خصوصیات شیمیایی و بافتی لعابهای قلع در سرامیکهای اسلامی شرق اسپانیا

Several productions of Islamic tin glazed pottery from eastern Spain have been studied under the chemical and microstructural points of view by means of WDS, SEM/EDX, XRD and XRF analyses. Samples of Islamic pottery from the workshops of Murcia 10th, Zaragoza 11th, Mallorca 11th, Denia 13th, Granada 14th and Córdoba 10th, which represent a wide range of local productions from medieval eastern Spain, have been studied in order to obtain the trends of the technical and compositional evolution. From the experimental data, some common features can be established, as well as some differences. All the Islamic Spanish opaque glazes are lead glazes with PbO contents from 37 to 56%, opacified with tin oxide in the range 4–15%. In all the cases, they were applied on a previously biscuited body made with a Ca-rich clay, probably to produce a buff colour less apparent through the glaze. The thicknesses range from 100 to 150 microns and the opacification is achieved by small crystals of SnO2(under a micron of size). The main differences are the size and distribution of such small crystals, being smaller in the early Islamic productions (Zaragoza and Murcia) and bigger in the late productions.

From the experimental results shown here after the study of samples from a wide range of different archaeological sites (workshops and production areas), several points can be established concerning the Islamic opaque-glazes production: (i) All the Islamic white glazes produced in Spain are lead–silica glaze opacified with tin oxide, at least since the 10th century. Accordingly, they do not have a slip, as it has been largely referred. Neither alkaline nor mixed lead–alkaline glazes have been detected in any site, which is coherent with the fact that the Iberian Peninsula was one of the main galena producers at this epoch. Thus, there are not any reasons to make alkaline glazes in a region where lead is so abundant. (ii) Slightly different recipes were used to produce tin-glazes in each production centre, with different lead/silica ratios. A very low contribution of clay (low aluminium contents) was used in almost all of the white glazes. It seems that very pure components were taken for preparing glazes (iron percentage is always very low) and perhaps applied to the ceramic body with some kind of gum. (iii) The tin oxide contents ranges from 5 to 10%, which are the typical limits used traditionally for these glazes. Glazes containing less than 5% are not opaque enough and the colour of the ceramic paste would be seen through the glaze and would give a pinkish appearance, while contents over 10% are not necessary and we must keep in mind the high price of this component. (iv) Copper and manganese were used for colouring the green and brown designs on the white glaze. (v) The thickness of tin glazes ranges between 150– 200 m thinner than Islamic glazes from the Near and Middle East. (vi) In all of the cases, tin glazes were applied on a previously biscuited body and then fired again. As a result, a thin interface of reaction between body and glaze was developed. (vii) Furthermore, tin glazes of the 10–13th centuries were previously prepared as frits and then applied to the ceramic body. Meanwhile, the tin glazes of 14th century would be applied without friting raw materials. (viii)A lower lead proportion in Islamic glazes (Zaragoza), as compared with the other sites, is compensated for a constant quantity of potassium (about 3% K2O). Ground quartz was always added in the frit, but this glaze preparation stage was distinctly done in other Islamic glazes than in Zaragoza Islamic glazes, which seem to contain bigger grains than glazes from other sites. (ix) The ceramic paste is in all of the cases and sites a calcium-rich paste, which is coherent with two facts: (i) the development of calcium silicates incorporates iron in their structure and thus, a low development of iron oxides produces a buffcolour paste and less tin is necessary to opacify glaze than for a red paste (the colour of the paste is not seen through the glaze) (Vendrell et al., 2000); and (ii) the expansion coefficient of Ca-rich pastes is similar to that of the lead glazes, avoiding cracks during heating and cooling (Tite et al., 1998).