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_