*3.4. Cuerda-Seca Decoration*

Concerning *cuerda-seca* samples, we determined the composition of the black line which divided different areas of the drawing, and the glazes applied inside those areas.

The analyses of the black lines showed inhomogeneous compositions (Table 5) that made it not realistic to establish average proportions. However, the high content of SiO2 as major element in all the samples is remarkable, mixed with lead, manganese, aluminum, and iron. Manganese was the responsible for the black color of the line, probably in the form of different oxides. Lead could be added to enhance the adherence of the pigments to the paste [24], because the black line was applied directly on the non-fired paste, as can be deduced by the thickness of the interface (Figure 6).


**Table 5.** Chemical composition (%wt) of the black line, yellow and green glazes in *cuerda-seca* decorated ceramics by EDS.

<sup>1</sup> In this case, standard deviation (*s*) corresponds to the number of analyzed points (9) in one sample.

**Figure 6.** *Cuerda-seca* decorated glazes: (**a**) Secondary electron-SEM image of the glaze; (**b**) Optical Microscopy image of *cuerda-seca* decoration.

The glazes used to fill the spaces between black lines were rich in silicon and lead (Table 5). The green glazes had tin oxide in lower proportions than white tin-opacified glazes. Although there were not many *cuerda-seca* samples with yellow and green glazes among the studied samples, and the yellow ones showed great variation in lead content, the average composition of the PbO was similar to the secondary yellow/honey glazes (subgroup 2, Table 3) on green-and-brown decorated objects, and to green monochrome glazes (Table 4). However, the silicon proportion of *cuerda-seca* glazes differed more in both colored glazes.

Once again, the presence of copper and iron oxides with percentages around 2–3% contributed to the final color of the glazes. Only a sample showed *cuerda-seca* decoration with honey and green-turquoise colors. In this case, the color was due to the presence of copper, but its green hue was shifted to turquoise because the glaze had alkalinelead composition with a significant contribution of sodium (2.5% Na2O) and potassium (2.9% K2O) and lower lead content (23.3% PbO). These differences in green hues were already highlighted in *cuerda-seca* ceramics of the 12th century CE produced in the Iberian Peninsula [25].

Studies of pottery with *cuerda-seca* glazes manufactured in the Iberian Peninsula during the 10th–12th centuries CE revealed the use of silicon and lead as main components [6,24,25]. The amounts of lead decreased from the 10th century (43–55% PbO) to 12th century CE (34–43% PbO), mainly in productions from the south of al-Andalus [25]. Nevertheless, samples manufactured during the 11th century CE in the NE workshop of Zaragoza were characterized by high lead proportions [24]. Contrary to other types of decoration, pottery decorated with *cuerda seca* produced in Albarracin in the 11th-12th centuries CE was more similar to the recipes employed in the south of the Peninsula with lead contents around 35–38% PbO.

Examinations by SEM of polished sections of *cuerda-seca* glazes confirmed the presence of abundant inclusions scattered homogeneously in the glaze (Figure 6). The inclusion sizes varied from 50 to 70 μm and their composition was mainly quartz. The use of such an amount of inclusions and their size, plus the tin-oxide content, could be responsible for the opacity. Opacification can be reached either with undissolved inclusions into the glazes or with tin-oxide crystals scattered in the glaze, and even with both of them [26,27]. *Cuerda-seca* ceramics manufactured in the same period in al-Andalus employed both techniques [6,24,25], although only tin oxide was employed as an opacifier in later centuries. Therefore, the inclusions found in the glazes played an important role in the opacity of the glazes. The thickness of the glazes reached values higher than 200 μm and the thickness of the clay-glaze interfaces was also higher (>45 μm), which implied that a single firing technique was carried out during the pottery's manufacture. The type of firing process (single or double) depended on the workshops; both techniques were used without a clear tendency. Single firing process was more frequent, especially in workshops such as Almeria and Zaragoza [24,25]. This characteristic was also followed in the studied production from Albarracin *Taifa*.

As it has been explained before, the technology of opacification also underwent an evolution from a double method (tin oxide and inclusions) employed in the 10th century CE to the opacity technique due exclusively to tin oxide used in the 12th century CE. However, in our case, the double methodology was followed, as in the case of the southern workshops.

It seems clear that the *cuerda-seca* technique underwent an evolution during the Islamic period with variations in recipes, firing processes, and methods of opacification. The evolution did not follow a regular extension to all the production centers, but we can highlight that Albarracin was influenced by the southern centers of al-Andalus. Although archaeological studies based on the decoration motifs of *cuerda seca* during the period of *Taifas* emphasized that the same type of vegetal designs was also employed in Zaragoza [6], both influences could be possible taking in mind the good relationships with northern and southern *Taifas*, and the wide commercial distribution of Islamic glazed ceramics [28,29].
