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.