Edward Dillon - Porcelain

The application of the Decoration under the Glaze is essentially a Chinese method. To it we owe the important family of ‘blue and white’ ware. The superiority of the Chinese in the management of the blue colour has been attributed to various causes. The result is no doubt influenced not only by the constitution of both paste and glaze, but also by the fact that the colour is painted upon the raw paste.

An important factor also is the care exercised by the Chinese in the selection and preparation of the blue pigment, by which not only the desired intensity but the richness of hue is secured. The quality of the blue depends in great measure upon the presence of a small quantity of manganese in the cobalt ore employed.

The only other colour that the Chinese have succeeded in using under the glaze is the red derived from the sub-oxide of copper. The full development of this colour has for long been a lost art, but a less brilliant red from this source, often little better than a buff colour, is sometimes found in later examples combined with the blue.

In the application of colours under the glaze there is one difficulty that the Chinese have surmounted even in their commonest ware, and this is the tendency of the cobalt blue to dissolve and ‘run’ in the glaze, giving to the design a blurred and indistinct appearance. It would seem that the sharpness of outline depends upon the consistency of the glaze at the moment when it first melts. At that point the glaze should be viscous and not inclined to flow, and this is what occurs in the case of the highly calcareous glazes of the Chinese.

Before passing to the enamel colours, we must say something of a class of glazes which may be looked upon as to some extent of an intermediate character. These are the glazes associated with the ‘San tsai,’ the ‘three colours’ first used in combination by the Chinese.

These coloured glazes were applied, not, as is usually the case in China, to the raw paste, but they were, it would seem, painted on the surface after a preliminary firing. Being applied with a brush, the whole surface of the biscuit was not necessarily covered, and glazes of all these colours could be used upon the same piece of porcelain. Glazes of this class were rendered more fusible by the addition of a certain quantity of lead, and on this ground, and still more in their historical relation, as we shall see later on, these ‘painted glazes’ may be considered as a link connecting the old refractory glazes of the monochrome and ‘blue and white’ wares on the one hand, with the fusible enamels which were at a later time superimposed upon the glaze on the other.

The three colours which are applied in this way by the Chinese are: (1) A turquoise blue derived from copper with the addition of some soda or potash. (2) The manganese purple, often described as aubergine. (3) A yellow prepared from an iron ore containing some amount of antimony. None of these colours would stand the full heat of the furnace, and for a reason which will be explained further on, they are known as the colours of the demi grand feu . 21 21 With these colours a dark blue is sometimes associated. Is this derived like the turquoise from copper? It is a curious fact that we have here exactly the same range of colours that we find in the little glass bottles of Phœnician or Egyptian origin, with zig-zag patterns (1500-400 B.C.).

Coloured Enamels. We have now to describe

PLATE II. CHINESE MING PORCELAIN, BLACK GROUND

the decoration that is applied to the surface of the glaze. In these coloured enamels the colouring matter is dissolved in a flux which contains a large quantity of lead. The comparatively gentle heat at which such enamels fuse allows of the use of a much larger palette than is available for the decoration under the glaze.

It is well to point out at the outset the marked distinction in composition and in appearance between the brilliant enamels of the Chinese and the dull tints of the ‘porcelain colours’ found in the hard pastes of Meissen and Sèvres. To make clear the cause of this difference it will be necessary to enter into some little detail.

The colouring matter in the European enamels may amount to as much as a third part of the total amount of the flux with which they are incorporated. As there is not enough of this flux to dissolve the whole of the oxides, the enamel remains dull and opaque after firing. The flux, in fact, is only used as a vehicle to attach the colour to the surface of the porcelain. The effect in consequence is inferior in brilliancy to that obtained by the Chinese with their transparent enamels in which the metallic oxides, present in much smaller quantity, are thoroughly dissolved to form a glass. There is, unfortunately, a practical obstacle to the application of these glassy enamels to the hard pastes and glazes of Europe. It is impossible to ensure their firm adhesion to the subjacent glaze. The Chinese, however, do not appear to find any difficulty in effecting this. The following explanation has been given to account for the difference of behaviour:—the tendency of the enamel to split off in cooling, as has been proved by experiment, arises from the small amount of contraction at that time of the highly kaolinic paste, compared with that of the superimposed glassy enamel. The more silicious paste used by the Chinese contracts, on the contrary, at the same rate approximately as the enamels that it carries, and these enamels may therefore be laid on in sufficient thickness without any risk of their subsequently splitting off. 22 22 See Vogt, La Porcelaine , p. 219. The problem is really more complicated. For simplicity’s sake we have ignored the changes that take place in the glaze that lies between the enamels and the paste. To appreciate the difference in the decorative value of these two classes of enamels it is only necessary to compare the brilliant effect, say, of a piece of Chinese egg-shell of the time of Kien-lung with the tame surface of a contemporary Meissen plate, elaborately painted with landscapes or flowers.

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.