will occasion it to shrink in the proportion of one part in twelve during the drying.

Another description of clay, much prized for the manufacture of finer kinds of earthenware and porcelain, was found in Cornwall by Mr. Cookworthy, as already mentioned, and is commonly denominated China clay. This is very white and unctuous to the touch, and is obviously formed by the gradual disintegration of the felspar of granite. There are found in Cornwall large mountains of this mineral, some of which are thus partially decomposed; this China clay proves, on examination, to be identical with the kao-lin of the Chinese. It was found by Mr. Gerhard in the course of some experiments upon granite (which is a compound of quartz, felspar, and mica,) that the felspar was melted into a transparent glass, that the mica was found lying under it in the form of a black slag, while the quartz remained unaltered.

The China clay of Cornwall is prepared by the clay merchants on the spot where it is found. The stone is broken up into pieces of a small size, and then cast into a running stream: there the light argillaceous parts are washed off and held suspended in the water, while the more ponderous mica and quartz remain at the bottom of the stream. At the end of the rivulet the water is stopped by a dam, and the pure clay gradually subsides. When this deposit is completed, the clear water is drawn off, and the solid matter dug out in square blocks, which are placed on shelves, and exposed to a continued current of air until sufficiently dry to be packed in casks for shipment. This clay, which is then in the state of a fine powder, is very smooth, and of an extreme whiteness. Mr. Wedgwood found by analysis that it contains sixty parts of alumina and twenty parts of silica. The manufacturers are required to pay a much higher price for this than for any other of our native earths, but for some finer purposes it is altogether indispensable.

A portion of undecomposed Cornish felspar is often

added to the clay, on account of its fusibility and tenaciousness, by which it binds, as it were, the whole ingredients more closely together. The fusible quality of felspar is owing to the presence of about an eighth part of potass. If this alkaline substance be separated by decomposition, as is the case with the China clay above described, the fusibility no longer exists, and the body remains unaltered in the greatest heat of a porcelain furnace. The use of this material has of late been very much increased in our porcelain works. It is a curious and very useful fact, that although neither clay, flint, nor lime can be separately melted, yet when mixed together in due proportions, the mass is fused without difficulty, the one mineral acting as a flux to the other.

Steatite, or soapstone, has of late years been very much employed in the composition of porcelain. When present, in even a small proportion, it limits the contraction of the ware in the furnace. Steatite is a subspecies of mica, which is found abundantly in Cornwall, and is met with also in the island of Anglesea. The mineral which forms the porcelain earth of Baudissero, was long considered to be a superior kind of clay, until it was discovered by M. Geobert that it contains not a particle of alumina in its composition. This chemist, on endeavouring to convert the substance into alum, found, to his great surprise, that he obtained only crystals of sulphate of magnesia (Epsom salts). Proceeding thence to analyse it carefully, he ascertained its composition to be, magnesia 68, carbonic acid 12, silica 15.6, sulphate of lime 16, water 2.8. The soapstone of Cornwall differs from this substance, yielding on analysis, magnesia 44, silica 44, alumina 2, iron 7.3, magnesia 1.5, chrome 1.2. It also contains traces of lime and muriatic acid.

In a published letter addressed by M. Proust to M. Vauquelin from Madrid, mention is made of a beautiful kind of porcelain produced in that city, and which is described to be of a texture even harder than the porcelain of France. Instead of employing kao-lin,

D

the body of the ware is made with spuma maris, a species of pot-stone found in the neighbourhood of Madrid, and the glaze is composed of felspar brought from Gallicia. The pot-stone when taken from the quarry is sufficiently soft to admit of its being cut with a knife like soap. Besides magnesia, silex, and some particles of argil and lime, it contains a portion of potass, the presence of which, in the competent opinion of M. Proust, contributes not a little to the superior quality of the manufacture.

CHAP. III.

ON THE PREPARATION OF MATERIALS.

DILUTION OF CLAY. CHEMICAL EXAMINATION OF WATER NECESSARY. RAIN WATER.CAREFULNESS OF GERMAN MANUFACTURERS. BLUNGING.

MACHINERY.

PREPARING FLINTS.-BURNING.-BREAKING. GRINDING.-DRY GRINDING. BRINDLEY'S IMPROVED MILL. - CHERT.CARE REQUIRED IN SELECTING GRINDING STONES. - DILUTION OF FLINT-POWDER. PROPER CONSISTENCE OF DILUTIONS. ADMIXTURE IN DUE PROPORTIONS. AFFINITY OF ALUMINA FOR SILICA. SLIP.- SLIP-KILN. - METHOD OF EVAPORATING SUPERFLUOUS MOISTURE. WORKING THE PASTE. -TIME NECESSARY FOR TEMPERING IT. - PROPORTIONS WHEREIN CLAY AND FLINT ARE UNITED. DIFFICULTY OF ASCERTAINING THIS.- SLAPPING. - FRENCH MANUFACTURERS. PRO PORTIONS OF INGREDIENTS USED BY THEM. KAO-LIN. --FLINT. GYPSUM. BROKEN PORCELAIN.-CALCINED BONES. -TENDER PORCELAIN. -ITS COMPOSITION. - PORCELAIN

EARTH USED IN BERLIN. FRENCH POTTERS BUY THEIR MATERIALS READY MIXED. ADVANTAGES OF THIS PLAN. INELIGIBLE IN ENGLAND.

IN preparing the clay, the first operation of the pottery is that of mixing it with water to the consistence of cream. It is well known, that water collected from springs, and from many streams, contains various foreign matters, some of which would be injurious to the composition of porcelain. It is therefore necessary to examine chemically the properties of water before it is employed for this purpose, in order to make choice of that which is purest; and to correct, by some of the well known means, any bad qualities that may be present. The French manufacturers are accustomed to employ only rain water, whose near approach to purity fits it for the object. In Germany, still more precise in his operations, the manufacturer prepares his materials only twice in the year, at the vernal and autumnal equinoxes; persuading himself that at these

seasons there is some peculiarity in the rain, which better qualifies it for the purpose of his manufacture. Although the grounds for this nicety are not apparent, it would yet savour of presumption to attribute the practice entirely to prejudice. The observations of men practically engaged in manual or chemical operations have frequently led to improvements in processes, long before the reasons whereon these should have been founded have been revealed by scientific researches.

The mixing of the clay, which is called blunging, is effected in a trough five feet long, three feet wide, and two and half feet deep. In order fully to break down the clay, and incorporate it with the water, a long wooden instrument, formed with a blade at one end and a cross handle at the other, is moved violently about in the trough in all directions, so that this becomes an operation of great labour. In large establishments, where machinery is adopted for the abridgement of labour, the blunging is thus effected: -The clay is thrown into a cast iron cylinder, four feet deep, and twenty inches in diameter. Through the centre of this cylinder runs an upright shaft, furnished with knives placed as radii at right angles to the shaft, but so arranged upon it that their flat sides are in the plane of a spiral thread, so that by the revolutions of the shaft, the knives perform the double office of cutting whatever stands in their way, and of forcing downwards the contents of the cylinder in the manner of a screw. Another set of knives is inserted in the interior surface of the cylinder, and these extend to the shaft in the centre, parallel to, and corresponding with, the revolving knives: thus the two sets, the one active and the other passive, have the effect of shears in cutting the clay into small pieces; while this, in its reduced state, is at the same time forced through an aperture at the bottom of the cylinder, and transferred to a vat for the purpose of being mixed with water; a process which this previous dividing of the clay is found materially to facilitate.

The vat where this mixture is performed is likewise