Porcelain. Dillon Edward
prevailed. I take from the excellent article on the manufacture of pottery in the Penny Cyclopædia the following account of the process in use in England at the beginning of the last century:—
‘The mould is made in two parts, and each is separately filled by laying in a cake of clay which has been beaten out to the proper thickness on a wet plaster-block; it is pressed into the mould by repeated blows from a ball of wet sponge, then squeezed into all the angular parts and smoothed with sponge, wet leather, and horn. When both sides of the moulds are thus lined with clay, they are joined together, and the man lays a roll of clay along the inside of the joining, which he works down until the whole is smooth and solid.’ The mould is then carried into a stoveroom, and the plaster here absorbs the moisture so as to release the clay. The contents are carefully taken out, and the empty mould returned to the stove previous to being filled again. The seam that remains on the outside of vessels after fitting the two parts together11 is removed by scraping and burnishing with wet horn; the handles and other appendages are then attached.
This is the process that is called ‘hollow-ware pressing’ or ‘squeezing.’ In ‘flat-ware pressing’ the mould is used to give the shape to the inside of the vessel only. The mould is placed on the extremity of the ‘whirler,’ a vertical revolving spindle provided with a circular table, similar to that of the thrower’s wheel. The plate-maker takes a cake of clay, which he has previously flattened out with his ‘batter,’ places it on the mould, and presses down with his hand. The upper surface of the cake of clay (what will ultimately be the bottom of the plate) is now shaped by an earthenware ‘profile.’ The mould is now taken off the whirler and at once replaced by another. Flat-ware, especially when greater finish is required, is also made in a double mould, and the clay may then be first thrown on the wheel so as to approximate to the shape required before being placed in the mould.
Processes very similar to the hollow and flat-ware pressing are largely used by the Chinese. Dr. Bushell has unearthed a passage from a technical work, written in the time of the Chou dynasty, more than two thousand years ago, in which a distinction is made between the ordinary potters who worked with the wheel, and the moulders who made oblong bowls and sacrificial dishes. In a somewhat later work (19-90 A.D.) the writer notes both the advantage resulting from regularity of size, and the obstacles arising from the shrinkage of the parts in firing, when vessels are made in moulds.12
Casting.—There is yet another process which is largely resorted to in European works, but which appears to be unknown to the Chinese. It depends upon the rapidity with which dry plaster of Paris will absorb the water from a slip of creamy consistency, without allowing any of the solid particles to pass along with the water absorbed. The slip-mixture is poured into the plaster mould, which at once absorbs the water, leaving a uniform deposit upon the surface of the mould. After pouring or otherwise drawing off the water, a second and thicker slip may be added so as to form a second layer. The paste of the porcelain so prepared is likely to be of a lighter and more porous consistency than when made by throwing or pressing. This process was used in the eighteenth century at Derby, and doubtless elsewhere, and it was preferred to moulding for making statuettes. Some account of it is given by Haslem, a good practical authority, in his Old Derby China. For small objects, ‘casting’ has long been employed in France, and more lately Ebelmen and Regnault have so improved the process, that vessels of all shapes and dimensions are made by it. This has been rendered possible by the introduction of compressed air into the interior of the vessel, by which means the paste is kept in position until it is sufficiently dry to support itself. A still better way of doing this is to exhaust the air on the outside, by placing the mould in an air-pump; the upper part can then be left open, and the whole operation is under the eye of the workman. M. Vogt (La Porcelaine, pp. 157 seq.) laments that in France the increased use of these mechanical processes had so reduced the demand for skilful potters, that the race is nearly extinct.
Firing and Furnaces.—So far in our treatment of the operations involved in the manufacture of porcelain, the same general description has been applicable, with trifling exceptions, to the processes in use both in Europe and in the far East, and to soft as well as to hard paste. But now that we have to describe the firing of the ware, a division into three classes is necessary:—
1st. The Chinese system. This is the simplest plan. The glaze is applied at once to the air-dried ware, which is then subjected to but one firing—that of the ‘grand feu.’
2nd. The French system for hard paste. The unglazed vessel is exposed to a heat varying from dull to full red, generally in the dome over the main body of the furnace. It is then glazed, and again fired to the full point required by the paste. This is essentially a French process, and the preliminary fire is known as the feu dégourdi.
3rd. The English system used for bone pastes. In this case it is the first firing that is the most severe. The ‘biscuit oven,’ therefore, in which this is effected, must not be confused with the feu dégourdi just mentioned. After dipping, the ware is heated again in the ‘glozing’ or glazing oven, but only to a temperature sufficient to melt the glaze.
In the case of ware decorated with enamel colours over the glaze, there will be required in all these cases one or more additional firings at comparatively low temperatures in the muffle-stove.
The furnaces, ovens, or kilns in which porcelain is fired are always of the reverberatory type; that is to say, the fuel is burned in a separate chamber or fireplace, and the products of combustion pass over or among the ware that is being fired. Such furnaces differ on the one hand from the arrangement in a blast furnace, or that often used in the burning of bricks, where the fuel is mixed with the material to be heated, and on the other hand from the muffle-stove, where the object exposed to the heat is protected from the direct flame by the box of fireclay or iron in which it is placed.
Kilns of many shapes and sizes have been used for firing porcelain, but they may most of them be included in one or the other of the following broad classes.
1st. The old bee-hive ovens of China, the use of which appears to have been abandoned in that country by the end of the seventeenth century. These ovens were generally small, in some cases only holding one vase. A row of them may be heated from one fireplace, and they are then built on a rising slope. This type has survived to the present day in Japan.
2nd. The oblong horizontal furnaces, often of considerable dimensions, used during the present dynasty in China. They resemble in section the ordinary type of reverberatory furnace found in metallurgical works. A very similar form was long employed at Meissen.
3rd. The large conical furnaces, now in general use in the porcelain factories of Europe. They may be heated by either direct or by reversed flame.13
In China the fuel is generally pinewood, in billets of uniform size. In many European kilns wood is still used: birchwood, cut in lengths of fifteen to twenty inches, is the only fuel used at the present day at Sèvres. In England, however, the difficulties attendant on the use of coal appear to have been overcome.
The reader will find in the third volume of Brongniart’s great work (Traité des Arts Céramiques, Paris, 1877) several plates giving plans and sections of all these types of furnaces. From a careful examination of these engravings more is to be learned than from any amount of verbal description. A thorough grasp of the process of firing is of the greatest assistance in understanding the problems and difficulties that arise in the manufacture of porcelain, and we shall have to return to the subject when we come to treat of the several wares.
Whatever differences there may be in the shape of the furnaces, when it comes to filling the interior with the ware to be baked, there is one precaution which has been adopted in nearly every country.14 The ware must be protected from the direct heat of the flame by means of a case of fireclay in which it is placed. These are the seggars (French cassettes; the process of filling and arranging them is called encastage), to the preparation of which so important a department has to be set apart in all porcelain works, and whose manufacture adds so much to the working expenses.
The seggar
This seam is often visible on vases of old Chinese porcelain, and may be taken as a sign that the object has been moulded.
Porcelain in China followed, as we shall see, in the wake of the more early developed arts of the bronze-caster and the jade-carver. Hence the prevalence in the early wares of shapes unsuitable to the wheel.
I think that this is a more practical division than the one made by M. Vogt and adopted by Dr. Bushell.
An important exception is to be noted in the case of the firing of large vases in China.