Enamel itself consists of a rather dense glass coloured with metallic oxides, and must not be confused with the enamel colours which are employed in painting on porcelain. The latter are vitrifiable but not vitrified material; that is to say, in them the metallic colours in powder are mixed with powdered glass, and the whole becomes fluxed together when the work is fired in the kiln or muffle, while in true enamel the glass and its colouring are intimately combined from the first. By way of definition it may be said that enamel work is therefore an arrangement of one or more layers of coloured glass on fused metal. In Champlevé work the design is first outlined on a metal plate, usually copper, and then, by means of gravers and chisel-like tools called scorpers, the space which is to receive the enamel is cut out to the necessary depth, from ¹⁄₃₂ to ¹⁄₂₀ of an inch, the rather rough surface that is left by the scorper serving as an additional key to the enamel (see Fig. 173), which having been ground to a powder and moistened with water, is then placed in the cavities prepared for it, and, after being carefully dried, is put into a muffle raised to a red heat, and thus fired until the enamel is fluxed. The work is then allowed to cool, is smoothed and polished, and the metal parts may then be gilt. Champlevé is usually executed in opaque enamel; that is, in glass made opaque by an admixture of oxide of tin. In very early specimens two or more colours are found in contact in one space, but this is extremely difficult to do, and the resulting line is a somewhat ragged one in European work, though wonderful effects of opaque colours are produced by the Japanese. Since the thirteenth century each colour, in Western work, fills its own space. [Illustration: FIG. 173.--Plate prepared for Champlevé Enamel.] The design, must, from the nature of the work, be kept very simple without too much subtlety of line that might be lost in the cutting, and the laying of the enamel will be facilitated if the angles of small spaces are not made too pointed. The principal qualities of form will consist in good distribution and well-balanced masses rather than in expression of detail. The outlines of charges which are in colour on metal, or vice versa, are formed by the edges of the sunk spaces and such further definition that maybe required, such as the marking of the junction of the further legs with a lion’s body, is effected with a line of the ground colour laid in a groove cut for the purpose. This perfectly natural method of drawing with the materials that are immediately concerned has resulted in some singularly inept modern examples of heraldry, where the limbs are deliberately represented as detached from the bodies, as though there were something mysteriously mediaeval in such an unreasoning travesty of a perfectly simple expedient.