90 EARTHS. 



Very beautiful designs are now fixed upon the common ware by 

 &id of the copperplate printing press. The design, first painted, and 

 then engraved upon copper, is printed with a metallic color, mix- 

 ed with prepared linseed oil, upon silver paper, which, with the figure 

 upon it, is immediately applied to the biscuit, and then rubbed with 

 a hard roll of flannel, to make it adhere, and after about an hour, the 

 article is immersed in water, which softens the paper, so that it is easily 

 removed, and leaves the colored figure ; the piece is next heated 

 moderately in an oven, to dissipate the oil, and is then prepared to 

 receive the glaze. 



The porcelain is not always painted in the biscuit ; sometimes it is 

 painted on the glazing, and I believe this is generally done, on the 

 most beautiful porcelain ; it is then necessary to heat the vessels again, 

 in the enameller's oven, that the coloring matter may be melted, and 

 incorporated with the glazing. 



Glazing. To prevent the absorption of fluids, and to make the ves- 

 sels more cleanly, they are covered with a vitreous coat, a thin glassy 

 film, which, as long as it lasts, protects the ware below. In the case 

 of the common stone ware, it is produced by throwing into the hot 

 furnace, common salt, which is raised in vapor, by the heat, when the 

 soda vitrifies the outside and forms a perfect covering, which is also 

 safe and cheap. 



The glazing, used on the common yellow ware, is composed of 40 

 pounds of ground flints, and 100 of litharge,* or of 100 of litharge, 

 and 80 of Cornish granite. 



For porcelain and the finer kinds of earthen ware, it is composed of 

 white lead, ground flint glass, ground silex, and common salt, 



The materials of the glaze are reduced to an impalpable powder, 

 and suspended by agitation in water ; the vessels are dipped in them, 

 and they retain enough to form a perfect covering when they are 

 again exposed to the heat of the furnace. This glazing is dangerous 5 

 on account of the poisonous nature of lead : lava and pumice stone, 

 have been substituted in France with good success ; and even 

 ground flint glass, mixed with clay and water, has been found to an- 

 swer | indeed, no protection would be better than the common mate- 

 rials of glass, was not the ratio of its contraction and expansion by 

 heat, different from that of pottery, which would cause it to break. 

 Metals and their oxides are sometimes mingled with the materials of 

 the glaze, to give it color, in certain parts, as on the edges of 

 plates, copper being used for green, and manganese for black. 



Porcelain is occasionally covered with gold or platinum in sub- 

 stance. The gold is dissolved in nitro-muriatic acid, which is evap- 

 prafed, leaving the metal in a state of minute division ; it is next mix-- 



The French use galena, the native sulphuret of lead, thence called potter"* 



