The heat for finishing may be rather more than that used in the first fire, 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 sur face appears bright and glossy.
We shall conclude our directions for firing the plates, with some general observations upon the accidents that are most likely to occur in both kinds of enamelling, and point out the best methods to remedy them when they happen. 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. This is no doubt owing to the unequal expansion of the two ena mels; and where the cracking takes place at the top or upper side of a plate, it requires very delicate treat ment to preserve the dial from being completely spoil ed.
To do this successfully, as soon as the crack is ob served, the plate must be withdrawn from the fire ; and if it extends only from the centre hole to the edge, it will, in most cases, bear mending ; but if it has taken place in two or three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 suffi cient 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 sometimes 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time when the crack has opened to its greatest width ; and before it unite or close at the bottom, the plate must be withdrawn front the furnace and allowed to cool. The opening must then be filled with fine ena
mel, 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 scam across the plate, will require much judgment; and, indeed, however well the opera tion may succeed, it will still remain visible, because the mending cannot be submitted to the process of using off, as the plate would, by such means, be rendered in parts very porous, and thereby contract dirt when the fingers touch its surface.
Another very common accident in making glass ena mel plates, is to over fire them, (as it is technically call ed.) Whenever this happens, the arsenic, which gives opacity and whiteness to the material, is converted into a flux by the extreme heat, and that part of the plate which has been so treated becomes semi-transparent and of a light blue colour. The only method that can be used in that case, is, to return the plate into the fire, and give it a longer continuance of heat, but as slight as possible, just keeping it red hot; and to do this con veniently, the turner should be brought nearer to the front of the muffle, as the greatest heat is always at the back. This treatment will restore the plate to a tolera ble degree of whiteness, by reviving the powers of the arsenic; but it must never be expected to look as white as if it had been properly fired. Should any crack ap pear on a hard plate, it would be in vain to try to mend it ; for as the shape of the dial in hard enamelling may be brought to such great perfection, when all parts of the process succeed, it is almost needless to say that the plate would suffer so very much in this point as to make it good for nothing. An accident, therefore, of this kindmay always, in hard enamelling, be considered as the close of an abortive attempt. (T. E.)