to blue by aspirating air at _m_, or after closing the opening, _d_, and removing the plate, _f_, the green ultramarine can be raked into the collecting chamber, _P_, and there oxidised. The retorts are then immediately re-filled with the mixture and the temperature raised. [Illustration: FIG. 28.] The heating of a charge lasts nine to ten hours; the larger the quantity of sodium sulphide in the mixture the shorter is the time required. The mixture is spread out in the muffles in a layer 7 to 9 centimetres thick, and brought to a moderate red heat. When the whole mass is uniformly hot it is more strongly heated; the change of the constituents then becomes visible. The mixture takes at first a brownish colour, and is somewhat similar in appearance to liver of sulphur. The colour soon begins to incline to green, and finally changes to a tolerably pure green, but always has a yellow tinge. Those parts of the hot mass which come into contact with air are more blue, and occasionally change to a pure blue. When crucibles are used the upper layer of the contents is always bluer than the lower, and when crucibles crack during the heating the portions near the cracks always show distinctly a more or less blue colour. When the calcination is finished the hot mass drawn out of the muffles is at once brought into washing vessels, in which it is treated with water so long as soluble materials are extracted. The compounds dissolved in the water are chiefly sodium sulphide and sulphate and a little alumina dissolved in caustic soda. The presence of a larger quantity of caustic soda in the wash waters indicates too violent heating. In this case the mass will not mix up easily in water; it contains sintered lumps. The washed material is strained, spread out on boards or linen stretched across frames, and completely dried by artificial heat. At this stage the product may be described as green ultramarine and sold as such, or it may be converted into blue ultramarine. For the latter purpose it is ground to a moderately fine meal, but green ultramarine must be ground to a very fine powder; the finer the powder the brighter the shade. In order to obtain blue ultramarine from the washed and ground substance, it is subjected for a short time to a moderate red heat with unrestricted access of air. The heating by which the blue colour is produced must be continued until the proper shade is obtained. This operation is conducted in special muffles; in many works it is customary to sprinkle powdered sulphur upon the heated mass during the roasting. The sulphur burns, forming sulphur dioxide, which escapes into the furnace through an opening at the back of the muffle. The object of the addition of sulphur may be to keep the temperature from rising beyond a certain point, or to prevent a possible reduction of the sulphur compounds in the ultramarine.