The Japanese porcelain painted under the glaze with the oxide of cobalt has been already described. Other qualities are painted over the glaze with colored enamels made from glass (or silica, litharge, and nitre) and white-lead. The coloring oxides are gold for carmine, copper, antimony, manganese, red oxide of iron, and oxide of cobalt. These are mixed and applied directly by the painter without any previous preparation, so that the colors do not show themselves until brought out by the fire. The method of decoration is peculiar. The design is first sketched in black lines, with strokes for the shades. When the enamel colors are opaque, they are laid on thinly; when translucent or resembling colored glass, so that the design appears under, they are laid on more thickly. Occasionally a white opaque enamel--but containing no admixture of tin--is first applied, and the colors are laid upon it.
Stone-ware is very seldom glazed by a "dip." The usual method is to combine the firing and glazing. When the ware has been exposed to the maximum heat for the necessary time, salt is thrown into the kiln. The heat vaporizes the salt, and of its constituent parts one, the chlorine, escapes; while the other, the soda, is, on coming in contact with the silex in the red-hot ware, formed into a silicate of soda, a perfectly transparent and intensely hard glaze.
In regard to the colors, the only ones now known which will bear the first firing--_couleurs de grand feu_--and are therefore put on before glazing, are blue from cobalt, browns from iron, manganese, and chromate of iron, green from chrome, and yellows from titanium and uranium. Between these and the more delicate _couleurs de moufle_, or enamel colors, are violets, reds, and browns from manganese, copper, and iron, which are designated as _couleurs au demi grand feu_. Beyond these, the colors used in decorating hard or natural porcelain are laid on the glaze, to which they adhere without incorporating themselves.
The great difficulties attending the manufacture of porcelain may now be estimated. The piece must pass through the kiln as many times as there are colors requiring different temperatures. Too much heat will blot out the delicate colors, too little will leave them dull. Those on artificial or soft porcelain sink into the glaze, and thus present a softness and creamy delicacy never seen on any other kind of ware.