The mixture of the materials used in making ultramarine must be most intimate in order that the constituents may act chemically upon one another. In some works the mixing is performed in a very laborious manner—upon a heap of the dried, levigated clay the remaining materials are thrown, and the whole shovelled about till it is completely homogeneous. Naturally, instead of this primitive and costly operation, mechanical mixers may be used, but the mixture can also be effected in a simpler manner, which almost dispenses with the necessity for a mechanical process. Of the substances to be mixed only kaolin and sulphur are insoluble in water; the others and also the by-products of previous processes are easily soluble. The soluble and insoluble constituents of the charge may be mixed in a very simple way by bringing the levigated kaolin in the form of paste, without drying, into a pan, adding the solutions of the salts and the powdered sulphur. The pans are most conveniently heated by the fire gases from the ultramarine furnaces. After mixing the solutions with the clay to a thick paste the heating is commenced, the solid substances are prevented from sinking to the bottom by continual stirring, the mixture is slowly evaporated, and an extremely intimate and complete incorporation of the constituents occurs. The heating is continued until a dry mass is obtained, which may be used for the second operation without powdering. The mixture of raw materials is heated for a long time at a clear red heat, or, in some works, at a white heat. The operation demands some care; air must be excluded and the whole mass must acquire a uniform temperature. If both these conditions are not obtained in heating it is very difficult to obtain a product of uniform colour. In different works different arrangements are used for the calcining process. In the older methods crucibles or dishes of fire-clay were used, of such a shape that the bottom of one crucible formed the lid of the crucible beneath, just as is the case with the saggers used in porcelain kilns. The mixture is closely pressed into the crucibles, of which piles are built in the furnaces. These must be so arranged that each crucible is surrounded by the fire. The furnaces are very similar to porcelain kilns. This method of calcining is obviously attended with many drawbacks; a large number of costly crucibles is required, a certain proportion of which is lost at each burning, either by breakage in the fire or in filling. The pressing of the charge into the small crucibles requires much labour, as also the placing of the crucibles in the furnace and their removal; at the end of the nine to ten hours’ calcining, it is necessary to wait some time to take the crucibles out of the furnace, or considerable loss will be caused by breakage of the crucibles due to rapid cooling.