La "lèpre" des carreaux de céramique glaçurée de la cathédrale "El Seo" de Saragosse (Espagne, XVIe siècle)

Résumé En

One can quite often notice, on glazed ceramics panels that cover the internal or external walls of a building, that certain tiles have lost their glaze or a part of it. What is the origin of this degradation and why does it affect some tiles rather than others? Such is the main question we tried to answer studying and comparing the physical characteristics of two tiles. One is a priori not affected by this type of degradation and another one, on the contrary, has almost entirely lost its glaze. The study was undertaken on 16th century tiles, coming from the "El Seo" Cathedral in Saragossa (Aragon, Spain). We also propose to determine some data concerning the manufacture technique of these tiles.The study shows that the physical characteristics of the two tiles (glaze, interface and body) are, considering the experimental uncertainties, very similar. It has revealed - as suspected - that the detachment of the glaze is the consequence of mechanical constraints caused by the formation of sodium chloride crystals, which are very abundant in the body of the degraded tile but absent in the other one. These crystals were absent at the initial state of the degraded tile and seem to have been introduced later, probably by infiltrations of sodium-chloride-charged water. These infiltrations start from the ground through a mortar made up of gypsum and quartz. From the data of the physicochemical analysis of the not-degraded tile, clues on the manufacturing technique of this production were given. The tiles were made in calcareous clay, decorated by stamping according to the "cuenca o arista" technique. Then, they were fired once (between 400 and 600°C). After the application of different glazing mixtures, depending on the desired colour of the decoration, the tiles underwent a second firing at 900 - 950 °C.