As to the methods of producing lustred pottery, the chemical investigations practised by Riocreux, Brogniart, Carand, and others, have shown that the metals used to produce the characteristic reflex which gives the ware its name were copper and silver, entering into the composition of an extremely thin glaze extended over the surface of the pottery, and employed, sometimes together, and sometimes separately. It is obvious that the lustre produced by copper would be deeper, redder, and less delicate than that produced by silver, while varying gradations would be obtainable by the mixture of both metals. It is also beyond doubt that the oldest specimens of this pottery extant to-day are those which contain the palest and most pearly lustre, and consequently the largest quantity of the costlier metal. In those of later date there is an evident inferiority, both in colour, lustre, and design. In fact, two separate, or nearly separate, epochs of this branch of Spanish pottery are pointed out by Señor Mélida, who gives the name of _Mudejar_ to lustred objects manufactured at an earlier time by Moorish artists working in the cities captured by the Christians, and that of _Morisco_ to the second or inferior class produced by Morisco craftsmen after the reconquest, and distinguished by the coarser and degenerate lustre, colouring, and draughtsmanship. The rarest and most beautiful examples of this ware are naturally those which belong to the former class, and consist of various kinds of plates and other objects in which elaborate devices such as lions, antelopes, and shields of heraldry, often combined with foliage and inscriptions in Gothic lettering, are coloured in bistre or pale blue,[82] and rendered doubly beautiful by the delicate nacreous lustre. [82] In lustred pottery these colours, and particularly blue, are far the commonest. It has been found that other colours, such as green and black, were ill adapted to the lustre process. In nearly every case it is extremely difficult to determine with any certainty the date of manufacture of these objects, as well as the locality. Wallis says he is aware of "no example of Spanish lustre pottery antecedent to those in the class to which the large Palermo jar belongs, and they are not likely to be much earlier than the end of the fourteenth century. Happily the celebrated plaque (Plate lviii.) formerly belonging to Fortuny, and now in the possession of Excmo Sr. Don G. J. de Osma, furnishes an early date, which, according to its owner, is between May 1408 and November 1417. Those who know the original will remember that it is no less remarkable for the quality of its golden lustre than for the grace and elegance of its fanciful Oriental design." It is also believed by Señor Osma that this plaque was manufactured in the kingdom of Granada; _i.e._ either at Granada or Málaga.