Thrown and turned goods are sometimes ornamented with figures in relief, which are made out of flat moulds by children, and fixed upon the ware by workmen, who, having care fully adjusted each figure to its place, run a little water under it with a camel-hair pencil, which unites it to the surface of the pot. Goods of an oval or angular Shape, which can not be turned, are made by pressing clay into plaster moulds, which give the outside form to the vessel : thii is called Hollow-Ware Pressing. Another kind, called Flat-Ware Pressing, is performed by giving the shape to the goods by moulds which fit the inside of the vessel; plates, dishes, saucers, cups, and hand-basins are made on this principle.
Casting is resorted to when a mould is so intricate as to be difficult for the workmen to fill by pressing. Slip clay is poured into the mould, which rapidly absorbs the water ; and a coating of clay is deposited upon the inside; the remaining fluid is then poured out or drawn with a syringe, and a thicker mixture is put in, and left rather longer than the first before it is withdrawn. The mould is then put into a stove to dry.
When completed by the workman, the goods are placed on boards to dry, before going to the biscuit-oven, in which they receive the first fire. The ware to be thus burnt iS placed in Saggers made of crucible clay; in shape they resemble hat-boxes, and being piled iu columns, each sagger covers the one beneath it, and protects the goods from the irarnediate con tact of smoke and flame. When the baking is finished and the oven cooled, the doorway is opened, the saggcrs brought out, and their contents submitted to a rigid scrutiny: all cracked and crooked pieces are rejected and thrown away. The ware is now called Biscuit, and in this state goes to the printer or biscuit painter to be ornamented.
The printing of earthenware is effected by transfer-papers from engraved copper-plates. The ink used in printing is made of linseed oil boiled with litharge, rosin, balsam of sulphur, or Barbadoes Us, and is tinted with any of the usual mineral colours—blue being the principal, and formed of oxide of cobalt. The colour having been ground'very fine, the printer blends it with his oil upon a hot stove, and filling the engraved plate with it, takes off the impression by the common rolling press. The tissue-paper used for this pur pose is first prepared with a solution of soap. As soon as the print is taken, a girl cuts out the engraving with scissors, and hands it to the transferer, who carefully places the print upon the biscuit-ware, which being absorbent, holds it with great tenacity. The transferer
then passes it to her assistant, who with the end of a cylinder of flannel, tightly rolled and bound with twine, rubs the print with such force as to work the ink into close contact with the biscuit. The goods thus coated with paper are then put into a tub of water, and the paper being wiped off with a sponge, every minute point of the engraving is found accurately transferred to the earthenware. When dry, the goods are packed close in a large muffle, or kiln, round which a fire cir culates, and brings the whole to a low red heat. By this means the oil is burned out of the colour, which would be injurious to the process of glazing which follows. Some pat terns are executed on biscuit by painters, who lay on the colours in gum-water.
The biscuit-ware, thus ornamented, is car ried to the dipper, who dips each piece into the tub containing the finely-ground mixture which, when melted, forms the glassy coating to the ware. This glaze is blended in water, which, being absorbed by the biscuit, leaves a thin cover of glazing-powder upon the surface ; a dexterous shake of each piece in a circular motion, as it emerges from the fluid, prevents the glaze from setting unequally, and throws off all that is superflous. The material of the glaze differs according to the kind of ware : white lead is a general ingredient.
When the goods have been dipped in the glazing mixture, they are dried, and placed in naggers, which are washed on the inside with a compound of glaze with lime and clay. Every piece is carefully placed so as not to touch another ; otherwise, when the glaze melts, they would stick together. When drawn from the oven, the ware is carried in baskets to the glossed warehouse, where it is again subjected to a close examination.
Porcelain is a finer species of pottery, in which the ingredients aro so selected that they act chemically upon each other, and are brought to a state of vitrification ; the fracture has a dense or greasy surface, like that of a flint stone, and is therefore not liable to be acted upon by acids. When the porcelain is coloured by metallic matter, it is called stone ware ; jasper and some drab-ware are of this description : but when it is perfectly free from colouring matter and is translucent, it is called China ; of which there are two species, hard and soft china.