The composition of the eastern or proper chinaware, according to accounts that have great marks of authenticity, is from two earths; one of which is, as was before mentioned, called petunse; the other a refractory earth, called kaolin.
The preparation of the petunse, or aluminous earth, is by pounding the stone till it is reduced to a very fine powder, and then washing it over to bring it to the most impalpable state, which is thus performed: After the stone is rendered as fine as it can be by pounding or grinding, the powder must be put into a large tub full of water, and, being stirred about, the upper part of the water must be laded out into another tub, by which means the finest particles of the powder will be carried into it. The water in the second tub must be then suffered to stand at rest till the powder be subsided, and as much as can be laded off clear must be put back into the first tub, and there being again stirred about, and loaded with a fresh quantity of the most subtle part of the powder, must be laded again into the second tub as before, and this must be repeated till none be left in the first tub but the grosser part of the stone, which, not being of a due fineness, must be again pounded, and treated as at first. The fine powder obtained in the second tub, must be then freed from the water, by lading off the clear part, and suffering what remains to exhale, till the matter becomes of the consistence of soft clay, when it will be fit to be commixed with the kaolin for use.
The kaolin is prepared in the same manner by washing over; but some specimens are so fine, that there is no occasion for this or any other purification.
From these two mixed together, the clay or paste is formed; but it is said that the proportion of the respective quantities is made to vary according to the intended goodness of the ware, the best being made from equal quantities, and the worst from two of the kaolin to one of the petunse.
_To make Saxon or Dresden China._
The Saxon composition, of which the chinaware is formed, is greatly similar to that of the eastern. In the place of the petunse, a stone is used, which is improperly called in the German language, bleyspatt, or spar of lead. It is a stone of a very opposite nature, as spars are calcareous, and will, on calcining, become lime; on the other hand, this stone is of a vitreous nature. This spar is of a very hard texture, and of a light flesh-color, or pale whitish red. It is prepared by pounding and washing over, which may be done as above directed and it is then ready for compounding with the mica. The mica is employed in the Saxon composition for the other ingredients; and is likewise prepared by grinding and washing over, when it is not in a perfect and pure state, but when it is entirely clean, it may be tempered with the texture, thoroughly broken, and it will be of the consistence of soft clay.
The two kinds of earth being prepared in the state of a soft paste, they are to be incorporated and blended into one mass, which is done by rolling and stirring them well after they are in the same vessel, and then kneading them with the feet till they are thoroughly united. When the compound mass is formed, it is made into cakes, or square pieces, and put by layers into cases of wood or stone, which must be placed in a moist situation, and left for 2 or 3 months; during which time a kind of ferment enters into the mixture, by which the parts of the different matter combine and form a substance with new qualities, unknown while separate. This change shows itself upon the whole mass by a fetid smell, and a greenish or bluish color, and a tenacity like that of clay, or the argillaceous moistened earths. If the time of keeping the paste in this condition be prolonged to a year or more, it will further improve its qualities, but great care must be taken to prevent its becoming dry; to prevent which, there may be occasion to water it. When, however, the described qualities are found in the matter, it is fit for use, and vessels, etc., may be wrought of it without any other preparation, the case below excepted.
_Composition of English China._
The following composition will produce wares, which will possess the properties of the true china, if judiciously managed.
Mix the best white sand, or calcined flints, finely powdered, 20 lbs.; of very white pearlash, 5 lbs.; of white calcined bones, 2 lbs. Temper the whole with the gum Arabic or senegal, dissolved in water.
This requires a considerable force and continuance of heat to bring it to perfection, but it will be very white and good when it is properly treated. Where mica can be obtained, it is preferable to calcined bones, and as it will form a kind of paste for working, a weaker gum-water will answer the purpose.
_To Bake Chinaware._
The furnace for this purpose may be constructed in the same manner as the potter's kilns usually are. The size of the furnace should be according to the quantity of ware required to be baked; but it must not be too small, lest the body of fire may not be sufficient to produce the requisite heat.
The caffettes, or coffins, to contain the pieces when placed in the furnace, are the most material utensils. They should be of good potter's clay, with a third of sand, and are generally made of a round form, with a flat bottom, the rim forming sides, being adapted to the height of the pieces to be inclosed.
The furnace and caffettes being prepared, the ware to be baked must be sorted in the caffettes in the most advantageous manner as to room, and as many caffettes must be set upon them as the furnace will conveniently contain, leaving space for the free passage of the fire betwixt the piles: take care to cover over the uppermost caffettes in each pile, then close the mouth of the furnace, and raise the fire so as to heat the caffetes red hot in every part, and keep them red hot for 12 or 14 hours. It is then to be extinguished, and the furnace left to cool gradually; and when little or no heat remains, the mouth may be opened, and the pieces taken out of the caffettes; when they will be in a condition to receive the glazing, or to be painted with such colors as are used under the glaze.
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