This kind of enamel is done on a smooth plaque of thin copper or other metal which has been slightly domed for the purpose. It is covered with a coat of enamel which forms the ground, the back also being enamelled in order to equalize the contractile strains, and fired. The design is then painted in white of suitable thicknesses, which on being fired form tones with the ground colour according to their relative opacity, and the result is called grisaille. This is a very charming form of decoration in itself, and is frequently done with that intention. Usually, however, coloured enamel is painted over it, the work being carefully fired at each stage, a very anxious process, and finally it may be heightened with gold after the manner of the illuminations. An additional method of obtaining decorative effects, in diapers, etc., presents itself in the power of scratching the painted gold with a needle before firing. Fig. 176, executed in this manner, is a plaque that is the property of W. H. Weldon, Esq. From the capacity of painted enamel to imitate the effects of the illuminations it presents few technical difficulties as design. Its practice, however, is another matter, as may be imagined when the risk of the numerous firings is taken into account. A form of enamel that is occasionally used for badges and similar heraldic subjects is that which is called bassetaille. In this method the subject, usually in the form of a medallion, is carved in low relief and the transparent enamel fluxed completely over it, the surface being afterwards ground and polished. The varying thickness of the enamel, from the deepest part of the relief to the highest, gives a quality of gradated colour that is extremely beautiful. In a badge or jewel of this kind there is, of course, a rim raised to the height necessary to contain the enamel and extending above the highest point of the carved subject. [Illustration: FIG. 176.--Enamelled Plaque. Crest of W. H. Weldon, Esq., C.V.O., Norroy King of Arms. Geo. W. Eve.] Still another method is that which came into use in the fifteenth century for jewels, and is known as plique-a-jour, a kind of transparent Cloisonné which is said to have been produced by Cellini, who certainly knew of it. It is an extremely difficult process, but the effect is remarkably precious and jewel-like, the enamel being fired into a design which has been built up of bent wire and soldered together like Cloisonné without the bedplate, so that the work looks like a miniature piece of stained glass (as indeed it is) and of great beauty. In a similar way a design may be cut out or saw pierced through a plate of metal and filled with enamel _à jour_.