## 3 Results and Discussion
### Lead and Raw Materials for Glazes  
Lead isotopes ratios were measured by ICP-QMS in a selection of representative glazed samples, because of the important information in lead sources that could be provided. Natural lead consists of four isotopes: three of them (\({}^{208}\)Pb, \({}^{207}\)Pb and \({}^{206}\)Pb) come from the radioactive decay of U and Th, but the fourth one (\({}^{204}\)Pb) has a non-radiogenic origin and natural lead changes its isotopic composition. The results of twenty-three samples from Albarracin, plus two samples from Islamic workshops of Zaragoza and one sample from Pechina are shown in Figure 7.  
The lead-isotope ratios of the samples, plotted as \({}^{208}\)Pb/\({}^{206}\)Pb vs. \({}^{207}\)Pb/\({}^{206}\)Pb and \({}^{206}\)Pb/\({}^{204}\)Pb vs. \({}^{207}\)Pb/\({}^{206}\)Pb ratios (Figure 7), offered the possibility of establishing several groups and distinguishing different features of the used lead for the production of glazes. Samples from Albarracin can be clearly separated into two groups. One of the groups was related to the fragments with yellow/honey glazes (Figure 7a,b, on the right of both plots). The second group was formed by white tin-opacified and green glazes from Albarracin (Figure 7a,b, on the left of both plots). Moreover, these last two types of glazes showed slight differences between them, but both had in common being tin-opacified glazes. If we observed the results of the three samples from other areas (Zaragoza and Pechina), the white tin-opacified glaze from Pechina was grouped with the tin-opacified samples from Albarracin. However, the glazes from Zaragoza indicated differences in their source of lead, because both samples were closer to the honey glazes from Albarracin, and even the white tin-opacified glaze from Zaragoza (marked with a red circle in Figure 7a,b) was unexpectedly similar to the transparent honey glazes from Albarracin.  
Therefore, all of these glazed ceramics, manufactured in Islamic Albarracin, had lead isotope abundances very well characterized in two groups related to the type of glaze (tin-opacified or transparent) applied on the ceramics. This suggested diverse lead sources for the glaze raw materials depending on the type and quality of glaze; then, it could be related to the use of a type of commercial lead for producing tin-opacified glazes and another different lead product for preparing transparent glazes. The first one could be linked to obtaining a best-quality glaze or to the preparation (fritting) of tin-lead glazes. In later periods (16 th century CE), differences in the commercial lead supplied to potters were documented: it was explained that the lead used to produce glazed cooking pots was cheaper than the lead acquired to prepare tin-opacified glazes [30].  
It is important to underline that this possible source or supplier of lead for tin-opacified glazes from Albarracin was quite similar to that for the white glaze from Pechina; as such it could perhaps come from one of the lead sources in the south/southeast of the Iberia Peninsula. However, lead used in Zaragoza workshops seemed to be different, or closer to that introduced in transparent glazes; slight differences could be highlighted between these two samples (honey and white) (Figure 7a,b), but there are not enough results to find significant dissimilarities.