SECTION VII.
Of the manner of laying on and burn*
ing the enamel grounds*
T H E matter of the enamel ffiuft be firfi finely levigated and fearced : and the body fo be enameled fhould be made perfectly clean. The enamel muft be then laid on as even as poffible by a brufh or pencil, being firft tempered with oil of fpike; and the dis-tance of time betwixt the laying on the ground and burning the piece fhould not be too great j becaufe the oil will exhale and leave the matter ©f the enamel a dry incohering powder; which will be liable to be rubbed or fhaken off by the leaft violence. This is the common method ; but there is a much better way of managing this part of the work by means of a fearce; in which the enamel is fpread with very little trouble; and the greateft part of the oil of fpike laved. The method of performing this is, to rub the furface to be enameled over with oil of fpike ; and then, being laid on a Iheet of paper or piece of leather, to fave that part of the enamel which dbes not fall on a proper objedt, to fearce the matter upon the oiled lurface till it lie of a proper thkknefe ; but great care muff be taken, in this method of proceeding, not to lhake or move too forcibly' the pieces of work thus covered with the powdered enamel.
It is ufualto add oil of turpentine to the oils of fpike or lavender, in order to make them go further, and fave the expence attending the free ufe of them ; and others add alfo a little olive or linfeed oil, or lome, in the place of them, crude turpentine. The ufe of the Ipirit of turpentine is very allowable ; for it is the feme for this purpofe as the oils of fpike or lavender; except that it wants the glutinous quality which makes them lerviceable in fpread- ing the enamel: but with refpedt to the ufe of the oils of olive and linfeed, or any other fub- ftantial oil, it is very detrimental tending to reduce the metalline calxes; and leaving a fmall proportion of black coal or alhes, which muft neceflarily injure the white colour of the ground. ' .
When plates, as in the cafe of pictures, dial- plates, &c. are to be enameled, they fhould always be made convex on the outerlide, and concave within; and all pieces of enamel formed of metal, where the figure does not admit of their being thick and folid, fhould be of the fame kind of form: otherwife they will be very apt to warp in the heat; and cannot be brought ftreight, aftei* they are taken out of the fire, without cracking the enamel. For this reafon, like wife, it is proper to enamel the work all Over, as well on the wrong as right fides, to prevent the heat from calcining the metal j which would both contribute to its warping, -and weaken the texture of it.
The enamel being laid on the body to be enameled, when the fixt muffle is ufed, the piece muft be gently lifted on to the falfe bottom; and put in that ftate into the muffle fixt in the furnace defcribed p. 235, by thrufting the falfe bottom into it as far as it will go. But it is beft to defer this till the fire be perfectly in order, which muft be known by putting a bit of tile or China with fome enamel on it of the fame tone with that ufed as a proof j and another proof of the fame kind may be alfo put along with the work into the muffle ; which, being taken out, may fhew how the operation proceeds.
When coffins are ufed, the fame general method is to be perfued : the pieces of work to be enameled are to be laid on the bottom of the coffin nH it be covered : and then the fecond flooring or falfe bottom is to be fixt in ife place, and covered in the fame manner; after which the lid is to be put on ; and well feeured, in the joints it forms with the fides of the coffin, by fire-lute. The proof, in this cfcfe, fhould be laid on the lid, on the part next the'fide door of the furnace ; and it may be expedient, efpecially till the working of the furnace, and the kind of enamel uled be very well underftood, to have two or three of thefe proofs. The enamel work being put thus into the coffins, they fhould be feton the dome of the furnace, which muft be of the kind defcribed p. 239, that they may be moderately heated, before they be put into the furnace j. which would otherwife endanger their cracking; and when they are fo heated, proof having been made/ by means of a fmall bit of China, or copper covered with the enamel, fliat the fire be of a due force, they muft be conveyed into the furnace through the fide dfeor, and muft reft on the pieces df fire-ftone placed for that purpofe on the flooring in the back part of the furnace. It is requifite never- thelefs, that the conveying diem into the furnace fhould be managed with particular care to prevent the fhaking off the enamel; and it muft be done by means of the peel or inftru- ment adapted to this purpofe; on which the coffin being laid, it muft be gently thruft into the furnace till the coffin be in its proper X fituation
'fixation, with refpeft to diftance from t$ut fides of the furnace; and then the further end qf the peel muft be turned flowly towards the front of the furnace ; the coffin being at the fame time fhoved off from it by means of the flat end of the tongs before defcribed, intro* duced through the door for feeding the fire, till it ftand intirely on the fire-ftones, when the peel muft be withdrawn. The operation being finilhed, the peel muft be again intro* duced under the coffin, by railing firft the neareft end of it, by means of the tongs thro’ the door in the front; and then the other parts gradually, till the peel fupport it; and then they muft be drawn out together; and may be beft placed on the dome of the furnace, that by cooling more gradually the temper of the enamel may be improved. .
If it be required to burn a lingle piece or two in this kind of furnace, it may be done by means of the common or loofe muffle defcribed p. 241. In which cafe the enamel work being laid on the bottom or flooring of the muffle, and the muffle put over it, the whole muft be conveyed into the furnace, by means of the tongs with turned points, thro’ either of the doors; and a proof may be at the fame time put in, on a piece of tile or lpare bottom of. a muffle : and the proceedings in other refpedts may be the fame as with the fixt muffle, or the coffins. •
Where there is no furnace, and it is defired to burn enamel work on an open hearth, preparation
paration muft be made according to the directions in p. 240 : and the flooring of the muffle being laid at a proper diftance from the' nozzle of the-bellows, the work muft be laid on it; and covered' with the muffle: which being done, pieces of charcoal muft be heaped over them ; and the fire being lighted, muft be blown up with the bellows, till it be fufiicient to flux the enamel; which muft be examined by the proof put into the fire along with the work. The coals muft then be taken off from the muffle; and the muffle, with the flooring and enameled work, removed out of the fire; but kept! near it to prevent their cooling too faft : and, if there be more work to be burnt, another muffle, &c. may be immediately put in the place of the other, and the fame operation repeated; for it is a matter of indifference, whe-" these the coals be burning when the work is put: on the hearth, or kindled afterwards.
Pit eoal may be ufed in the furnace, where enamel'is burnt with the fixt muffle, or in coffins : which is indeed one principal conveniency attending the ufe of them; as it faves a con- fiderable expence of charcoal: but where the open muffle is ufed, charcoal alone fhould be employed : as the fumes of mineral coal are very detrimental to fome colours; and deftruc- tive; of the grounds, if whitened by arfenic, as the common white glafs.