As previously mentioned, calcined ferric oxide is very inert, chemically, so that, when the calcination has been strong, prolonged boiling with the most powerful acids is needed to bring the oxide into solution. If the heating has been continued up to the strongest white heat, and the ferric oxide maintained in that condition for several hours, even hot sulphuric acid will have only a slight effect on the oxide, and the only way to make it more readily soluble is by fusion with potassium bisulphate.
Now indifference to chemical action is just the property required of a pigment for fine work; and in this respect, the ferric oxide colours are superior to all others. The gradations of tone that can be obtained from ferric oxide by varying the calcination are very numerous, comprising all between iron red, red-brown and pure violet.
The author has tried heating ferric oxide for a considerable time at a very high temperature, equivalent to the strongest white heat, and obtained a product which was no longer pure violet, but had a decidedly blackish colour. Perhaps, by greatly prolonging the heating, it might be possible to get a pure black; but, even if this were so, the matter would be of no special interest, because black pigments for paints can be prepared in a much cheaper manner. All that would be accomplished would be the proof that ferric oxide actually undergoes an extensive molecular modification when heated.
Ferric Oxide Pigments from Alum Sludge
Alum is manufactured from alum shale and alum earth, the former being a carbonaceous clay shale interspersed with pyrites, and the latter a clay charged with pyrites and bitumen. The raw materials are left in heaps for several years, the pyrites being thereby oxidised with formation of free sulphuric acid and ferrous sulphate. This free acid reacts further on the clay, which it transforms into sulphate of alumina; and by leaching the heaps with water, a solution is obtained which contains the sulphate of alumina and the ferrous sulphate. On the liquor being concentrated, a basic ferric sulphate is deposited, which is worked up into red pigment.
For this purpose it is first levigated in a special manner, the sludge from the pans being placed in a large vat, suffused with water, and kept in slow circulation by stirrers, which distribute the particles in the water, forming a turbid liquid. This liquid is conducted into a gently sloping shute, the sides of which are perforated with openings at certain intervals, to allow part of the liquid to run off into large collecting vessels underneath.
The heaviest of the suspended particles settle down first and are flushed out by the water escaping through the first opening. The finer the particles, the longer they remain in suspension, so that the liquid escaping through the last holes carries off only a very fine powder. The liquid collected in the different vessels is allowed to subside and is then drawn off from the firm deposit. The operation is repeated with fresh quantities of sludge until sufficient sludge has been collected for further treatment. The collecting vessel furthest away from the intake of the shute contains the finest levigated material, and this is used for making the best ochres.
The levigated mass is dried in a very simple manner, being usually spread out on boards, which are exposed to the air in open sheds, covered with a roof to keep out the rain. Here the sludge is left until it forms a pasty or earthy mass, and is then calcined.
The best calcining furnace is of the type used for colcothar; but the pipes must be connected to an exhaust pipe for carrying off the vapours disengaged during calcination.
