## 3 Results and Discussion  
### Body Compositions  
Chemical compositional data of ceramic bodies obtained by ICP-OES were treated statistically by hierarchical clustering analysis using eight variables (Na, Mg, Al, K, Ca, Ti, Mn, and Fe) and Euclidean distances between samples (Ward method) to group them. This multivariate statistical treatment (Figure 3) revealed several differentiated groups whose compositions are summarized in Table 1 (oxides were calculated by stoichiometry). Calcium was one of the elements whose content varied between groups and, together with other components, allowed the characterization of each group. The groups presented calcareous matrix whose composition ranged from 5 to 18% CaO. Group 3 presented lower average calcium proportion (7.5% CaO) than the other calcareous matrixes, but potassium, magnesium, and aluminum contents were higher. The other two groups with calcareous bodies (group 1 and 2) had similar calcium quantities, but different minor element contents. All studied decorated ceramics were produced with calcareous clay, only a less frequent group of slip-ware decorated with red/brown painted designs (not included in this paper) was made with non-calcareous clay (0.5-1% CaO) [10]. The use of calcareous clays for manufacturing glazed pottery was expected in order to obtain light-colored bodies [11]. The reference groups obtained by cluster analysis, according to the chemical composition of the bodies, could be related to the decoration types manufactured in Albarracin. Groups 1 and 2 corresponded to ceramics decorated with white or green glazes. Most of the _cuerda-seca_ fragments were included inside group 1. Group 3 was mainly formed by honey-glazed objects, although some yellow fragments were also included in groups 1 and 2. This fact suggested that yellow-glazed ceramics could be manufactured with different raw materials or in different workshops. Figure 3: Dendrogram from hierarchical clustering analysis of the body composition data (CS: _cuerda seca_; GB: green and brown decoration on white glaze; G: green glaze; H: yellow/honey glaze). According to chemical results, it seems that selection of the raw materials was performed depending on the decoration. In groups 1 and 2, it can be assumed that the same type of illicit clay was employed for the manufacture of the pottery. However, the raw materials used in the ceramic production of group 3 differed in the chemical composition from those employed for groups 1 and 2. Therefore, ceramics included in this last group can belong to another production workshop or even another area. Comparing the results obtained from Albarracian ceramics with analysis of clay bodies of Islamic productions from other areas of the Iberian Peninsula [4; 12; 13; 14; 15; 16], it can be pointed out that all the clays used in decorated ceramics were also Ca-rich (8-22% CaO) and had similar contents of iron (4-6% Fe\({}_{2}\)O\({}_{3}\)) and fluxing agents (4-5% Na\({}_{2}\)O+K\({}_{2}\)O). The use of calcareous clays for decorated pottery justified the buff colors of the bodies due to the growth of calcium silicates, which incorporated some iron atoms to their structure and decreased the content of iron oxides, responsible for red color [11]. A great similarity was found when we compared these objects from Albarracin with other pottery productions from Muslim centers, such as Cordoba, Denia, Granada, Mallorca, Murcia, Pechina, or Zaragoza [4; 13; 14; 15; 16]. For instance, the Islamic production from Zaragoza during the 11-12 th centuries CE was studied [13] and four reference groups were described, based on the type of body clays employed. In these workshops from Zaragoza, differences among the calcareous groups were also based on the amounts of alkalis and the groups were related to the final decoration and to the utility of the pottery. The selection of the raw materials implied great knowledge of their characteristics and properties. In the south of al-Andalus, the production of ceramics was very popular with a great number of workshops.