A manageable red had long been a desideratum. There was no more treacherous material than the basic copper oxide, whether painted under or mixed with the glaze. As an over-glaze source of red this pigment was of course unavailable, while the opaque brick-like tints obtained from iron, though in keeping with the rougher, picturesque decoration of early times, did not harmonise well with the delicate style of painting now in fashion,[65] so that it is not surprising that the beautiful pink tint obtained from gold carried all before it. The gold was probably incorporated with the enamel flux in the form of purple of Cassius, which is readily prepared by dissolving gold in a mixture of nitric acid and sal-ammoniac and adding some fragments of tin. The colour had been known for some time in Europe--we can perhaps even trace this pink tint on enamelled Arab glass of the fourteenth century (see p. 89).[66] A very small quantity of this material goes a long way, especially when used to give a gradated tint to a white opaque enamel, as on the petal of a flower. As a colour it is singularly harmonious, and in a period of decline helped to ‘keep together’ the motley array of enamels used along with it. There is nothing more popular in the work of this time than the little egg-shell plates, decorated with flowers and birds, for which such high prices are given by collectors. The original type, for both ware and decoration, is probably in this case to be found in the ‘chicken-cups’ of Cheng-hua’s reign.