## 3 Results and Discussion
### White Tin-Opacified Glazes  
Different types of decoration on the Islamic pottery from Albarracin _Taifa_: white tin-opacified glazes, colored glazes, and _cuerda seca_ were studied for their characterization. In the case of white tin-opacified glazed samples, the compositions of white glazes on the main side and the yellow-honey glazes on the secondary side of these fragments were studied. Further, brown and green decorations on the white glazes were analyzed in most ceramic samples. Tin-opacified glazes presented an average chemical composition that was very homogeneous in the main side, as can be seen in Table 2. The major elements found in the white glazes were silicon (SiO\({}_{2}\) 42.8%), lead (PbO 36.2%), and tin (SnO\({}_{2}\) 7.0%), mixed with contents below 5% of aluminum, iron, alkalis (potassium and sodium) and calcium oxides. Glazes produced during the medieval period in the Iberian Peninsula were characterized by lead-rich contents. The low percentage of aluminum (Al\({}_{2}\)O\({}_{3}\) \(<\) 4.5%)  
could be attributed to the use of small amounts of clay to produce the glaze, or also to the diffusion of aluminum from the paste to the glaze during firing [17]. Iron and calcium were not added to the recipes of glazes as main components. Their very low proportions (FeO \(<\) 1% and CaO 2.86%) were also due to the migration of iron and calcium into the glazes. Higher contents of iron would induce yellow or brown colors in the glaze. Therefore, raw materials employed in the production of tin-opacified glazes were carefully chosen in order to obtain a white color of glaze. Green and brown decorations on tin-glazed samples from Albarracin _Taifa_ did not differ in their compositions from the white ones, except for the presence of metallic oxides responsible for the colors (Table 2). Analyses carried out on green areas were rich in copper oxide, and brown zones had higher contents of manganese oxide. The content of CuO was in the range of 1.4-3.4% and MnO varied between 1.2 and 6.0%. The presence of copper in brown decorations can be explained due to the thin brown design that made it difficult to determinate only the composition of brown areas without including green zones. Opacity was accomplished by the presence of small crystals of cassiterite (SnO\({}_{2}\)) within the glaze that reflected and scattered the light [18]. These crystals were achieved by the addition of tin oxide to the raw materials of the glazes. During the heating (above 700 \({}^{\circ}\)C) of the mixture, the tin oxide was dissolved and began to recrystallize as cassiterite [19]. The amount of tin, the size of tin oxide crystals, their distribution in the glaze, and the thickness of the glaze were factors with great influence on the grade of opacification [12; 18; 19] and they were related to the process of manufacture. The average amount of tin oxide (7% SnO\({}_{2}\)) employed in the studied samples was in good agreement to obtain enough opacity (5-10% SnO\({}_{2}\)) without increasing too much the price of the product. The size of tin oxide crystals in Albarracin samples varied from 500 to 800 nm. These dimensions of the crystals indicated that high temperatures (above 800 \({}^{\circ}\)C) were reached during the firing process and the cooling occurred fast, as it was demonstrated in the literature [19]. The homogeneous distribution of the cassiterite crystals suggested that the tin glazes were prepared by fritting the raw materials before applying them to the ceramic body [15; 20]. Fritting was an extended method used since ancient times to obtain glasses and glazes, which consisted in melting the raw materials (lead, tin, and silica) previously to use them as components of the glazes. Finally, these fired materials were ground to powder before preparing the glaze suspension that was applied on the surface of the body.