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.