in any. is difficult to ascertain the precise quantities employed The grinding and dilution of the materials is effected in the manner already described, and when these have separately been thus treated, their union is commonly effected in the proportion of about eighteen measures of argillaceous to fourteen measures of siliceous fluid. Any larger proportion of flint would render the compound more difficult to be worked; and if much less were used, the baked ware would not present a sufficiently smooth and shining appearance. The best descriptions of this ware have very long been made in the potteries at Lambeth, the proprietors of which procure their supply of clay from Devonshire and Dorsetshire, and purchase flint already ground from Staffordshire, where this material can be afforded at a cheaper rate than would attend its preparation so near the metropolis. The plan introduced by the two brothers Elers is still pursued, of glazing these goods by the decomposition of common salt thrown into the kiln, at or near the conclusion of the baking process. Other glazes are likewise employed for many articles, and, according to the usual system, the particular components of these are also veiled in mystery; ground glass is, however, understood to be the basis of all. One of the most considerable of the Lambeth potters has recently announced the discovery of a glazing compound, which is sufficiently fusible without containing a particle of lead, and which has been proved capable of altogether resisting the action of acids; it is used by. him for coating the insides of jars and bottles of all descriptions. The different colours observable on the outer surface of drinking jugs and other articles, is owing to the partial use of a glaze, the part to which this is applied becoming dark in the heat of the kiln, while the glazing of the light coloured portion is caused, as before mentioned, by the introduction of salt. The fashioning of stone ware pottery is, in all re spects, conducted similarly to the processes already fully described. Seggars are mostly employed for enclosing the pieces during the baking, which usually occupies about forty-eight hours. Vessels of considerable size, some being capable of containing sixty imperial gallons, are made in this manner, and are found highly useful in performing many chemical operations connected with the arts. CHAP. VI. ON THE ART OF APPLYING COLOURS AND ENGRAVINGS TO EARTHENWARE. -- ANTIQUITY OF ENAMEL COLOURING. -SPECIMENS FROM ANCIENT ENAMELLING KILNS. THE art of painting on earthenware, although of comparatively recent introduction into England, is by no means a modern invention. It is well known that the ancients manufactured coloured enamels; and some specimens of the art, performed by the Egyptians more than three thousand years ago, have been preserved to the present day, which is an evidence of this fact; showing us, at the same time, that, in so remote an age, the artists of Egypt were possessed of sufficient practical knowledge of chemistry, to avail themselves of processes which have been brought to light anew by scientific investigators of more modern times. Recently, the art has been carried to an admirable degree of perfection in Europe. Some specimens are preserved in the porcelain works at Sèvres, of which the French artists are justly proud; while the performances in our own potteries at Worcester, in Staffordshire, at Derby, and yet more recently in Yorkshire, are such as entitle them to be placed in an equal rank with those of our continental neighbours. When, about seventy years ago, Mr. Wedgwood commenced the series of improvements, by which his name has been rendered so deservedly celebrated throughout Europe, no attempts at embellishment had been made in the English potteries; and if ornamented services of porcelain were seen on the tables of the wealthy, they were always of foreign, and generally of oriental production. So soon, however, as, by the intrinsic merit of his wares, this enterprising manufacturer had secured not only the patronage of royalty, but the more solid support of his countrymen in general, he called into action the crucible of the chemist, and the pencil of the artist, and led the way in bestowing that degree of outward embellishment on his productions, which converted them into objects of elegance, and at once encouraged and gratified the growing taste for luxury among the higher classes in this country. The system of mystery still preserved in the English potteries, in all that respects the composition and glazing of wares, was likewise long practised with regard to the preparation of colours. If, at any time, a manufacturer had found out a preparation more advantageous than that in use before, he always endeavoured to limit the benefit of the discovery to his own works; a desire more easy of accomplishment formerly than it has become since the more general diffusion of the light of science. M. Brongniart, at one time director of the national manufactory of porcelain at Sèvres in France, has the merit of being the first who published a correct statement of the most approved plans for preparing and combining the metallic oxides used in colouring porcelain and glass. The employment of these substances for such purposes had been long before and commonly practised, and the art was even carried by some of its professors to a high degree of perfection; but, as M. Brongniart observes, no attempt had been made at the time his essay was written (1801), to apply to it the principles of chemical science; and such pretended descriptions as had then been published contained no theory, and consequently no general principles. Where even the authors did offer explanations, these were founded upon the most ridiculous hypotheses; so that all improvements were rather the offspring of chance than the result of systematic enquiry. It would afford but little satisfaction, or information, to relate by what steps the art now under consideration was reduced into a system, and thus became, in some measure, deserving of the name of a science. It will be sufficient to state the means by which that art is now rendered available in the best conducted porcelain works. In a great part of the following description, the lucid statements given by M. Brongniart in his essay will be closely followed; since, with scarcely any variation, the same processes have been used in both the French and English establishments to the present day. In this branch of the art there are various objects to be considered, a proper acquaintance with which is necessary to success. Such are, the composition of colours; the fluxes which are necessary to render these fusible, which unite them to the wares, and in many cases impart brilliancy to their tints; the vehicle employed in laying on the colours, and the course to be pursued in fixing them on the porcelain by means of heat. Metallic oxides form the bases of all vitrifiable colours, but every metallic oxide is not proper for being employed in decorating porcelain. Some are highly volatile, as the oxides of mercury and of arsenic. Others part so freely with the oxygen they hold in combination, that their colour proves uncertain, and varies with every application of heat; such are the puce-coloured and red oxides of lead, and the yellow oxide of gold. Oxides |