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.
However, since alum manufacturers do not usually go in for making the highest-grade pigments, simpler calcining furnaces are used, consisting of reverberatory furnaces in which the heating-gases are allowed to act directly on the materials of the charge. A front elevation and section of such a furnace are shown in [Figs. 30] and [31]. The furnace is constructed with several arches, one above another, marked c, k, d. The charge is introduced through the openings b and b’. The furnace chamber is at a, and the ashpit at g. The gases of combustion flow over the charge on the hearths of the several arches and escape, at the top, into the stack, along with the acid vapours liberated from the glowing mass.
Fig. 31.
The further the hot gases get away from the fire, the cooler they become, and therefore the less strongly heated the charge on the upper hearths. Consequently, the resulting product (ferric oxide) from the different stages of the furnace differs in colour; and a number of gradations can be obtained by blending. The ferric oxide pigments prepared in this way are not pure oxide, but also contain small quantities of sulphuric acid and metallic oxides which were present in the original crude sludge. However, by reason of the simple process of preparation employed, these pigments are usually sold at lower prices than those from colcothar; and for less fine work they are excellent.
CHAPTER VII
BROWN EARTH COLOURS
In point of chemical composition, the majority of the brown earth colours are closely allied to the reds, both kinds containing ferric oxide. The main difference consists in that, in the brown earths, the ferric oxide is combined with water to form ferric hydroxide.
Many of the brown earth colours, however, are of entirely different chemical composition, and either consist mainly of organic matter derived from the decomposition of plants—and therefore very similar to brown-coal or peat—or else contain varying quantities of inorganic substances mixed with these dark-coloured organic decomposition products.
The brown earth colours form a highly important group, some of the members of which are used in the finest paintings, and, for certain purposes, cannot be replaced by other pigments. Those containing ferric hydroxide are found—though not very frequently—in natural deposits, the most celebrated being the terra di Siena, occurring in the vicinity of that city.