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.
Bremen Blue.—This is a more or less pure hydrated oxide of copper, varying in its qualities according to the method of preparation. When made by precipitating a neutral salt of copper from solution, it forms a dense and compact mass; whereas a porous and pulverulent pigment results when basic and insoluble copper salts are treated with alkalies.
(1) The foundation of the manufacture of this colour was waste copper scrap, such as ship’s sheathing, from which, in various ways, was prepared a basic chloride or oxy-chloride. Some of the methods adopted were:—(a) 100 lb. scrap copper, 99 lb. powdered sulphate of potash, and 100 lb. salt, moistened with clean water; (b) 100 lb. copper fragments, 60 lb. salt, and 30 lb. diluted sulphuric acid (3 volumes of water to 1 of acid); (c) a solution of copper oxide (scales) in pure hydrochloric acid poured over the scrap copper. The method (a) produces a chloride of copper which, in contact with more metal, becomes a sub-chloride; this, absorbing more oxygen from the air, is converted into the basic green “oxide” of the factories. By the (b) process, the hydrochloric acid set free, and the atmospheric oxygen produce the same result. In the (c) process a similar effect is obtained.
It is of primary importance that no trace of this sub-chloride of copper shall be allowed to remain, as it undergoes decomposition by caustic alkalies, and throws down an orange-yellow sub-oxide of copper. Hence it has sometimes been the practice to prepare the basic oxy-chloride twelve months in advance, and to stir it frequently before use. Complete oxidation, however, can be as satisfactorily accomplished by alternately wetting and completely drying the mass.
An interesting phenomenon takes place during the transformation of this green magma into a hydrated oxide of copper. On this magma being introduced by degrees into a caustic potash or soda lye of about 22° B., the thoroughly washed and dried product is exceedingly fine, with great covering power, and deepens on addition of a little water. When the magma is diluted with an equal volume of water, and the mixture at once poured into an excess of caustic lye, with constant agitation, a few minutes’ rest will suffice for the mass to assume a most compact consistence. The colour thus produced, when washed and dried, is much lighter in colour, and has less body. A blue derived from any of these products is unsatisfactory as regards freshness and intensity of colour; whereas by adding a small quantity of concentrated solution of sulphate of copper to the magma before treating it with the alkaline lye, apparently a highly basic sulphate of copper is produced which deepens the colour.