(3) A richer and more velvety blue is got by using oxide of cobalt and substituting phosphate of lime for the alumina (Boullai-Marillac).
(4) Binder’s process is as follows:—Dissolve by boiling 6 lb. alum, free from iron, in a leaden or earthenware vessel, and filter it into a vat 5½ ft. high and 3 ft. across, one-third full of clean water. Precipitate the alumina by solution of potash, fill up the vat with water, settle, decant the clear liquor, and wash repeatedly till barium chloride gives no precipitate. Dissolve ½ lb. sesquioxide of cobalt in 1½ lb. hydrochloric acid at 22° B., and evaporate to dryness. Dissolve the residue in 3 lb. hydrochloric acid, and pass a stream of sulphuretted hydrogen through it, to throw down any foreign metals. Filter clear, evaporate again to dryness, and dissolve the residue in enough water to produce 4½-5 lb. of solution. Next precipitate the cobalt solution (3 to 6 lb., according to depth of tint required) by ammonia, avoiding excess. Wash the precipitate, and add it to the water, holding the gelatinous alumina in suspension, stirring thoroughly for ½ an hour. A reddish tint in the supernatant liquor shows that some of the cobalt has been dissolved. Add a little ammonia, and allow the precipitate to settle. Decant and add new waters repeatedly. Finally collect on a fine filter cloth, drain, press, stove dry, and calcine for 2-2½ hours at red heat in clay crucibles: then cool, grind first in a mill and then on a slab, and sift.
Smalts.—This pigment has not maintained its position in competition with artificial ultramarine. Formerly it was very largely used to correct the yellow tone of cottons, papers, and pottery. It has a pale violet-blue tint, which, however, is not constant in artificial light. Being a silicate it is very permanent, and proof against the action of acids, alkalies, and sunlight, besides being inert when mixed with other pigments. It can be used with either water or oil as a medium, but is not a successful paint owing to its weak colouring power. It is virtually a double silicate of cobalt and potash, or a cobalt glass, containing a few impurities, of which the chief are aluminium, iron, and lead oxides. The colour varies somewhat according as these impurities fluctuate, and the finest ground sample is always the palest. It is hardly ever adulterated, and the chief point to secure is that it be ground to the finest possible degree.
Its manufacture is most extensively and successfully carried on in Saxony. The raw materials used are cobalt speiss (an arsenide of cobalt and iron), potash, and sand. The ore is broken up into convenient sized pieces and roasted at red heat in a reverberatory furnace provided with a tall shaft for discharging the sulphurous and arsenical fumes at a high altitude. When the evolution of these fumes has ceased and the mass begins to assume a pasty consistence, the roasted ore is removed from the furnace, cooled, reduced to a fine pulverulent condition (then known as “zaffre”) and passed through a silken sieve. Should it be necessary,
Fig. 12.—Furnace for Roasting Cobalt Ores.
the cobalt ore is first spalled and hand-picked to remove the ores of foreign metals which are associated with it; and then reduced to a very fine state in an edge runner or mortar mill, and freed from earthy impurities by washing. The concentrated ore is then dried and dead-roasted in small charges at a time (about 4 cwt.) in a specially designed reverberatory furnace such as shown in [Fig. 12], of which a is the hearth on which the ore is spread; b, the fireplace, the products of combustion from which pass over the ore on the hearth, and thence into the flues c, which repeatedly circle round the furnace so as to provide abundant opportunity for the arsenious oxide derived from the combustion (oxidation) of the arsenic in the ore to condense; this highly poisonous arsenious oxide is collected in a solid form from the flues at convenient intervals by means of the doors d. The ore is charged and discharged at the door e. The roasting should not be carried to such a point that the whole of the sulphur and arsenic are removed when making smalts, as by leaving a portion of these substances in the ore at this stage, the ultimate purification is better accomplished.
The next stage is to fuse the roasted ore with potash and silica so as to form a blue glass. The proportions in which the ingredients are mixed depend upon the depth of colour in the zaffre operated upon and the tint desired in the finished smalts; hence it is always determined by a preliminary experiment, and is then most carefully adhered to, each material being accurately weighed out. Only the best potash can be used, as it must be quite free from soda, and iron or other metal; the effect of soda is to render the blue greenish tinted. Quartz affords the requisite silica, and is hand-picked to ensure freedom from alumina, iron, and lime, which import dullness into the colour, and then ground to a fine powder in an edge runner mill. The duly weighed quantities of the several ingredients are intimately mixed in wooden or cement lined vessels, so as to preclude the possibility of any metallic iron finding its way in; and as a further protection against this risk a little white arsenic is often added so that the iron may be carried down in the regulus which is formed during the fusion in the crucible.
These crucibles are of refractory earthenware quite free from lime, and measure about 18 inches across at top, gradually diminishing to 14 inches at bottom, so that an ordinary charge is about ¾ cwt. They are placed in rows in a furnace which generally bears a close resemblance to a glass furnace, the operation being very similar. The form of furnace common in Saxony, where most of the smalts is made, is shown in [Fig. 13]. By means of a series of openings a in the walls of the furnace the pots b are introduced on to the hearth of the furnace, whereupon the openings a are bricked up again and remain closed during the operation. The ingredients are charged into the pots b