If ores are employed which contain only a small quantity of cobalt, the same quantity of glass is melted time after time with fresh quantities of the poor ore. At each operation cobalt is taken up. In place of glass a mixture of potash and very fine quartz sand may be used with advantage. It produces that species of glass which most easily takes up cobalt oxide. The quartz sand is generally made by igniting flint and quenching in water to facilitate the grinding; it is then powdered. River sand is generally so impure that it would produce a brown or green coloured glass, which, when fused with cobalt ores, would give smalts of a bad shade. =2. Fusion of the Charge.=—This process is very similar to the fusion of ordinary glass. The materials, in very fine powder, are carefully mixed and then ”fritted,” that is, heated until the mass sinters together without melting. The real fusion is accomplished in glass furnaces. The fritted mass is fused in small ”glass pots” made of fire-clay, in a furnace similar in principle to the ordinary glass furnace. The glass pots are made rather small, to hold about 50 kilogrammes, in order to obtain as homogeneous a product as possible. They are placed to the number of six in a furnace heated by coal, in which a very high temperature can be obtained. This is essential, because the charge has a high melting point and must be made quite fluid in order that impurities may settle. The furnace must be slowly heated at first to prevent the pots from cracking. When they are once heated the temperature is raised to fuse the charge, which is repeatedly stirred so that the residues may sink to the bottom. These residues consist chiefly of metallic sulphides and arsenides. After six to seven hours’ fusion the stirring is discontinued, and the temperature raised for a short time to the highest it is possible to attain, so that the smalts may be as fluid as possible. When this is the case it can be drawn out into long thin threads. The melted glass is then taken out and poured into cold water. The rapid cooling makes it extremely brittle, and considerably facilitates the powdering. When new pots are used for the fusion, their walls are considerably attacked by the alkalis contained in the glass. In order to prevent this a small quantity of common smalts is fused in them before the introduction of the real charge; the inner surface then becomes covered with a protective glaze.