PORCELAIN, a fine sort of earthen*. ware, chiefly manufactured in China, and thence called china-ware.
The combination of silex and argil is the basis of porcelain: and, with the ad dition of various proportions of other earths, and even of some metallic oxides, forms the different varieties of pottery, from the finest porcelain to the coarsest earthenware. Though siliceous earth is the ingredient which is present in largest proportion in these compounds, yet it is the argillaceous which more particularly gives them their character, as it commu nicates ductility to the mixture when soft, and renders it callable of being turned in to any shape on the lathe, and of being baked.
The clays are native mixtures of these earths ; but they are often rendered unfit for the manufacture of at least the finer kinds of porcelain, from other ingredi ents which they also contain.
The perfection of porcelain will depend greatly on the purity of the earths of which it is composed ; and hence the purest natural clays, or those consisting of silex and argil alone, are selected. Two substances have been transmit ted to Europe, as the materials from which the Chinese porcelain is form ed, which have been named N and PET uNSE, which see; it was foisid difficult to procure, in Europe, natural clays equally pure, and hence, in part, the difficulty of imitating the porce lain of the east. Such clays, however, have now been discovered in different countries; and hence the superiority to which the European porcelain has attain ed. The fine Dresden. porcelain, that of Berlin, the French porcelain, and the finer kinds which are formed in this coun try, are manufactured of such clay, which, from the use to which it is applied, has received the name of porcelain earth, and which appears, in general, to be derived from the decomposition of the felspar of granite. It appears, also, that natural earths, containing magnesia, are used with advantage in the manufacture. The proportion of the earths to each other must likewise be of importance ; and from differences in this respect arise, in part, the differences in the porcelain of different countries, as well as the necessi ty frequently of employing mixtures of na tural clays. The argil communicates te
nacity and ductility to the paste, so that it may be easily wrought : the silex gives hardness and infusibility ; and on the pro per proportion of these depends, in a great measure, the perfection of the com pound. The proportion of silex in porce lain of a good quality is, at least, two. thirds of the composition ; and of argil, from a fifth to a third. Magnesia is of utility, by lessening the tendency which the composition of silex and argil alone has to contract in baking, and which is convenient in the manufacture. In the manufacture of the finer kinds of porce lain, the ingredients are carefully washed, dried, and ground by a mill to a very fine powder, which is passed through a sieve. This is made into a paste with water, which is well kneaded, so as to he uni form in composition. The vessels shaped from this paste are baked in earthen pots, to render them tolerably hard and com pact: they are then covered with the ma. terials for glazing, which, in the better kinds of porcelain, consist of a mixture of earths, which form a compound more vitrifiable than the porcelain itself.
These materials are diffused in a very fine powder in tvater, into which the baked vessels are dipped : the surface is thus coveted with a thin crust, the water being absorbed. When dry, they are again placed in the earthen pots, and ex posed to a very intense heat. The solid matter of the porcelain undergoes a semi vitrification, whence it possesses all the hardness of glass, and has an additional value, in being less brittle, and much more able to bear sudden alterations of temperature : it derives also much beauty from its semi-transparency and white co lour. The glazing on the surface is, from its greater fusibility, more com pletely vitrified, and is, of course, more smooth and impervious. See GLAZING,