[Illustration: Fig. 13.--FURNACE FOR MAKING SMALTS.] by means of long iron ladles which are introduced through the small square apertures _c_, which can be temporarily closed by a half-brick or other simple article. The fire is then lit in the fireplace _d_, and the products of combustion circulate around the pots _b_, and finally escape at the orifices _e_ at the top of the furnace into flues leading to the chimney _f_. After about 8 hours’ firing fusion commences in the pots, whereupon the contents are thoroughly stirred by rods inserted through the working holes _c_. The temperature is then increased till a white heat is attained, this being necessary for the formation of a glass. The fused mass is repeatedly sampled, and when it has become quite homogeneous, and the regulus or speiss containing the iron, antimony, bismuth, arsenic, copper, nickel, sulphur and other impurities has completely separated itself and collected at the bottom of the pots, the blue glass is ladled out and dropped at once into cold water, by which it is disintegrated and rendered very brittle ready for the subsequent grinding. The regulus is then drawn off from the pots through holes provided for the purpose, and removed by the orifices _g_, after which the pots are ready for another charge. They ordinarily remain serviceable for about six months. The grinding needs to be done with great thoroughness, and is accomplished partly by stamps and partly by edge-runner mills in the presence of water. The particles as reduced are floated off by the water to a series of settling tanks communicating one with another. The portion which settles in the first of the series is too coarse for use, and is returned to the edge-runner for further grinding; while the portion in the last of the series possesses such a weak colour that it is rejected, or put into the crucible to undergo a second fusion. The selected portions are dried ready for the consumer. =Copper Blues.=--These form an unimportant class, being unstable and not endowed with great colouring power. Their tint is pale and greenish, and though opaque in water, they are not particularly so when mixed with oil. Exposed to the action of sulphur or its compounds, whether present as sulphuretted hydrogen in the air, or in combination with a metal, forming another pigment with which they may be mixed, copper blues undergo an important chemical change, the carbonates and oxides of copper being converted into the sulphide, which is black. Under the influence of heat too the blue carbonate will lose its carbonic acid, and be turned into the black oxide. Ammonia and the acids dissolve them, but other alkalies are resisted until heat is applied. The chief kinds of copper blue are Bremen blue, cæruleum, lime blue, mountain blue, Péligot’s blue, and blue verditer.