"Crustae sunt tabulae marmoris. Unde et marmorari parietes et constati dicuntur. Qui autem marmora secandi in crustas rationem excogitaverunt non constat. Fiunt autem arena et ferro serraque in praetenui linea premente arenas, tractuque ipse secante: sed crassior arena plus erodet marmoris. Nam tenuis fabricis et polituris accomodata est." [Illustration: LI DOOR OF THE MIHRAB (_Showing mosaic-work. Cordova Cathedral_)] It is impossible to affirm with any confidence that glazed earthenware, whether in the form of tiles or other objects, was manufactured by the Spanish Moors during the Cordovese Caliphate, or the period of the kinglings of Taifa. No trace of it has been discovered among the scanty ruins of Medina Az-zahará[67] and Az-zahira--ancient palaces of Cordova--or in the marvellous mosque. We know, however, that towards the seventh century the Arabs borrowed from Byzantium the mosaic-work of tessons known as _psephosis fsefysa_, and this, or something similar, was used, though probably to a small extent, among the Muslims of the Spanish Caliphate. Although, towards the middle of the thirteenth century, the historian Aben-Said, a native of Granada, recorded that in Al-Andalus "is made a kind of _mofassass_ which is called in the East _alfoseifesa_," remains of this elaborate product only exist to-day at Cordova, where patches may yet be seen lining the dome of the _mirhab_ in the vast _aljama_ (Plate li.). The mosaic in question is stated to have been a gift from the Byzantine emperor to the sultan Al-Hakem, and was set in place by a skilled workman, a Greek, who, like the offering itself, proceeded from Constantinople. [67] Among these ruins, at five miles' distance from the city, pieces of common brick have come to light; but no glazed pottery of any kind, whether as _foseifesa_, _azulejos_, or mosaic. During his stay at Cordova this Greek was helped by certain of the Sultan's slaves, who thus acquired the secrets of the craft, and practised it thereafter.[68] [68] Dozy's version of _The History of Almagreb_, by Ibn-Adzarí the Moor; p. 253. Rodrigo Amador de los Rios contends, however, that this decoration is in no sense a true mosaic, but just a tempera painting executed on the wall and overlaid with cubes of glass. In any case, no other specimen of such work has been discovered in any part of the Peninsula.