the use of raw materials based on alumina and silica. According to literature [
31,
32], alumina is considered the characteristic oxide testifying a Chinese provenance. In addition, the presence of calcium magnesium silicates (e.g., diopside, CaMgSi
2O
6) as neo-formation minerals, indicating a high firing temperature, is conceivable for samples, like ours, that present such minerals in their raw materials, as occurs for ancient Chinese pottery [
33].
Based on the aforementioned considerations, a Chinese provenance is hypothesized for all the investigated fragments.
Interestingly, a high content of CaO is observed for AZR3 sample with respect to the other fragments, for which the amount of these elements is almost comparable. This excess can be associated with the presence of calcite (CaCO
3) of secondary origin [
7].
As far as external glazed surfaces are concerned, the results indicate, for samples AZR5 and AZR7, a glaze composition based on Pb- and Si-oxides [
34,
35], with iron-oxides responsible for the yellowish pigmented layers [
35,
36]. In the case of AZR3 shard, the dark coloration can be hypothesized to be due to iron oxides. The presence of such oxides, as reported in Reference [
7], suggests that firing was carried out in an oxidizing atmosphere at temperature around 850 °C. The glazed surface could be of alkaline nature [
37]. The glaze for AZR1 sample turns out to be Si- and Na-oxides rich, whereas Cu can be hypothesized as responsible for the blue pigment.
Finally, it is worth remarking that, contrary to PIXE, RBS has allowed, with fairly good accuracy, for the estimation of the content and the depth distribution of the metallic elements.
3.2. Ion-Microbeam Analysis
μ-PIXE elemental maps have allowed us to obtain the distribution of the major elements.
Figure 6 displays the micro-beam images of some elements (Ca, Al, Fe and Si) present in the cross-section close to the external glazed surface of AZR3 sample. Mesovoids are observed, with a size ranging from some hundred of μm down to tens of μm. The sample is highly damaged on the surface, with the dark glaze present only in some points, and presumably, responsible for the red spot observed in the Fe distribution map. Si and Al are widespread in the ceramic body, whereas Ca is visible in a low amount and appeared mainly concentrated on the surface, supporting the presence of calcite of secondary origin, as previously hypothesized [
7]. Ti is present only in trace.
Figure 7 shows the Ca, Pb, Fe, Si, and Al μ-PIXE elemental maps of a 1 × 1 mm
2 portion from the external glazed surface of AZR5 sample. A general inspection of the maps reveals evident changes in the distribution of all elements at ~200 μm below the surface, allowing us to identify two main zones that can be ascribed to the glaze (external) and the ceramic body (internal). Si appears to be the main element, widely spread in all the investigated area, with a major concentration in the ceramic body. Ca is detected in low amount, mainly concentrated on the external layer, whereas the opposite occurs for Al. Interestingly, as far as Fe is concerned, it appears concentrated on a thin layer marking the two different observed zones, furnishing an evidence of the yellowish pigmented layer underlying the glass. Below it, the distribution map of Pb evidences presence in some amount of this element down to –50 μm, and then the element tends to disappear. According to this, the existence of an engobe as an intermediate layer between glaze and ceramic body, hypothesized in previous measurements [
7], is confirmed. In addition, Pb is not uniformly distributed on the glazed surface, testifying a not good conservation state.
AZR 7 sample exhibits a uniform distribution of Si, Al, Ca and Mg-elements (data not shown). Worth of note appears, instead, the distribution gradients of some elements such as Fe and Pb observed in the cross-section close to the external glazed surface, reported in
Figure 8. In particular, the surface layers show a significant Fe depletion, whereas Pb is detected in a very high amount.
As far as AZR1 sample is concerned, it is too small size avoided any cross section investigation, so the mapping was performed scanning a 1 × 1 mm2 area on the external glazed surface, including both the zones with and without pigment.
The obtained distributions for some of the main elements (Ca, Cu, Fe, Si, Ti, and Zn) are reported in
Figure 9.
For all the detected elements, the distribution appears homogeneous, Si and Ca representing the main constituents of the glassy layer, Cu the main responsible of the blue coloration. TiO
2 and ZnO could both have been used as opacifiers to tone down the classic shades in the case, for example, of dark blue [
38].
3.3. SR-Based FTIR Spectroscopy Analysis
As already reported in Materials and Methods, the examination of small powdered portions of the samples by optical microscopy allowed distinguishing among grains from the bulk and from different pigmented surfaces. In both cases, the chemical characterization was performed by SR FTIR microscopy.
In the case of grains coming from the bulk, the analysis was aimed at the identification of current and neo-formation mineralogical phases in order to achieve information on the production process, in terms of raw materials and firing conditions.
As far as samples belonging to Group 1 are concerned,
Figure 10 reports the photomicrographs, in white reflected light, of AZR3, AZR5, and AZR7 samples relative to the bulk area, after being pressed within the DAC, together with their corresponding average SR-FTIR micro-spectra.
The SR-FTIR spectra are rather similar, indicating a compositional homogeneity of these shards for what concerns the mineralogical phases. All spectra evidence a main peak at ~1087 cm−1, together with a shoulder at ~1162 cm−1, that can be ascribed to the presence of quartz (SiO2), together with another main peak at ~1047 cm−1, and a less intense one at ~1633 cm−1, and features at ~3450 cm−1, associated to montmorillonite ((Na,Ca)0,3(Al,Mg)2Si4O10(OH)2·n(H2O)). In addition, the characteristic band of calcite (CaCO3) at ~1445 cm−1 is observed in the spectra of AZR3 and AZR5 samples.
The high quality of our spectra has allowed for a quantitative analysis of the observed profiles, by means of deconvolution into symmetrical Gaussian profiles and curve-fitting. This is particularly helpful in the (800–1400) cm
−1 spectral range, since, as has been already demonstrated [
39], the complex broad band at ~1000 cm
−1 contains several components that are fingerprint of the main mineralogical phases. The results of the best-fit procedure in the case of AZR3 and AZR7 shards are reported in
Figure 11 as examples.
From the best-fit, other than quartz, montmorillonite and calcite (already mentioned), typical vibrational bands of oligoclase ((Ca,Na)(Al,Si)4O8, bands at ~1009 cm−1, ~1135 cm−1), diopside (CaMg(Si2O6), bands at ~944 cm−1, ~1059 cm−1), and anorthite (CaAl2Si2O8, band at ~975 cm−1) have been recognized.
Concerning AZR1 fragment, belonging to Group 2,
Figure 12a shows the photomicrograph of the sample, relative to the bulk area. The corresponding SR-FTIR micro-spectrum is reported in
Figure 12b.
With regard to the mineralogical phases, a first examination of the spectrum has allowed us to clearly recognize the presence of montmorillonite, as indicated by the peaks at ~1034 cm
−1, ~1630 cm
−1, ~3348 cm
−1, and ~3414 cm
−1. In particular, the former two peaks are typically assigned to the hydration water molecules of this mineral. Looking further into the spectrum, we could postulate the presence of quartz by the peaks at ~1087 cm
−1 and ~1164 cm
−1. Metal carboxylates (ν
sym COO
− at ~1424 cm
−1) and oxalates (ν
sym COO
− at ~1312 cm
−1, respectively) peaks are also detected, that can derive from the degradation of some organic compound. About this, it is worth noting that the C-H stretching contributions that appear in the same spectrum at ~2846 cm
−1 and ~2909 cm
−1 support the presence of a proteinaceous material subjected to degradation processes [
40]. Finally, the shoulder at ~1448 cm
−1 is associated with the presence of calcite.
Deconvolution into symmetrical Gaussian profiles and curve-fitting of the (800–1250) cm
−1 spectral range was performed, and the results of the best-fit procedure are reported in
Figure 13.
Other than the aforementioned montmorillonite, quartz and calcite, also orthoclase (KAISi3O8, bands at ~1002 cm−1, ~1118 cm−1), diopside (bands at ~947 cm−1, ~1056 cm−1, ~1203 cm−1) and anorthite (band at ~978 cm−1) have been identified.
In
Table 6, the qualitative mineralogical phase composition relative to the ceramic bulk of all the investigated shards is reported.
Diopside and anorthite are reported to be formed at (850–950) °C through the reaction between silica and carbonate materials [
41]. Hence, their presence testifies a maximum firing temperature around these values, whereas quartz and feldspars can persist above 1000 °C [
42]. This hypothesis is also supported by the absence of gehlenite, that, as is well known, tends to vanish at –900 °C, as well as of high-temperature neo-formation minerals. Going on, montmorillonite could be derived from the hydrolysis process occurring during the burial period of the findings [
43]. Finally, as far as the presence of calcite revealed for AZR3, AZR5, and AZR1 samples is concerned, it is well known that at temperatures above (750–800) °C calcite dissociates in calcium oxide (CaO) and carbon dioxide (CO
2). Its observation for these three samples, could then testify a firing process performed at low temperature, around (650–750) °C, or incomplete, probably due to an inhomogeneous heat distribution inside the kiln, that didn’t permit the completion of decomposition and formation reactions. Nevertheless, decarbonation of calcite may extend to (1000–1100) °C for calcite rich systems and/or in case of coarse mineral grains and rapid heating rate [
44]. Again, calcite can probably be of secondary origin, in agreement with what observed for AZR3 fragment by PIXE and ion-microbeam analyses, due to conservation circumstances such as burial conditions in humid soil, because of the gradual interactions of hygroscopic lime (CaO)-moisture and calcium hydroxide [Ca(OH)
2]-atmospheric CO
2.
Hence, the whole observations suggest a firing temperature surely higher that 850 °C and in particular in the range (850–950) °C for all the investigated samples, despite the presence of calcite.
As far as the grains coming from the different pigmented surfaces are concerned, the analysis was aimed at the identification of pigmenting agents used for decoration.
Figure 14 shows the photomicrograph, in white reflected light, of sample AZR3, relative to the black pigmented area, together with the corresponding average SR-FTIR spectrum.
The collected average absorption spectrum reveals several peaks, that, based on comparison with database and literature [
29] could be attributed to burnt umber (features at ~1032 cm
−1, ~1458 cm
−1, ~3393 cm
−1, and ~3625 cm
−1), a deep brown color pigment obtained by heating raw umber, a natural clay pigment consisting of iron oxide, manganese oxide and aluminum oxide (Fe
2O
3 + MnO
2 +
nH
2O + Si + AlO
3), mixed with some organic binder that seems to be a fatty acid (features in the ~1300 ÷ 1700 cm
−1 range, at ~2854 cm
−1 and ~2923 cm
−1).
As far as sample AZR5 is concerned, the average SR-FTIR spectrum collected (data not shown) on the dark yellow pigmented area unfortunately only gave evidence of a vitreous matrix, hindering the identification of the pigment.
In
Figure 15, we report the photomicrograph, in white reflected light, of sample AZR7, relative to the yellowish pigmented area, together with the corresponding SR-FTIR average spectrum.
Based on the literature [
45], the collected SR-FTIR average spectrum can be mainly ascribed to limonite, a yellow earth pigment known since antiquity, composed of a mixture of various iron oxide and hydroxide minerals, the most common among them being goethite, α-FeOOH and lepidocrocite, and γ-FeOOH.
Figure 16a reports the photomicrograph of sample AZR1, relative to the blue pigmented area. The corresponding SR- FTIR average spectrum is reported in
Figure 16b.
Once again, the comparison with database and literature [
29] has allowed us to identify as pigmenting agent the cuprorivaite (CaCuSi
4O
10) mineral (features at ~1019 cm
−1, ~1044 cm
−1, ~1168 cm
−1, and ~1207 cm
−1). This pigment is a compound produced since the ancient Egypt time by means of a high temperature (>800 °C) synthesis starting from siliceous sand, a copper compound, calcium carbonate and sodium carbonate as a flux. It appears bounded together with some organic binders of proteinaceous origin (features in the ~1400 ÷ 1700 cm
−1 range, at ~2846 cm
−1, ~2926 cm
−1, and ~3392 cm
−1).
The photomicrograph relative to the area without pigment of sample AZR1 is shown in
Figure 17a. The corresponding SR- FTIR average spectrum is reported in
Figure 17b.
The composition of the area without pigment seems similar to the blue pigmented zone, but more homogeneous, as can be seen by the relatively low standard deviation. A more detailed comparison of the SR-FTIR spectra collected in the (800–1400) cm
−1 wavenumber range in the blue pigmented area and in the area without pigment (see inset of
Figure 17b) reveals, in this last case, the strong decreasing of the contribution at ~1019 cm
−1 typical of cuprorivaite.