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.