Ochres as By-Products
In the manufacture of certain chemicals, substances of divergent composition are obtained which are sold under the name of ochre and are used as painters’ colours. Whereas ochre, properly so-called, consists of either ferric hydroxide or ferric oxide in association with clay, lime, etc., the products now under consideration are basic ferric salts composed of varying quantities of ferric oxide in combination with certain proportions of sulphuric acid.
These ochres are obtained as by-products in the manufacture of green vitriol from pyrites, and in alum manufacture; and, according to their origin, they are classed as vitriol ochre, so-called alum sludge, and pit ochre. All the basic ferric sulphates of which they are composed form fairly large crystals, and, therefore, in most cases, the covering power is small. On this account the products are of low grade and are put on the market at low prices, for which reason they are largely used in making cheap paints.
Vitriol Ochre.—Commercial green vitriol is, for the most part, manufactured from native sulphides of iron. When many of these sulphides are piled in heaps and left to the action of the air, oxygen is gradually absorbed and green vitriol is formed which is dissolved out by rain and is collected in large clarifying tanks.
In the case of pyrites, however, the mineral must first be roasted in a current of air, since otherwise its conversion into green vitriol would only proceed in a very sluggish manner. In any event, the aqueous solution of ferrous sulphate has to be concentrated, by evaporation, to the point at which the green vitriol crystallises out.
Both in the clarifying-tanks and—still more so—in the evaporating-pans, a rusty-looking sediment forms at the bottom, consisting of basic ferric sulphate. This originates in the partial oxidation of the ferrous oxide (first formed) while the pyrites is exposed to the air, and since the quantity of sulphuric acid present is insufficient to saturate all the ferric oxide, basic salts are produced.
The yellow-brown sludge deposited in the pans during the concentration of crude green vitriol liquor, constitutes the product termed vitriol ochre, which contains varying amounts of ferric oxide, sulphuric acid and water, according to the quantity of ferric oxide resulting from the oxidation of the pyrites and the character of the latter, e. g.:—
| Ferric oxide | 65–70% |
| Sulphuric acid | 14–16% |
| Water | 13–16% |
Although the colour of these ochres is not particularly handsome, they can be transformed, by firing, into colours of fairly good quality. As this subject will be more thoroughly gone into when dealing with the preparation of the red iron pigments, the applicability of these ochres will only be casually referred to here. During the burning process, these ochres, of course, part with the whole of their contained water; and by protracted, high calcination, the whole of the sulphuric acid can also be expelled, so that finally nothing but pure ferric oxide is left.
Alum Sludge.—Solutions of crude alum always contain a certain amount of ferric oxide which settles down at the bottom of the pans during concentration. This sludge, too, consists of basic ferric sulphate, but is inferior in covering power to vitriol ochre, the crystals being of coarser grain. On the other hand, the ochreous sediment from the alum concentrating-pans has the valuable property of being readily transformable into red-brown to pure red tones by burning. For this reason, particular attention has been devoted to this sludge in a number of alum works.
Since the products are only of value when burned, and the shades thereby obtained are always red, they will be dealt with more fully along with the red earth colours.
Pit Ochre.—Springs containing small quantities of ferrous sulphate and other salts are met with in many iron mines, but, in most cases, the amounts are too small for their recovery by artificial concentration to be contemplated. If, however, the conditions allow of the springs being easily diverted, they may often be utilised for the preparation of low-grade ochre.
The chemical composition of these pit ochres varies considerably, and depends on the geological character of the locality. Water can only dissolve such minerals as occur in the form of fairly readily soluble compounds; and for this reason pit waters are always solutions of the metals which are found in the mine.
The variety of compounds that may be present in an ochre can be seen from the subjoined analyses of ochres deposited from pit waters at Rammelsberg. As elsewhere, two distinct classes of ochre are met with, having a conchoid and an earthy fracture respectively. The latter usually contain rather more ferric oxide, and, in particular, a higher content of foreign substances, the most important of which is quartz sand. In the Table, the ochres with conchoid fracture are marked A, and those with an earthy fracture, B.
| A. | B. | |
| Ferric oxide | 68·75 | 63·85 |
| Zinc oxide | 1·29 | 1·23 |
| Copper oxide | 0·50 | 0·88 |
| Sulphuric acid | 9·80 | 13·59 |
| Water | 15·52 | 18·45 |
| Clay and Quartz | 4·14 | 2·00 |
The preparation of the ochre is a simple matter, consisting in collecting the mass and sorting out the loose, earthy portions of a pure yellow colour from the denser and darker parts. The former are dealt with separately, usually by a simple process of levigation, for the sole purpose of getting rid of the earthy matter, quartz sand in particular.
The denser varieties require much more work, but yield a far superior product, which, by suitable treatment, can be converted into the finest grades of ochre. The first operation consists in a very careful crushing, and as the pieces are often very hard, they are treated in ordinary or stamp-mills, edge-runners being also employed with advantage.
The product reduced by any of these means is passed through a number of sieves, to separate the fine particles from the coarse; and the finest dust is burnt. This last treatment causes a considerable loss in weight, both the accompanying water and most of the sulphuric acid being volatilised. However, since, as already stated, all varieties of ochre can be obtained, the process is consequently very remunerative notwithstanding the loss in weight it involves.
Yellow Earth.—From the particulars given in the general description of the earth colours, yellow earth may also be regarded, to some extent, as an ochre, but one containing a large proportion of foreign substances. It might, however, be more accurately termed a clay contaminated with a considerable amount of quartz sand and a certain proportion of ferric oxide. The method of preparation is on the same lines as for ochre, but burning is never practised, nor is the treatment so careful as for the better grades of ochre, the low price of the colour making this unremunerative.
CHAPTER VI
RED EARTH COLOURS
The number of minerals that can be directly used as red earth pigments is comparatively small, and by far the greater proportion consist of ferruginous colours, a few of which are obtained by the mechanical treatment of native iron ores or clays coloured red by ferric oxide, the majority, however, being formed by burning certain materials of another colour. To these belong nearly all the materials mentioned in connection with the ochres and the brown iron colours, together with a few by-products of the chemical industry.
In addition to the foregoing, which have ferric oxide for their pigmentary principle, is the native mercury sulphide, occurring, as scarlet, crystalline masses, under the name of cinnabar (vermilion). The only reason for including natural vermilion with the earth colours is to make the list complete, the largest proportion of this pigment being prepared by artificial methods. The product sold as “Chinese” vermilion may, in former times, have really been introduced from China into Europe, and prepared there by grinding and levigating the best-coloured lumps of the natural cinnabar; but, at the present time, all the vermilion made—in Europe at least—is from sulphur and mercury, by artificial processes, and the name Chinese vermilion is merely retained to designate a particularly fine grade.
On the basis of occurrence and chemical properties, the red earths can be classified into several groups. The first comprises natural products requiring only mechanical preparation, such as the minerals known as hematite, micaceous iron ore, Elbaite, etc., and the special modification of red ironstone termed raddle. All these minerals consist almost entirely of ferric oxide in a pure state. The mineral, bole (red chalk, terra sigillata, Lemnos earth), is chemically allied to the ochres, being, like them, composed of alumina, frequently accompanied by lime and small quantities of magnesia, but differing in that ferric oxide is always present in bole, whereas the ochres always contain ferric hydroxide.
The second group consists of the artificial reds obtained by burning or calcining raw materials, whose ferric hydroxide is more or less transformed by heat into ferric oxide, such as vitriol ochre, pit ochre and alum sludge.
Of late years the artificial earth colours have attained a high degree of importance. They are obtained in large quantities in the manufacture of sulphuric acid from green vitriol. Formerly, it is true, they were also used as pigments under the name of caput mortuum or colcothar, but were not held in much esteem; and it is only within recent times that it has been discovered that these inferior by-products can be converted into very handsome and brilliant colours, which now form important articles of commerce.