The glassy enamels used by the Chinese resemble the pastes used for artificial jewellery. They are essentially silicates of lead and an alkali. The composition of the flux has to be modified to ensure the full development of the colour of the different metallic oxides which are either made up with it or added subsequently. But in a general way we may say that the colourless fluxes which form the basis of the coloured enamels are prepared by melting in a crucible a mixture of pure quartz sand and red lead, and adding more or less alkali. In certain cases the lead predominates, as when it is proposed to make an emerald green enamel by means of copper, or when the flux is to serve as a basis for the ruby colour given by a minute quantity of gold. On the other hand, if copper be added to a flux containing an excess of either soda or potash, we obtain a turquoise blue. A fine purple, again, can only be obtained from manganese with an alkaline flux; if too much lead is present only a brown tint is obtainable.
To melt these enamels and to ensure their adherence to the subjacent glaze another firing at a gentler temperature is necessary; indeed in many cases more than one such firing has to be resorted to. The comparatively high temperature required to develop the colour of one enamel may be sufficient to decompose or otherwise damage another part of the decoration. The lowest temperature of all is that of the muffle-fire in which the gilding is fixed. This is therefore the last decoration to be added.
The oven in which these enamels are melted on to the surface of the already glazed porcelain is called a muffle. The ware in this case is protected from the direct action of the flame by the closed rectangular box of fireclay in which it is placed, like bread in a baker’s oven. The muffle is placed over the fireplace of a rectangular furnace, and the flame plays round the sides in such a way as to ensure the uniform distribution of the heat. For the sake of greater cleanliness and the avoidance of dust, the pieces to be fired are placed upon tiles of porcelain rather than upon biscuit or fireclay supports. The temperature may vary from a dull to a full red heat (600° to 1000°C.), and the firing lasts from four to twelve hours.
We have already mentioned incidentally many of the so-called ‘muffle-colours’ or enamels. Those used in China were carefully studied some years ago by Ebelmen and Salvétat at Sèvres. It would appear that the opaque white of the Chinese is obtained from arsenic—the merits of the use of tin for this purpose appear to be unknown to them. The blacks are made from the already mentioned cobalt-manganese ore (wad), mixed with white lead—when oxide of copper is added a more lustrous black is obtained.[23] For the blue enamel, a very small quantity of cobalt suffices to give a brilliant colour. The various tints of the greens and blues derived from copper depend on the nature of the flux; of this we have already given an instance. Antimony in combination with lead gives a bright yellow, which tends to orange when a little iron is present; by the addition of more iron the colour of old bronze is imitated. Iron in the state of the sesqui-oxide is the source of many shades of red, but as this iron oxide will not readily combine with silica to form a transparent glass, it has to be applied as a more or less opaque paint, and thus differs from the other colours in being in perceptible relief. Hence the importance of the ruby red derived from gold, which was first introduced into China in the early part of the eighteenth century, and soon became the predominating colour in the decoration of the time (the famille rose).
The palette of the European enameller is a more extensive one, and each large porcelain manufactory has its book of recipes. The composition of the enamels and the relation of the metallic oxides to the fluxes employed have been systematically studied in more than one laboratory. It is only at Sèvres, however, that the results obtained have been made public. It has been the pride of successive generations of chemists—of Brongniart, of Salvétat, of Ebelmen, not to mention living men—to devise fresh sources of colour for the decoration of porcelain. First chromium, then nickel, cadmium, uranium, iridium, and platinum have been added to the list of metals from which enamel pigments have been derived. Among the colours of the muffle-stove the chief gain has perhaps been the discovery of the quality possessed by the oxide of zinc of altering the tints of other metallic oxides with which it is mixed.
CHAPTER V
THE PORCELAIN OF CHINA
Introductory—Classification—The Sung Dynasty (960-1279)—The Mongol or Yuan Dynasty (1280-1368).
‘La porcelaine de la Chine! Cette porcelaine supérieure à toutes les porcelaines de la terre! Cette porcelaine qui a fait depuis des siècles, et sur tout le globe, des passionnés plus fous que dans toutes les autres branches de la curiosité.... Enfin cette matière terreuse façonnée dans les mains d’hommes en un objet de lumière, de doux coloris dans un luisant de pierre précieuse.’—Edmond de Goncourt, ‘La Maison d’un artiste.’
In any work on porcelain it is something more than the premier place that must be given to the ware of China. We are dealing with an art Chinese in origin, and during a succession of many centuries Chinese in its development. It was only at a comparatively late time that the knowledge of this art spread over the whole civilised world. We in England have, as it were, acknowledged the pre-eminence of that country by adopting the word ‘china’ as an equivalent, more or less, to porcelain.[24]
It was under Imperial patronage that the art was developed in China, and the excellence of the porcelain of that country has in a measure varied with the taste and intelligence with which that patronage was exercised in different reigns. The native scholar and connoisseur has for ages been a collector of choice pieces, and his influence has always been exercised in a conservative direction. There is, indeed, in the whole world no such consistent laudator temporis acti, and it is this conservative spirit, resulting in a constant ‘returning upon oneself,’ that it is essential to bear in mind if we are to understand the involved relation of the old and the new in the history of the arts of China.
But the Chinese potter was not working only for the court or for the learned connoisseur, or again for the supply of the towns and villages. From the earliest times, or at least for the last thousand years, there has been a demand for his ware, small at first but slowly spreading, from the outer barbarian. Porcelain, or something akin to it, has been exported from China, by one path or another, from the time of the first Arab settlements at Canton and Kinsay in the eighth or ninth century; and thus a countervailing influence, acting in the direction of variety and change, at least as far as the decoration of the ware is concerned, has always been present. To give but two instances of this influence—we shall return to the subject later on: in the intimate connection of the Chinese court with Western Asia, and especially with Persia, in the thirteenth century, we may probably find the occasion of the first introduction into China of the blue decoration under the glaze; and with more certainty—the fact is indeed acknowledged by the Chinese—we may attribute the second great revolution in the decoration of porcelain, the use of enamel colours over the glaze, to European or Arab influence.