PORCELAIN AND THE ART OF ITS PRODUCTION. 315 



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 por- 

 celain 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 fix- 

 ing 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 glaz- 

 ing of the porcelain and the materials employed to decorate it. Sev- 

 eral different methods are applied for this purpose, but they may be 



