The manufacture of mineral and lake pigments

by Josef Bersch

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.

For these reasons in most works crucibles have been abandoned in favour of muffles, a large number of which are built in one furnace. The charge is placed in the muffles; after completion of the calcining it is quickly raked out, and the still hot muffle at once recharged, thus the losses of time and heat are reduced to a minimum. The muffles are generally about 1 metre long, 1 metre wide and 30 to 40 centimetres high. They are made of fire-clay, and, when kept in uninterrupted use, last a long time. The muffles have a small opening at the back which communicates with the furnace, so that the gases evolved have a free outlet without reaching the working place. The front of the muffle is closed by an iron plate in which are small openings, through which the contents can be examined without the entry of much air.

Three muffles are generally built in a furnace, but, by suitable alterations in the construction, a considerably larger number may be heated in one furnace. Whatever the number of muffles, the furnace must be so arranged that the fire can be controlled at will. This is best done by a good damper; all the muffles must be uniformly heated.

J. Curtius recommends cast-iron cylinders, lined with a thin layer of fire-clay. For this purpose a covering of fire-proof cement is used to protect the retorts. This covering, as it is gradually destroyed, may be renewed by smearing over the damaged places with fire-proof cement, mixed with water or some binding material. Aluminium silicate, graphite and coke may also be used. The retorts, a ([Fig. 28]), project through the wall of the furnace at one end. They rest on fire-proof stone bridges, and can be put into communication with the air by the pipe, m, which can be closed air-tight. Short pipes, k and d, which can be closed, connect the interior of the retort with the cooling and collecting chambers, P and g, the latter of which is connected by r with the vessel, h, where the gaseous products are absorbed. The porous plate, f, prevents the charge in the hinder part of the retort from becoming open and falling to pieces during stoking. In order to obtain a regular heating of the retorts the fire gases passing through the space, q, are led away at two opposite places by flues into the main flue, F. The intimate mixture of finely-powdered materials is introduced into the hinder part of the retort, the porous plate, f, placed in position, the cover, t, closed up air-tight, and the connection with the chamber, P, closed. On heating, the volatile products pass through the porous plate and the pipe, d, into the chamber, g, the more volatile portions, without mixing with the furnace gases, passing direct to the absorption vessel, h, or to a lead chamber, whilst the sulphur which distils over remains in g. When the reaction is finished, by opening the cover, t, and the tube, m, the crude green ultramarine in the retort can be rapidly oxidised 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.

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.