The vessel being thus printed, its permeability must be removed by glazing, or by covering its surface with a vi treous substance. An almost endless variety of materials may be used for this purpose, according to the colour or the transparency required, or the quality of the vessel that is to undergo the operation. One species of stoneware is glazed simply by throwing sea-salt (muriate of soda) into the furnace in which it is biscuited. The salt, it is pro bable, is decomposed, the acid flying off, while the soda combines with the earth of the pottery, forming a vitreous coating. This pottery might be extended for culinary pur poses. From not being in saggars, as previously stated, the vapour and smoke come into immediate contact with the ware; and hence it is of a brownish colour. Pounded glass, also, forms an excellent glazing; it is very trans parent, and hence the colour of the vessel is easily seen through it. Ware, glazed with this latter substance, is termed cream-coloured, as it exhibits a yellowish hue, from the presence of a small quantity of oxyd of iron. Glaze may be rendered of a bluish tint, by the presence of tin or arsenic, and a small portion of oxyd of cobalt. Glazing, except in the first case, when sea-salt is used, is uniformly performed by mixing the substances of which it is composed with water, so as the whole may assume the consistence of cream; and the vessel, when in the state of biscuit,being dipped into this liquid, the water of the glaze is absorbed by the pores of the biscuit; and if the vessel has been turned with sufficient regularity, a coat of glazing, of uniform thickness, will be deposited on the surface. The vessel also, when taken out of the liquid, must still be continued to be turned, to prevent the glaze from running into ridges. The ware is again placed in the saggars, as before, and removed to the kiln ; but the fire is neither so strong nor so long continued as before, the object being only to bring the glaze into perfect fusion. These glazes, however, are subject to some objections, particularly their slot expanding and contracting equally with the ware ; hence the vessels are frequently known to crack, and even the glaze to peel off; and the surface thus rendered per meable to fluids, as when in a state of biscuit. The oxyd of lead, however, removes this objection; but this oxyd, even in its vitreous state, and when combined with flint or clay, is easily soluble in acids, and possesses poisonous qualities; so that it is now as little used as possible, and bad consequences have often taken place from eating pickles kept in jars glazed with lead. Lead, however, cannot be entirely dispensed with. All the coarser kinds of pottery, at least, arc glazed with this substance, which, it may be remarked, promotes the fusion and vitrification so rapidly, that a very low degree of heat is required to ef fect the operation. When the ware is removed from the kiln, it is considered as finished and as fit for use.
The modern mode of glazing seems to be decidely infe rior to that practised by the Romans. Modern glaze, as already shown, cracks, and often scales off; and, besides, it is easily destroyed by acids. The Roman glaze, on the contrary, from specimens of it seen on urns dug up in se veral places, was entirely free from this defect. The in gredients of which it was composed cannot now be ascer tained, though some have supposed that it was made of some species of varnish; while others have insisted, on the authority of some vague expression from Pliny, that it was obtained from bitumen. However this be, it is evident, at least, that it never lost its original beauty, or, probably, that it improves by time, and that it was so much valued among the Romans, that some statues were, at length, glazed with it.
There is, however, a species of very coarse porcelain, which does not require to be glazed at all, and which is of a yellowish tint, from a portion of oxyd of iron being used in the composition of it. It does not undergo glazing, be cause, without this operation, it is extremely and imper meably dense and compact in its texture; a property which results from using a comparatively small quantity of flint in the manufacture of it, and from giving it a greater de gree of heat than usual in the burning. Glazing is really
but a miserable imitation of a polished surface; and the pottery in question, scarce as it is, is the more beautiful on account of its being devoid of the vitreous covering. This species of ware is confined chiefly to bottles, parti cularly those used for soda water and artificial mineral waters.
A new species of pottery has of late been introduced, denominated lustre ; which consists in fixing gold or pla tina on the surface of the glazing, in the following man ner: Dissolve platina in equal quantities of the nitric arid muriatic acids, with heat. When to this solution a solution of muriate of ammonia is added, the yellow precipitate will fall to the bottom. Continue to add the latter till no no more is precipitated ; drive off the water by heat, and the powder thus obtained must be ground with a small portion of enamel in oil of turpentine; and, after this pre paration, it is in the state fit to be applied to the earthen ware. It must be spread thinly over the glazed surface; and the vessel, having afterwards been exposed to the heat of an enamel kiln, or a red heat, the patina assumes its metallic form, and acquires a greater brilliancy from the presence of the glaze. The precipitates of gold are ap plied exactly in the same way; but gold does not afford nearly so brilliant a lustre as the platina, and exhibits, in deed, more the colour and symptoms of copper than of gold.
The preceding discussions on the manufacture of pottery are, in every respect, applicable to the cognate subject of porcelain. Substances, the same naturally in form, and re quiring a similar mode of preparation as the kaolin and petuntse of the Chinese, are unknown in this quarter of the world. European porcelain, like stoneware, is, as specified in the foregoing observations, made of the finest species of clays and silex, or flint, substances analogous to those of which the China ware is formed: and these ingredients are prepared, amalgamated, biscuited, glazed, and printed, exactly by the process already illustrated with regard to pottery. Some elegant kinds of European porcelain are reckoned as beautiful and valuable as those of the East : yet they are made on the same principles as pottery; and the two articles, indeed, may, with propriety be regarded as species of the same manufacture, differing in elegance and in estimation, according to the coarseness of the ma terials of which they are respectively constructed. For this reason, we did not, under the head PORCELAIN, give an account of the manlier in which that commodity is made in Europe, but referred our readers to the present article, for suitable information on the subject.
We need not mention here, that the most elegant and perfect pottery and porcelain yet made in this country were manufactured by Mr. Wedgewood, and his sons who succeeded him. The most celebrated, probably, of all his productions, were his imitations of jasper, which were manufactured into vases, medallions, and other ornamen tal forms, and which soon found their way into the col lections of the curious in every quarter of Europe. He also made some cameos of exquisite workmanship; the most famous of which was that of a'slave in chains, in I the attitude of supplication for liberty, with the motto n scribed underneath, Jim /not a man and a brother ? Of these he distributed many hundreds, to excite the humane to assist in the abolition of the slave trade. We cannot, however, at present, give any account of the inventions of this celebrated manufacturer, but defer our observations on this head, till we come to give an account of his life. See the articles PORCELAIN and WEDGEWOOD, Josiaa, and the works there referred to. (Et;')