The foregoing is an outline of thefor glass enamel plates. The firing of the Venetian hard enamel is but the heat applied to melt it must not be so great, and the plate must be aken from the fire as soon as the enamel is found to form one tolerably compact body, as any longer con tinuance would have a tendency to spoil the intended shape of the plate, which is always considered a most essential quality in those of hard enamel. The heat for finishing may be rather more than that used in the first fires, as in that instance the intention was only to unite the particles of enamel into one solid mass : but the principal object in finishing being to raise the flux to the surface as much as possible, a greater heat may be used with advantage, but the plate must be taken from the furnace the instant that the surface appears bright and glossy. Where good Venetian enamel cannot be obtained, and mixtures of various kinds are resorted to, it frequently happens that the glass enamel plates crack when they are brought to the second fire. When this is the case, as soon as the crack is observed the plate must be withdrawn from the fire ; and if it extends only from the centre hole to the edge, it will, in most cases, bear mending; but if it has happened in two or three places, it will be useless to make the attempt, as it will rarely succeed. If the dial plate was to continue in the fire after it is cracked a sufficient time, the enamel would close, and the plate become sound again ; but as the copper on its surface is in a state of oxidation, the oxide of copper uniting with the enamel would rise to the upper surface of the plate, producing by its union a faint and some times a dark green line, which would evidently render the plate useless. The
operator, therefore, must observe the time when the crack has opened to its greatest width, and before it unite or close at the bottom the plate must be withdrawn from the furnace and allowed to cool. The opening must then be filled with fine enamel, laid sufficiently high to allow for its running down in the fire ; but to adjust the quantity so as to prevent the appearance of a seam across the plate will require much judgment ; and, indeed, however well the operation may succeed, it will still remain visible.
The method of painting in enamel is performed on plates of gold, silver, and copper, enamelled with the white enamel ; whereon they paint with colours which are melted in the fire, by which they acquire a brightness and lustre like that of glass. This painting is the most prized of all for its peculiar brightness and vivacity, which is very permanent, the force of its colours not being effaced or sullied with time as in other painting, and continuing always as fresh as when it came out of the workman s hands.