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_).