640 



PORCELAIN. 



en body. Natural or hard porcelain is made 

 of pure kaolin, feldspar, and silica usually from 

 the finest powdered quartz ; no trace of iron 

 or other metals is permitted in its composition. 

 The glaze is of the same materials as the body, 

 though in different proportions, and with the 

 possible addition of a little lime or chalk to 

 make it flow more readily. The finest and 

 best qualities of this natural or hard por- 

 celain owe their perfection and comparative 

 indestructibleness to the methods of baking 

 or firing. All glazed fictile wares require two 

 bakings or burnings, and, if decorated after 

 glazing, a third, to fuse and fix the metallic 

 colors. In the production of hard porcelain, 

 the first baking, by which the molded material 

 is brought into the condition of biscuit- ware, 

 is conducted in the upper story of the great 

 kilns, where the heat is not so intense as in 

 the lower story, and seldom reaches more than 

 1,800 Fahr., and in the case of the thinner 

 wares is even less. The biscuit-ware thus 

 produced is porous, somewhat friable, and can 

 be turned perfectly smooth in a lathe. When 

 ready for the glaze, which is an impalpable 

 powder, kept suspended in a large tub or vat 

 of water by constant stirring, it is plunged 

 into it, and, from its porosity, absorbs the 

 water, leaving the glaze in a fine, pasty pow- 

 der on the surface. It is dried in the air, and 

 then carefully packed in the seggars and 

 placed in the lower story of the kiln, where 

 it is subjected to a heat of from 4,500 to 

 5,000, the later stages of the baking being 

 carefully watched, and the temperature reduced 

 at the point where the glazing begins to flow 

 freely, and the body is so far vitrified that 

 body and glaze form a homogeneous mass. 

 The porcelain is suffered to cool gradually, and 

 is not taken from the seggars under four or 

 five days. When taken out, it is pure white, 

 translucent, not readily frangible, and, when 

 broken by a sharp blow, is found to be per- 

 fectly homogeneous in its composition, so 

 that it is impossible to tell where the glaze 

 ends and the body begins, and there is no 

 danger of chipping, crazing, or cracking of 

 the glaze, or absorption of grease or acids. 

 The glazed porcelain and biscuit-wares of 

 China, Japan, Dresden, Berlin, Sevres, and 

 Limoges, and of the Union Porcelain Works 

 in Brooklyn, N. Y., belong to this class of nat- 

 ural or hard porcelain. 



The soft or artificial porcelain differs from 

 this in very many particulars. Miss Young, 

 in her work on " The Ceramic Art," makes 

 three subdivisions of artificial porcelain, some 

 of them of great beauty, but all fragile and 

 possessing qualities which, for most purposes, 

 render them undesirable. The first subdivis- 

 ion has the body of alkaline paste some kao- 

 lin, but, mingled with it and the feldspar, is 

 very nearly an equal quantity of bone ground 

 fine ; the phosphoric acid from which, combin- 

 ing with the other ingredients, makes the paste 

 alkaline, and, when baked, translucent but fra- 



gile. The glaze is also alkaline, and is com- 

 posed of feldspar, carbonate of lime, borax, 

 and white-lead. Sometimes silica and pearl- 

 ash '(bicarbonate of potassa) are added to the 

 ingredients of the body, to make it still more 

 alkaline. The alkaline glaze may be either 

 colored or colorless. To the former class 

 belong some of the alkaline wares of Persia, 

 China, and of Deck and Haviland to the lat- 

 ter those of Persia, China, St.Cloud, Tournay, 

 Sevres, and Haviland. The old Sevres statu- 

 ettes in biscuit- ware also belong to this class of 

 alkaline pastes. 



The second subdivision of artificial porce- 

 lains includes those having a body of calca- 

 reous or chalky paste in which chalk and lime 

 take the place of bone, and are combined 

 either with kaolin or feldspar. The glaze of 

 these is boracic and colorless. This includes 

 English china, the Minton, Worcester, and 

 Copeland wares. 



A third subdivision is Parian wares, not 

 glazed. The paste is mainly feldspathic, but its 

 exact composition is variable, and in some cases 

 secret. The paste for Parian wares is stiffer 

 and more solid before baking than that of the 

 glazed wares generally, and is always cast in 

 molds. The Parian figures and statuettes of 

 Copeland, and those of Minton and the Wor- 

 cester potteries, England, are included in this 

 subdivision. 



It is to be noticed that not only is there 

 this radical difference in the composition, both 

 of the body and glaze of the artificial or soft 

 porcelain, from that of the natural or hard 

 porcelain, but there is a difference, as marked 

 and as important, in the methods of baking 

 or burning them. All of the soft porcelain 

 wares are baked at a very intense heat the 

 first time. The temperature of the biscuit- 

 wares is raised to 4,000 Fahr., or higher, and 

 they are cooled gradually for. several days. 

 The reasons assigned for this intense heat are 

 various: where the body contains a large per- 

 centage of bone, the burning must be carried 

 to a temperature high enough to destroy all 

 traces of animal tissue in the bone, and reduce 

 it to a pure phosphate of lime. When thus 

 thoroughly burned, the body, though profess- 

 edly infusible, is carried almost to the point of 

 vitrification. In the case of Parian wares, they 

 become so hard and dense that they can be 

 rubbed, polished, or turned in the lathe, till 

 they acquire a marble or ivory-like surface. 

 With those wares which are to be glazed, there 

 is the still more potent reason that the alkaline 

 and boracic glazes will become vitrified at a 

 much lower temperature than that to which 

 the body has been subjected. This is still truer 

 of all the plumbiferous or lead glazes, which, 

 however, are not used to any considerable ex- 

 tent in the true soft porcelains, or pdtes tendres, 

 of Europe. 



When these wares are glazed, they are sub- 

 jected to a heat varying from 800 to 1,200 Fahr., 

 and the glaze flows over the wares, forming a 





