The cobalt ores, rich in nickel, are exposed to slow oxidizement in the air, whereby the iron, cobalt, arsenic, and sulphur get oxygenated by the atmospheric moisture, but the nickel continues in the metallic state. This action of the weather must not be extended beyond a year, otherwise the nickel becomes affected, and injures the cobalt blue. The ore hereby increases in weight, from 8 to 10 per cent. [Fig. 291.] is a longitudinal section of the furnace: [fig. 292.], a horizontal section upon a level with the sole of the hearth. It is constructed for wood fuel, and the hearth is composed of fire-bricks or tiles. The vapours and gases disengaged in the roasting, pass off through the flues a a, into the channels b b, and thence by c into the common vent, or poison chamber. See the representation of the [poison tower of Altenberg], under the article [Arsenic]. The flues are cleared out by means of openings left at suitable situations in the brick-work of the chimneys.
The azure manufacture is carried on chiefly in winter, in order that the external cold may favour the more complete condensation of the acids of arsenic. From 3 to 5 cwt. of Schlich (pasty ore), are roasted at one operation, and its bed is laid from 5 to 6 inches thick. After two hours, it must be turned over; and the stirring must be repeated every half hour, till no more arsenic is observed to exhale. The process being then finished, the ore must be raked out of the furnace, and another charge introduced.
The duration of the roasting is regulated partly by the proportion of sulphur and arsenic present, and partly by the amount of nickel; which must not be suffered to become oxidized, lest it should spoil the colour of the smalt. The latter ores should be but slightly roasted, so as to convert the nickel into speiss. The roasted ore must be sifted in a safety apparatus. The loss of weight in the roasting amounts, upon the average, to 36 per cent. The roasted ore has a brownish gray hue, and is called safflor in German, and is distributed into different sorts. F F S is the finest safre; F S, fine; O S, ordinary; and M S, middling. These varieties proceed from various mixtures of the calcined ores. The roasted ore is ground up along with sand, elatriated, and, when dry, is called zaffre. It is then mixed with a sufficient quantity of potash for converting the mixture into a glass.
[Figs. 293.] and [294.] represent a round smalt furnace, in two vertical sections, at right angles to each other. The fire-place is vaulted or arched; the flame orifice a, is in the middle of the furnace; b is the feed hole; c, a tunnel which serves as an ash-pit, and to supply air; d, openings through which the air arrives at the fuel, the wood being placed upon the vault; e, knee holes for taking out the scoriæ from the pot bottoms; f, working orifices, with cast-iron plates g, in front of them. Under these are the additional outlets h. The smoke and flame pass off through the orifices i, which terminate in expanded flues, where the sand may be calcined or the wood may be baked. Eight hours are sufficient for one vitrifying operation, during which the glass is stirred about several times in the earthen melting pots.
The preparation of the different shades of blue glass are considered as secrets in the smelting works; and marked with the following letters:—F F F C, the finest; F C, fine; M C, middling; O C, ordinary. A melting furnace, containing 8 pots of glass; produces in 24 hours, from 24 cwts. of the mixture, 19 cwts. of blue glass; and from 1⁄2 to 3⁄4 cwt. of scoriæ or speiss (speise). The composition speise, according to Berthier, is,—nickel, 49·0; arsenic, 37·8; sulphur, 7·8; copper, 1·6; cobalt, 3·2 in 100. Nickel, arsenic, and sulphur, are its essential constituents; the rest are accidental, and often absent. The freer the cobalt ore is from foreign metals, the finer is the colour, and the deeper is the shade; paler tints are easily obtained by dilution with more glass. The presence of nickel gives a violet tone.
The production of smalt in the Prussian states amounted, in 1830, to 74521⁄2 cwts.; and, in Saxony, to 9697 cwts.; in 1825, to 12,310 cwts.
One process for making fine smalt has been given under the title [Azure]; I shall introduce another somewhat different here.