*3.5. 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 (208Pb, 207Pb and 206Pb) come from the radioactive decay of U and Th, but the fourth one (204Pb) 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 208Pb/206Pb vs. 207Pb/206Pb and 206Pb/204Pb vs. 207Pb/206Pb 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.

**Figure 7.** Lead-isotope ratios of glazes from different ceramics (white tin-opacified glazes, honey glazes, and green glazes from Albarracin; glazes from Zaragoza workshops (white tin-opacified glaze is marked with a red circle); and glaze in a sample from Pechina): (**a**) 208Pb/206Pb ratio vs. 207Pb/206Pb ratio; (**b**) 206Pb/204Pb ratio vs. 207Pb/206Pb ratio.

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 (16th 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.

#### **4. Conclusions**

The results of this study allowed the characterization of the pottery production from Albarracin *Taifa* in the 11–12th centuries CE, and establishing differences and similarities with Islamic productions manufactured in other *Taifa* kingdoms of the Iberian Peninsula during the same period. The analyses carried out on the ceramic bodies confirmed that the selection of raw materials depended on the decoration to be applied afterwards. All the samples had a Ca-rich body in order to obtain buff colors and to avoid cracking during firing process.

Firing process depended on the kind of decoration. Tin-opacified glazes were applied on pre-fired clay bodies, but *cuerda-seca* and transparent yellow/honey glazes were applied on the unfired clay body. Once again, this differentiation between the two techniques implies a specialization of the potters.

Concerning to the decoration, it can be pointed out some conclusions about Albarracin production. Glazes manufactured in Albarracin *Taifa* during 11th century CE were rich in lead and silicon oxides. Their composition ranged from 32% to 48% PbO and from 36 to 49% SiO2, but differences could be established between white tin-opacified and colored glazes. Secondary-side glazes had similar compositions, although the number of inclusions and the size of them were in both cases bigger. However, the thickness of the coatings was thinner on the less important side. Those features corroborated the idea of a specialization and knowledge of the techniques of production.

As well as silicon and lead as main components, tin-opacified glazes included cassiterite crystals that were responsible for opacity. The buffer color of the bodies required only ~7% SnO2. Tin-oxide crystals were well distributed inside the glazes, and the size of them (<800 nm) was an important factor to achieve the opacification. Green monochrome glazes were always opacified with tin oxide. However, opacity in *cuerda-seca* samples was reached by the combination of tin oxide plus undissolved grains of quartz. This technique of opacification was also used in the south of Iberian Peninsula in the 10th century CE.

Copper, iron, and manganese oxides were added to the glazes to obtain, respectively, green, yellow, and black glazes. Manganese oxide was employed to draw the black lines that described the designs of *cuerda-seca* specimens.

Comparing the lead/silicon ratios used in the manufacture of ceramics with transparent and tin-opacified glazes in Albarracin *Taifa* with other production centers of the Iberian Peninsula during the Islamic period, we can conclude that PbO/SiO2 ratios were lower in the northeastern workshops than in the southern ones. However, the potassium content was higher in the northeastern centers.

In all the studied ceramic workshops of the Iberian Peninsula, tin-opacified glazes were applied on a biscuited body, as in Albarracin *Taifa*. However, there existed differences in transparent glazes, because the southern workshops seemed to use a double firing for producing this type of glaze. In the case of *cuerda-seca* decoration, the production of Albarracin had more similarities in the method of opacity and in the lead/silicon ratios with southern workshops; however, for the firing method, Albarracin samples followed a single firing process, as with the Zaragoza and Almeria centers.

According to these results, it can be highlighted that Islamic ceramics with tinopacified and transparent glazed decorations manufactured in the *Taifa* of Albarracin during the 11th century CE were influenced by the production of the north *Taifa* of Zaragoza. Nevertheless, ceramic decorated with the *cuerda-seca* technique was influenced in some aspects by southern workshops, and in other ones by centers of production from Zaragoza or Almeria.

These possible influences and the Islamic potters' knowledge and specialization were also revealed by the lead-isotope ratio results, where two different lead sources or suppliers were shown: one to prepare tin-opacified glazes, and another one for yellow/honey glazes.

**Author Contributions:** Conceptualization, resources, writing—review and editing, supervision, project administration, funding acquisition, J.P.-A.; methodology, validation, investigation, data curation, writing—original draft preparation, P.M. and J.P.-A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Comunidad de Trabajo de los Pirineos (Aragon-Catalonia-Aquitaine) and the Diputación General de Aragón (DGA).

**Acknowledgments:** The authors would like to thank the Fundación Santa Maria de Albarracin and the Museum of Teruel for providing the samples, and Julián Ortega for selecting and describing the archaeological fragments. We would like to acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

