When the furnace is sufficiently heated, lay the en amelled plates upon the hearth, on a ring of a proper size, and there allow them to remain till the moisture is entirely evaporated. The planch is then to be with drawn from the fire, where it will have attained a white heat, and the plate and ring set on it as nearly in the centre as possible; when the whole is to be placed upon the turner, under the muffle, and kept in constant mo tion, until it assumes one uniform surface. It must then be immediately withdrawn with the tongs, and another treated in the same way, till the whole day's work is fired, which, from the various qualities that may be given to the work, although made of the same materials, may vary in number from six to three dozen.
It rarely happens that any plate will come out of the first fire without a number of small black or green specks, which, perhaps, were in the enamel, and had escaped the notice of the operator. In addition to this imperfection, the union of so many 4mall particles of enamel in one mass throws up to the surface a scummy and mottled appearance, which renders the plate, when cold, very unpleasing. It will be necessary to remove these imperfections by first carefully removing the specks with a square graver, whetted so that the face makes nearly a right angle with the plane of either side, observing to hold the index finger of the left hand immediately under that part of the plate where the speck is situated, so as to counteract the pressure given to the graver, and prevent any strain upon the whole, which might cause the enamel to crack when it again approaches the fire. When all the specks are removed, the centre hole of the copper, if all has been conducted right, will appear to rise above the surface 01 the ena mel. This must be very carefully filed clown, and the same operation practised at the edge, which will be found to have a similar appearance, and the plate is then ready to undergo a process, technically called using of This is performed by rubbing the surface of the plate on a grit stone, with line sand and water, until! all the glazed appearance is completely obliterated, and one uniform and equally rough surface is produced. The intention of this part of the process, is to remove the mottled appearance on the surface, and give a more equal convexity to the plate. It may likewise be observ ed, that the flux in the enamel being always in the great est quantity near the surface, gives a semi•transparent appearance to the plate. The using off removes this perfection, and greatly increases the intense whiteness of the enamel, adding, at the same time, much to its beautiful opacity. After the using off is eomplet d, the plates should be brushed with a stiff haired broth and wet sand, to remove any light stain; when, being well washed in clean water, they must be dried with a clean napkin, and they are ready to receive the finish ing fire.
As the specks that have been picked out generally leave some deep holes in the enamel, they must be fill ed up with some of the finest enamel, nearly as fine as flour; and being placed in a proper sized ring, must be set on the iron hearth of the furnace, gradually pla cing it nearer to the fire, until it attains such a heat as will permit it to be placed in the 'hottest part of the furnace without danger of cracking, which would be very likely to happen if this gradual annealing was not attended to. The ring and plate may now he lifted upon
the planch with the tongs, and kept in motion, turning it gently round until it attains a white heat. As soon as this is observed, it must be brought out of the fire, and blown upon with the breath for two or three seconds, and immediately returned into the furnace, where it may be suffered to attain a white heat, which will bring it to a most beautiful gloss, and a degree of whiteness not to be exceeded. The plate must not be permitted to re main one moment longer in the fire after it has arrived at the desired heat, but must be quickly withdrawn from the furnace, and set to cool gradually ; when, if it is found to be free from specks, the operation is finished, and the plate ready to have the figures painted upon it.
If the enamel should have any specks upon the sur face, they can only be removed by the graver, and the firing repeated, omitting the using off in this case.
This is the process for firing and finishing the glass enamel plates. \Ve shall now describe the method adopt ed to fire and finish the hard enamel dials.
The firing of these when the enamel is laid on, is the same as for the glass enamel, the apparatus being used for both purposes ; but the heat applied to melt the Venetian hard enamel must not be so great, and the plate must be taken from the fire as soon as the enamel is found to form one tolerably compact body, as any longer continuance would have a tendency to spoil the intended shape of the plate, which is always considered a most essential quality in hard enamel The method of using off the hard enamel plates is different from that made use of in the glass enamel ; for as the proportion of flux in the Venetian hard enamel is con siderably less than in the glass, and as diminishing the flux by using it off, (if carried to the extent we have before described,) would render the surface very po rous, great care is necessary to be observed that the flux may be raised to the surface in the first fire as little as possible ; which is done by giving the plate a very slight heat, barely sufficient to unite the particles of enamel into one solid mass. Should any specks ap pear, they must be taken out with the graver, as was directed in the glass enamelling, and the surface must be reduced to perfect evenness, by rubbing it with a grey stone, or what is still better, the water of air stone found in Scotland, and now much used by mathe matical instrument makers. This method of using off must be continued until the surface is smooth, and all the old or first gloss is removed. This part of the pro cess should be very carefully performed, for should the plate receive any strain, it would be cracked in the fire, and inevitably spoiled ; to prevent which, it will be best to hold the plate in the fingers of the left hand, while the stone is rubbed lightly over every part of its sur face. The plate must then be well washed with very clean water; and to prevent any particles of the stone remaining in the pores, it is customary to rub the sur face for about five minutes with a piece of soft smooth hoop wood, or what is still better, if it can be had, soft dog wood cut quite flat and smooth. When this is done, the plate must be well washed and wiped, and will then be ready for the fire.