English delftware (c. 1770) from Bristol, Lancaster and Liverpool: A composition study using Raman spectroscopy and electron microscopy

Raman identification of pigments and opacifiers: Interest and limitation of multivariate analysis by comparison with solid state spectroscopical approach—II. Arsenic‐based opacifiers and relation with cobalt ores

“…The first cluster is constituted by 10 of the 12 lacunar apatites (Na 1-x K x Pb 4 (AsO 4 ) 3 ) that the references count. 18 The hydrated wendwilsonite reference [Ca 2 Mg(AsO 4 )Á2H 2 O] observed in an English delftware glaze (c. 1770) could be due to some surface corrosion, 28 as well as the hedyphane reference [Pb 3 Ca 2 (AsO 4 ) 3 Cl] from Frost et al, 25 but which was also observed in glazes from English delftwares, 28 are also found in this cluster. Two Na 0.5 K 0.5 Pb 4 (AsO 4 ) 3 references are more isolated from this main cluster.…”

“…Spectra selected from the literature 18,[25][26][27][28] were used as reference. They are presented in Figures 1 and 2.…”

“…Whether for the spectra of our studies or those obtained from the literature, 18,[25][26][27][28]30,31 a linear segment baseline was subtracted using Labspec ® software (5.25.15, 2007, Horiba Jobin-Yvon 6,32 ) and the spectra were then decomposed into a sum of elementary bands using Peakfitting Origin ® software (6.0, 1999, Microcalc Inc.).…”

“…In the following, we will therefore use only the arsenicbased references for comparison. Figure 6A,B compare thus the two main PCA components of spectra of the (Figure 6A) 23 As-based references from literature 18,[25][26][27][28] (Table S2) and (Figure 6B) 52 measurements on enamelled artefacts (Table S1), once SNV normalization was performed on the defined interval. Figure 6C shows the loading plots of PC1 and PC2 resulting from this PCA.…”

“…Two clusters-A.1 (Apatite 1) and A.2 (Apatite 1)clearly correspond to Raman signatures similar to that of apatite (Figure 4C). Cluster A.1 represents the samples with the positive values on the two principal components F I G U R E 6 Diagrams from PCA analysis with the first and second components calculated on the spectra within the 730-to 880-cm À1 interval from the corpus of the studied objects plus all the phases from the literature featuring the main As-O peak around 820 cm À1 : (A) references 18,[25][26][27][28] only; (B) references 18,25-28 and studied objects 5,17,[21][22][23][24] ; (C) loading plots showing the projection of PC1 and PC2 on the initial variable space. The inset circles represent the contributions of three of the wavenumbers used in the PCA calculation.…”