Porcelain

by Edward Dillon

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.