Casting is resorted to when n 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 tho water, and a coating of clay is de posited upon the inside ; the remaining fluid is then poured out or drawn with a syringe, and a thicker mixture in put in, and left rather longer than the first before it is withdrawn. The mould is then put into a stove to dry. Goods thus made are very light.
When completed by the workman, the goads are placed on boards to dry, before going to the biscuit-oven, in which they receive the first fire. The details of the process are described in the article already cited ; and the ware, now in the state of biscuit, is ready for the blue printer or for the artist, according to its quality and intended appearance.
In the eld days, before the expoditioua and cheapening process of electrotyping was known, a set of engraved plates for a table-service commonly cost 130(. or 1501. • and would, with proper care, print 2000 dozen services before it wanted recutting. The ink used in printing is made of linseed-oil, boiled with litharge, rosin, balsam of sulphur, or Barbadoes tar; almost every printer has his favourite recipe for making this tenacious oil, which is the vehicle of the colour to be used. Blue colour is made of oxide of cobalt, mingled with flint or carbonate of lime so as to dilute it to the proper tint. Lilac, of smalts 2 parts, man ganese 1. Brown, zaffre 2, litharge 2,antimony 1, manganese 1. Red brown, manganese 12, litharge 2, flint 2, glass 1, borax 1. Orange, litharge 6, antimony 4, oxide of tin 1, oxide of iron 2. Pink, sub chromate of tin and carbonate of lime, equal parts. Green, oxide of chromium, the tints varied with cobalt or tin. Black, red-lead 60, antimony 25, manganese 15, fritted together; then add oxide of cobalt 40, oxide of tin 5.
Some patterns are executed on biscuit by painters, who lay on the colours in gum-water, in which case the firing in the mulile is not required; but the choice of colours in this kind of painting is rather limited, as the heat of the glazing-oven and the chemical action of the materials of the glaze would be destructive to many colours which are used by the enameller upon the glaze. For a descrip tion of the modes of printing and glazing, see The • following glazes are excellent, and have been successfully used in the Staffordshire potteries; they must bo ground very fine at the mill. Cream-colour glaze.-White-lead 66, Cornish granite 22,
flint. 12. Printed-ware !,laze.-White-lead 45, Cornish granite 28, flint 13, flint-glass 14. In these forms, the ingredients are simply ground together, and are therefore called raw glazes, in distinction from such as have a portion of tho materials first united by calcination into a frit, or incipient glass ; by this practice, a more complete combination Is effected, a thinner coating of glaze will suffice, and the ware in con sequence is less liable to craze with change of temperature. Fritted ghue.-Cornish granite 30, flint 16, red-lead 25, soda 12, borax 17; mix and calcine in the easiest part of the gloss-oven, in mggers lined with flint; then take of the frit 26, Cornish granite 15, flint-glass 10, flint 0, white-lead 40 ; grind the whole with a little oxide of cobalt, to increase the whiteness. Drab-ware glaze.-Litharge 56, Cornish granite 20, flint 24. Blue glaze.-Flint 40, borax 21, red-lead 16, Cornish granite 7, sena 5, oxide of tin 5, oxide of cobalt 5 ; calcine, then grind with the addition of a little pearl-ash. Green glam.-Dissolve six pounds of sulphate of copper, and precipitate it with a solution of borax; to which add 10 quarts of white glaze. Yellow glaze.-Colour a white glaze with king's yellow, to the depth of tint desired; or chro mate of lead. Black glate.-Red-lead 74, flint 14, manganese 10, pro toxide of iron 2.
We proceed to point out the chief differences between earthenware end pores/aim.
Porcelain is a finer species of ware, In which the ingredients are so selected that they act chemically upon each other, and are brought to a state of vitrification. The fracture low a dense or greasy surface, like that of a flint stone, and is not liable to be acted upon by aids. 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 in translucent, it is called China of which there are two species, hard and soft. It is necessary to bear In mind, however, to prevent confusion, that many writers treat poredais and china as convertible terms, applying the former name an well as the latter to translucent ware only.