Page:Popular Science Monthly Volume 21.djvu/327

Rh put in for that purpose. These pieces become glassy a few hours before the baking is completed, but are apt in that condition to fly to pieces. If the baking is arrested at that point, the pieces would all be excessively brittle, and the batch would be spoiled; so the heat is kept up till the trial-pieces come out glazed and clear, without being brittle; but, if the cooking is prolonged much beyond that point, it will be at the risk of changing the character of the porcelain, and of other serious accidents. It is a very delicate matter to determine the right point for stopping the heat without running upon one danger in the effort to avoid the other. When the cooking has been judged complete, the fire is covered up, the vents are stopped, and the furnace is left to cool of itself—a process requiring from four to eight days.

An equally important consideration with that of the temperature is that of the nature of the gases existing within the furnace. If only white porcelain is baked, it is generally best to have an atmosphere of reducing properties, because the small quantities of iron, titanium, etc., included in the clays, will then be least oxidized, and will not color the mass as would be done with an oxidizing flame; if, however, the porcelain is decorated, it is generally an advantage to have an oxidizing atmosphere; and, as both kinds are generally baked at once, it is only by the best management—by, for example, artificially introducing into the piles gaseous substances adapted to one or the other object—that a satisfactory result can be reached. The nature of the fuel employed is variable. Different kinds of wood and coal are used. Efforts are made to adopt gaseous fuels, with which alone we can expect to be able to obtain a complete mastery over the baking.

Having reviewed as succinctly as possible the principal points in the fabrication of porcelain, we now come to the description of the processes employed to enrich that beautiful material. The art of fixing colors on pottery differs essentially from that of fixing them on cloths, wood, and paper; besides, special qualities, distinguishing it from all other kinds, are exacted of ceramic decoration. A perfect adherence, an absolute resistance to atmospheric agents, a glaze that shall permit the colors to be confounded with that of the object itself, are the characteristic qualities of a handsome ceramic decoration. Since the glazing of porcelain is a rock, one of the hardest substances of the mineral kingdom, it is easily understood that absolutely special processes must be adopted to make a color adhere. The result can be reached only through the intervention of an elevated temperature; and 'this fact eliminates at once from the palette of the ceramist all organic coloring matters and all minerals of slight stability. We must have recourse to oxides, metallic silicates, and metals. The fixation of these colors is always the result of a chemical action, of a combination that takes place at a high temperature, between the body or the glazing of the porcelain and the materials employed to decorate it. Several different methods are applied for this purpose, but they may be