The production of _azulejos_ in Spain may thus be traced to as far back as the twelfth century. By far the most important centre of the craft was Seville. Here, from the twelfth until the fourteenth century, was made the glazed and decorative tiling which consisted of small pieces of monochrome earthenware--black, white, green, blue, or yellow--cut one by one, and pieced together in the manner of a true mosaic. This process, says Gestoso, was lengthy, difficult, and dear. In the fourteenth and fifteenth centuries the same mosaic would often take the form of a series of narrow, white, ribbon-like strips, with coloured interspaces. Specimens of this "ribbon-work tiling" exist to-day in the Patio de Las Doncellas of the Alcázar (Plate lii.). Towards the sixteenth century the Sevillano potters discovered a simpler way of making effective and artistic _azulejos_, which they called the _cuerda seca_ process. This novel method consisted in pressing a wood or metal mould upon the unbaked tile, in such a manner that the outline of the pattern remained in slight relief. This outline was next brushed over with a mixture of manganese and grease, which turns, in baking, very nearly black. The body of the pattern was then filled in with the various colours, which the greasy line completely separated, and thus prepared, the tile was rendered permanent by firing. [Illustration: LII MOSAIC OF THE PATIO DE LAS DONCELLAS (_Alcázar of Seville_)] This process, in which the patterns are nearly always geometrical, remained in general use until about the year 1550, when it began to be superseded by two others, known respectively as the processes of "cuenca" and "Pisano". The _cuenca_ tile was simple and of excellent effect. The pattern, stamped from a metal mould, remained in bas-relief,--a characteristic which caused these objects to be also known as _azulejos_ "_de relieve_". The shelving border of each hollow stamped into the tile thus formed a kind of natural barrier which kept the colour there deposited from mingling with its neighbours. When of a larger size, and joined in pairs to form between each two a single motive (_ladrillo por tabla_), these _azulejos_ were often employed for decorating roofs and ceilings.