The ground silver sand is used to give sharpness to the polishing stones, and wear away the enamel with greater celerity. The act of polishing is continued till all the gloss is ground off the surface. In this operation care must be exercised that the pressure is not too powerful, as the plates will crack in the fire, and can never, or very rarely, be properly mended.

When the enamel is sufficiently polished, which can easily be known by the criterion of all the gloss being removed, the plates must be clean washed, and all the specks of dirt, etc, picked out with a sharp graver. They are then well rubbed over with some fine ground glass enamel, either by means of a cloth, or perhaps a small piece of fir - wood cut smooth, in order to remove the stains that may be left by the polishing stones, and, clean water being suffered to run over them, they are wiped dry and placed on rings for firing, as described. The degrees of heat necessary for glossing plates are determined by the fine or coarse modes by which they were prepared, as the fusion is much facilitated by the enamel being free from scratches. When the surface is properly run - i.e. when it becomes perfectly smooth, even, and bright - the plate is completed, and when cold, is tit for painting on. The above description refers more particularly to the best kind of work. There are two other modes of enamelling watch - dials, which it will be required briefly to explain.

The plates made by the following methods are called technically "run - down plates," and " run - down second coats."

Run - down plates are those which are made by laying enamel upon the copper in sufficient quantities to form plates of the required thickness without putting on a second coat. Both labour and fire are thus saved, but the neatness, regularity, and 8 juareuess obtained by the first method are scarcely obtainable in this; and indeed flat plates can hardly be managed at all by this mode. Running - down plates require more "coddling " than the others, and a longer continuance of vivid heat is necessary to make the glass flow to a proper evenness of surface, the plates being wholly completed with one heat and without polishing. It is obvious that only common work can thus be manufactured. For work of the next superior description, the run - down coats are polished off with the ragstone and undergo a second firing. The run - down second coats are those which are. reduced to a comparatively even surface by a second firing, and then painted on without being polished off.

In enamelling hard plates for watches, the coppers and the first coats are prepared in the manner already described, excepting, perhaps, that the layer of glass is rather thinner than in glass work only. The hard enamel, which used to be much valued on account of its rich cream colour, is broken down and ground in the same way as the glass, if only a small quantity is wanted; but if otherwise, it is first broken from the cake with the hammer, and then pounded in a steel mortar till reduced to coarse grains. These grains are then exposed to the action of a magnet, in order that all the particles of steel that have been broken off the mortar in the act of pounding may be taken away, as they would infallibly spoil the work by rising in black specks to the surface of the enamel when exposed to the fire. As an additional precaution, it is also necessary to put the granulated enamel into a 8mall basin, and pour upon it a strong solution of oil of vitriol, or aquafortis, and allow it to stand for some time, so that any steel particles which may be in the enamel may be completely dissolved.

The enamel must then be very carefully washed till the water comes off pure and tasteless, for should any acid remain, the work would certainly blister.

The enamel is then ground to the necessary fineness in an agate mortar, as previously described, and afterwards spread over the first coats with a quill in small quantities, and as evenly as it can be laid, so that it may require the use of the spatula as little as possible. The water is then partly absorbed by a very fine, clean napkin, and the enamel is smoothly spread and closely compressed with the spatula, after which more water is absorbed, and the spreading is continued till the surface lies true and even. The plate is then put upon a ring and properly fired, and is afterwards polished by placing it upon a cork and grinding the surface, first by a fine file, or a smooth piece of steel with silver sand ground to an almost impalpable powder; secondly, by a fine blue - stone and sand; and thirdly, by the blue - stone alone. With the latter, a sort of half - polish should be given to the enamel, and the higher that polish approaches to complete glossiness the better, as the plate will then be finished in the fire with a less degree of heat than would otherwise be required. In this process, much caution is required to prevent scratches, which cannot be "run up" by the fire without giving the enamel a greater degree of heat than it will bear.

It should be mentioned that before polishing the face of the dial, the top edges should first be taken off with a fine grey stone.

When the polishing is completed, the plate is carefully cleaned with ground enamel, and should there be any specks, they must be picked out with a small sharp diamond, and the hollows very dexterously filled up with enamel from a quill point, so that they may neither rise above nor sink below the general surface when the plate is again fired. Should they actually do so, they must be made smooth with a Milestone, and the plate must undergo a fourth firing to render the surface of uniform texture and glossiness. Hard enamel dials are always considerably more expensive than glass enamel ones, through the greater labour, attention, etc, that is requisite in making them. In the polishing off of both hard and glass enamel dial-plates, much care is required 3 to prevent a separation of the enamel from the edge of the copper; for if too great a pressure is exercised, or if the stones which are employed to grind down the copper are too rough in the grain, the adhesion will be destroyed, and various black indents will arise round the edge of the enamel when the plate is again exposed to the fire.

In glass enamel dials these defects may sometimes be amended, but in hard enamel dials scarcely ever.

These directions for enamelling watch - dials may be concluded with some general observations upon the accidents that are most likely to occur in both kinds of enamelling, and in pointing out the best methods to remedy them when they do happen. When 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 due to the unequal expansion of the 2 enamels, and when the cracking takes place at the top, or upper part of the plate, it requires very delicate treatment to preserve the dial from being completely spoiled.

To do this successfully, as soon as the crack is observed the plate must be withdrawn from the fire, and if the crack extends only from the centre hole to the edge, it will in most cases bear mending; but if it has cracked in 2 or 3 places, it will be useless to make the attempt, as it will rarely succeed. If the dial - plate were to continue in the fire a sufficient time after it was cracked, the enamel would close, and the plate become sound again. But as the copper on its surface is in a state of oxidation, the copper oxide uniting with the enamel would rise to the upper surface of the plate, producing by its union a joint, and sometimes a dark green line, which would evidently render the plate useless. The operator must then observe the time when the crack has opened to its greatest width, and before it unites or closes at the bottom, the plate must be withdrawn from the fire and allowed to cool. The opening muni 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, because the new cement cannot be submitted to the process of "using off," as the plate would by such means be rendered very porous in some parts, and thereby attract dirt when the fingers touched the surface.

Another very common accident in making glass enamel plates is to " over - fire " them, as it is technically called. Whenever this happens, the arsenic, which gives whiteness and opacity 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 conveniently, the plate should be placed near the front of the muffle, as the greatest heat is always at the back. This treatment will restore the plate to a tolerable 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 at first. Should any crack appear on a hard plate, it would be in vain to try to mend it, for as the shape of a hard enamel dial can be brought to 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 render it good for nothing; therefore, an accident of this kind in hard enamelling is always regarded as the close of an abortive attempt.