of the very first necessity, in order completely to expel every air bubble, however minute, remaining in the clay, and which could not be so thoroughly driven out by its previous beating in the larger mass when taken from the slip-kiln. If the air were not thus thoroughly expelled, it would become so much rarified and expanded in the oven, that it would force out a passage for itself, and by blistering, spoil or much injure the goods.

Some of the more considerable among the potters, who employ steam power for blunging the clay and grinding the flints, perform this operation of slapping by the same agency, saving thereby both time and labour. In this case the expulsion of the air is effected by mechanical pressure, and the office is performed with perhaps as much efficiency as by hand.

Having undergone all these preparations, the clay is now fit for being shaped upon the wheel or fashioned by the aid of moulds into all the diversified forms which fancy may desire. This branch of the art is divided into three different departments throwing, pressing, and casting—either of which is used according to the form of the article manufactured.

The manufacturers of porcelain in France do not use so much secrecy as is preserved in our own potteries, with respect either to the materials whereof their ware is compounded, or to the proportions wherein these are employed. The clay which forms five parts in six of the whole mass, is the porcelain earth already mentioned as being identical with the kao-lin of China; to this are added in certain proportions flint and gypsum (plaster of Paris), both calcined and ground; and fragments of broken porcelain, which must be white, also ground to a fine powder. One rule for forming this composition assigns nine parts each of flint and broken porcelain, and four parts of calcined gypsum, to each one hundred parts of porcelain clay. Another authority recommends five parts of gypsum and only eight parts of ground porcelain ; while the flint and clay are used in the same proportions as in the first rule.

If at any time the manufactory should not afford broken porcelain for the purpose, it is recommended that pieces be compounded, about a quarter of an inch thick, of the other three ingredients, wherein the proportions of gypsum and flint are augmented; and these pieces, having been previously converted into porcelain by baking, are to be ground and mixed in the above mentioned proportions with the other ingredients.

It has of late years been the practice of some English manufacturers to use a considerable proportiou of calcined bones, together with a small quantity of gypsum, in combination with china-clay, flint, Cornish stone, and enamel. By this means porcelain of a brilliant and very transparent white is produced, which, however, is deficient in density, and very liable to crack on the application of hot liquids.

Several among the chemists and scientific manufacfacturers of porcelain in France have given recipes for the composition of tender porcelain, although this desscription of ware is no longer made in that country.

This kind of porcelain will support, without softening, a greater degree of heat than suffices to melt glass. It is semi-transparent, has a vitreous fracture, and returns a clear sound when struck by a hard body. It is harder but not so brittle as glass, and bears, without injury, sudden and very considerable alterations of temperature.

M. Brongniart recommends a mixture of one part of pure white clay, with three parts of a frit compounded of nitre, soda, alum and selenite (sparry gypsum), together with a large proportion of ferruginous sand and a little common salt. The ingredients of this frit are to be calcined together, and the whole intimately kneaded; when cold, the compound is to be reduced to powder, and in that state mixed with the clay.

This paste is not so cohesive or viscous as that which forms hard porcelain, and greater carefulness is, therefore, called for in fashioning vessels with it. Lime and selenite, or any similar earths which, if fused by themselves, produce a transparent and colourless glass, may answer

the same purpose as the frit just mentioned. In making choice, however, of the compound to be thus used, it must be borne in mind, that the paste must not be more stubborn in resisting fire than the seggars, or vessels wherein it is enclosed during the baking, and no greater proportion of any substance that promotes fusion must be used than can be supported by the clay without having its cohesiveness greatly diminished.

The porcelain earth used in Berlin is compounded with silicious sand and sulphate of lime in crystals. The constituents of their flux are varied in the proportions according to the quality of the ware it is intended to produce. In the greater part of the German manufactories felspar is used, and some employ a calcareous sand.

When tender porcelain was made in the works at Sevres, a small portion of arsenic entered into its composition. This was found very hurtful to the workmen. Few of the turners or moulders, after following their employment for some years, escaped severe pulmonary complaints; and to this disadvantage is ascribed the order of the French government under which this branch of the manufacture has been discontinued in that establishment.

Individual manufacturers of porcelain in France avoid all the labour of preparing their materials, and purchase these in a state of readiness from establishments at Limoges where the best porcelain earth is found. The price of the compound, when delivered in Paris, or at an equal distance from the place of preparation, does not exceed three sous - about one penny halfpenny of our

money-per pound.

This arrangement must considerably simplify the operations of the manufacturers, and lessens the amount of capital which they must employ in the establishment of their works; but it is doubtful whether the accompanying disadvantages are not fully equivalent to these benefits. There are many reasons to render such a plan ineligible in this country.

CHAP. IV.

THROWING.

ON THE FORMATION OF UTENSILS.

POTTER'S LATHE. THROWER. MODE OF PROCEEDING.-PROFILES.-SLURRY.-GAUGES.-TURNING LATHE.

-TURNING AND SMOOTHING -MOULDING DISHES, &c. TOOLS. STEAM MACHINERY.

ENGINE-LATHE. — MILLED

EDGES. -HANDLER.-FORMATION OF HANDLES, SPOUTS, &c. PRESSING. -SMALL ORNAMENTAL FIGURES. MODE OF AFFIXING THEM. METHOD OF MAKING MOULDS. BOILED PLASTER.GREAT USE OF GYPSUM IN MAKING MOULDS. STOVES. MODELLING. QUALIFICATIONS REQUISITE FOR A MODELLER. INCREASING SKILL OF ARTISTS. FOSTERED BY WEDGWOOD.- MOULD-MAKER. - METHOD OF HIS WORKING.-CASTING.-CAREFULNESS REQUIRED IN DRYING.

MR.

THE operation of throwing consists in shaping such vessels as have a circular form, and is performed upon a machine called a potter's lathe.

This consists of an upright shaft, about the height of a common table, on the top of which is fixed a circular piece of wood, whose breadth is sufficient to support the widest vessel that is to be made. The bottom of the shaft runs in a step, and the upper part in a socket a Fig. 1.

little below the circular board, so that the shaft and board turn together. The shaft has a pulley fixed upon

it by means of which it is turned, an endless band passing round the pulley from a wheel placed at a short distance, and which is ten times the diameter of the pulley; this wheel when turned by a handle sets the lathe in motion.

The clay to be thrown is first cut and weighed and formed into a ball. It is then placed on the face of the circular board, which being put in motion, the thrower; dipping his hands from time to time into water, or slip, that the clay may not adhere to them, fashions it first into a long thin column, which he forces again down into a lump, and continues these operations until assured that no air bubbles can possibly remain in the body of the clay. He then directs that the speed of the wheel shall be moderated, and proceeds to give the first form to the vessel. This is done either by his fingers alone, or with the aid of an instrument shaped according to the desired form. The instruments employed for this purpose are called profiles or ribs. By the assistance of one of these, the inside is smoothed and made to assume the requisite shape, and any inequalities, technically called slurry, are removed. When it is wished to make any number of vessels exactly similar to each other in shape and dimensions, certain pegs are fixed as a gauge without the circumference of the revolving board, but placed in such a manner, that whenever the plastic clay is brought to coincide at the requisite points with the gauge, the thrower knows that the article has attained the proper dimensions.

In this manner most circular-shaped vessels are formed. When finished to the artist's satisfaction, he proceeds to remove his work, cutting it from the lathe by passing a thin brass wire through the lowest part of the clay. The vessel is then lifted off and placed on a board or shelf, where it is left to dry partially before it is farther smoothed and shaped in the turning-lathe.

When the vessel is so far dried as to be in that particular state of hardness well known to the workmen, and which is called the green state, it is in the most