The enamels, after being finely ground, should be thoroughly dried; then mixed up with turpentine, and used like other colours with a pencil; after which fused again, and vitrified by fire. Spirits of tar may be substituted instead of turpentine in all enamels, with the exception of blue and colours prepared from chrome. With regard to the burning, the lustres will bear the highest temperature of an enamelling heat; the rose colour, cornelian red, and pomona green require a less degree of heat, and are generally placed in the middle of the kiln or muffle, as well as burnish gold; other colours are not so susceptible of being destroyed by heat, and will fire in any part of the kiln or muffle. The even surface of the various coloured grounds on china is produced by first laying the space wanted with linseed oil, previously boiled with a little red-lead and a small portion of turpentine; the enamel colour is then ground fine, and dusted on the oiled part with cotton wool, or laid on with a large camel-hair pencil.
The component parts of the different colours are as accurately stated as possible, but the preparation principally depends on observation, therefore experiments will be necessary that a proper judgment may be formed.
Take 2 parts flowers of sulphur, and 4 of turpentine; put them in a vessel over a slow fire until the sulphur is completely dissolved; after which add 8 parts linseed oil, and continue the same degree of heat for about one hour; previous to becoming cold, strain it through a piece of cloth.
Copper black is a very fine colour, the obtaining of which altogether depends upon a proper temperature of heat being applied, for nothing is more fickle and uncertain; if in the least degree overtired, the colour is destroyed, and becomes of a dirty green. The other blacks are called umber blacks, and will stand any degree of heat which is required in an enamelling kiln or muffle. The umber to be highly calcined in a biscuit oven, but particular caution should be observed that it is the real Turkey umber, and not the English, which is of an inferior quality. The two first enamel blacks to be calcined in the usual way; the materials of the two latter only want grinding.
1 part calcined umber, 1 1/4 calcined borax, 1/2 blue calx.
1 part copper calcined, 3 enamel flux (a).
For these the materials must be calcined in an air furnace or glazing oven, and caution should be observed that they are not too finely ground at the mill, in order to prevent them from crazing or chipping after being burnt on the pieces of ware, (a) 16 parts flint glass, 5 red-lead, 2 white enamel, 2 blue calx, 1 common salt, 1 potash. (6) 16 parts flint glass, 5 red-lead, 2 nitre, 2 potash, 2 1/2 blue calx.
(a) 30 parts refined regulus of zaffre, 1 piaster, 1/2 borax. (6) §0 parts refined regulus of cobalt, 1 plaster, 1/2 borax. These materials to be made very fine, and well mixed; put the mixture in earthenware biscuit cups 1} in. high, 3 in. diameter, and 1 1/2 in. thick, filled nearly to the top; set them in a furnace, the fire to be increased until the mixture is in a state of fusion; the same degree of heat must be continued for about six hours afterwards, and then the fire hastily slackened; this operation will occupy 12-13 hours; at the top of the cups will be found a blue calx separated from the nickel; but as a large proportion of blue will still remain in the nickel when sunk to the bottom of the cups, it will be necessary, in order to procure the whole of the blue contained, to pursue precisely the same method over again.
50 parts regulus of cobalt, 6 potash. Refine as for regulus of zaffre; the operation of refining must be repeated until the scoria is of a bright colour and of a slight bluish hue; then spread the purified metal, finely pulverised, 1/2 in. thick, on flat pieces of earthenware covered with flint; place in a rever-beratory furnace, and apply a moderate degree of heat for a few hours.
32 parts sand, 32 potash, 10 borax, 1 blue calx. These smalts, the materials of which are calcined in the usual manner when finely pulverised will produce a fine rich-looking blue powder.
112 parfa zaffre, 57 potash, 18 1/2 charcoal. The charcoal, being pulverised, and all the materials mixed up together, they are put into large-sized crucibles capable of holding 3-4 qt., and filled quite full, then placed in a strong brick-built rererberatory furnace, commencing with a slow fire, and continued for some time, but as soon as it is heated to a red-heat, it will require a considerably stronger fire before the cohesion . between the different particles is sufficiently destroyed. This operation will be complete in about ten hours, the weight of the regulus being 31-33 lb.; on examining the scoria, if there remains mixed with it small pieces of metal like small shot, or when pounded, if the scoria has a bluish cast, the fire has not been strong enough; there is but little danger to be apprehended from the most intense heat, provided the particles in fusion do not perforate the crucibles. At the bottom of each cake of regulus there will be bismuth slightly adhering, which is easily separated without the application of any great degree of heat, by placing the cakes upon an iron plate or pan, which will soon bring the bismuth into a state of liquefaction, and it can then be separated from the regulus.