(C) The Red Earths
Apart from the small quantities of native vermilion handsome enough for direct use as painters’ colours when reduced to powder, the red earths, with practically no exception, consist of ferruginous minerals, and it is only within a recent period that red painters’ colours have been prepared from certain chemical waste products from manufacturing processes. In all cases, however, compounds of iron and oxygen constitute the bulk of the red earths. In addition to ferric oxide, which is the chief material used for making the important red colours, are compounds of ferric oxide and water, i. e. ferric hydroxides. The ferric oxide pigments are among the most important in the entire series of earth colours, being on the one hand very cheap, and on the other so handsome in colour that ferric oxide can be used for the finest paintings.
Ferric oxide can also be shaded very extensively by a fairly simple treatment, so as to furnish a whole range of very handsome shades.
In nature, ferric oxide occurs in numerous varieties of one and the same mineral, red iron ore, which is also known as hematite, blood stone, raddle, etc.
Red Ironstone
Red hematite occurs native as rhombohedral crystals, which mostly consist solely of ferric oxide, and may be considered as pure oxide for the purposes of the colour-maker. The difference between the several varieties is due, not to any chemical variation, but entirely to changes in physical structure. The varieties with a radial, fibrous structure are known as red hematite, the colour of which ranges from blood red to dark brown and is frequently accompanied by metallic lustre. The scaly modification of this mineral forms micaceous iron ore, and is usually a deep iron black. In the neighbourhood of volcanoes it is frequently found as particularly handsome crystals.
Iron cream (frosty hematite) is the name given to a beautiful cherry red variety, which easily rubs off, has a greasy feel and is composed of extremely fine scales.
The so-called raddle occurs in Nature as a readily pulverulent earthy mass of ferric oxide contaminated more or less with extraneous substances. On account of its abundance and low market price, it is largely used in painting.
Although mixed with numerous foreign substances, certain clay ironstones, oölitic ironstones and siliceous ironstones may be regarded as ferric oxide in the sense understood by the colour-maker, all these minerals having a deep red to deep brown colour and being capable of finding advantageous employment as pigments.
Ferric oxide is distinguished by two properties which render it specially valuable to the colour-maker. When combined with water, its colour is no longer red, but a handsome brown; and, on the other hand, when heated, the colour passes through brown into a permanent dark violet. By suitable treatment of such minerals as consist mainly of ferric hydroxide, mixtures can be obtained which contain the oxide and hydroxide in variable proportions and give a whole range of shades between brown and red.
The preparation of these colours is easy when very pure red ironstone is available. The somewhat expensive pigment, Indian red, is—when pure—really nothing but a very pure ferric oxide of Indian origin. Ferric oxide, however, often contains impurities which considerably influence the colour of the product. Owing to the fact that large quantities of ferric oxide are formed as by-products in certain chemical processes which are carried out on a very extensive scale, this oxide, which is very pure, can be advantageously used for making iron pigments, especially as its application for other purposes is very restricted, and it can therefore be had at a very low price.
The following analyses show the composition of a number of red ironstones, Nos. 1, 2 and 3 being hematite from Froment, or Wetzlar, No. 4 from Wetzlar, Nos. 5 and 6 hematite from Whitehaven, No. 7 from Thuringia, No. 8 from Bohemia, No. 9 from Spain, No. 10 from N. America, and No. 11 from England.
| No. | Ferric oxide. | Manganese oxide. | Silica. | P2O5. | Alumina, lime and magnesia. | Water. |
| 1 | 94·00 | Trace | 2·00 | — | Trace | 2·00 |
| 2 | 80·95 | — | 16·74 | 0·51 | 0·97 | 0·83 |
| 3 | 73·77 | — | 23·16 | 0·45 | 1·41 | 1·21 |
| 4 | 92·45 | — | 5·63 | 0·19 | 0·65 | 1·08 |
| 5 | 96·27 | — | 4·20 | — | 0·59 | 4·40 |
| 6 | 63·05 | 5·29 | 22·76 | — | 0·49 | 4·40 |
| 7 | 85·00 | 1·601 | 3·304 | 0·457 | 8·795 | 0·633 |
| No. | Iron. | Manganese. | Alumina. | Lime. | Magnesia. | Silica. | Sulphur. | Phosphorus. | Loss on calcination. |
| 8 | 33·64 | 0·10 | 7·58 | 8·10 | 0·82 | 17·80 | Trace | 0·19 | 9·82 |
| 9 | 31·38 | 0·19 | 0·06 | 29·95 | 0·35 | 0·87 | — | 0·09 | 23·68 |
| 10 | 62·54 | 1·93 | 1·71 | — | 0·50 | 3·80 | 0·02 | 0·04 | 2·23 |
| 11 | 62·91 | Trace | 1·39 | 0·70 | 0·42 | 5·89 | 0·05 | 0·11 | — |
There are certain other minerals closely allied, both chemically and mineralogically, to red ironstone, namely, the brown hematites or ironstones used in the manufacture of iron. Brown hematite consists of ferric hydroxide, Fe2O3H2O, and occurs in a variety of forms in Nature, the most frequent being pea (oölitic) ore, which owes its name to the spherical shape of the grains. Some brown hematites are decomposition products of other minerals, and contain sulphur and phosphorus in addition to ferric hydroxide. Like the pure hydroxide, they are brown in colour, but differ therefrom considerably in their chemical behaviour when heated. This is particularly the case with the so-called bog ore, which is mostly found, as spongy yellow-brown to black masses, in swamps, and owes its origin to the decomposition of various ferruginous minerals. It varies greatly in chemical composition and occasionally contains up to about 50% of sand. The amount of ferric oxide in bog ore varies between 20 and 60%, and it also contains 7–30% of water, up to 4% of P2O5, small quantities of ferrous oxide and manganese hydroxide, together with, in most cases, mechanically admixed organic residues.
The phosphorus content makes bog iron a very inferior material for smelting, the resulting iron being of low quality. Nevertheless, it can sometimes be advantageously used in making earth colours, though the products cannot lay much claim to beauty of colour.
Bole
The native earth pigments known by this name form masses of the colour of leather to dark brown, with a conchoidal fracture and an earthy appearance. Bole chiefly consists of iron silicate combined with water, some varieties containing small quantities of alumina. The composition fluctuates very considerably, most varieties containing 41–42% of silica, 20–25% of alumina, and 24–25% of water, the remainder consisting of ferric oxide. Some kinds, such as Oravicza and Sinope bole, contain only 31–32% of silica and 17–21% of water.
Bole is used as a paint for walls, clapboards, etc., and is only mentioned here because of its relationship to the ferric oxide pigments.
Alum Sludge
Large quantities of clarification sludge are produced, in alum works, as the sediment from the red liquors. This sludge consists mainly of ferric oxide, with small quantities of other oxides and sulphuric acid (basic ferric sulphate), and would be an entirely worthless by-product except for the fact that it can be manufactured into pigments, some of them of great beauty.
All alum makers should treat this residue and convert it into pigments, which they could put on the market at a low rate, the cost of preparation being small. Since the material is chiefly composed of ferric oxide, the resulting colours are very similar to those obtained from iron ores; and all shades, from yellow-brown, through red, to the darkest brown, are represented.
Mine Sludge
The water frequently present in iron mines sometimes contains large quantities of sediment, which consist mainly of iron ochre and can be advantageously worked up into pigments. There is scarcely any need to mention that all substances containing ferric oxide can be used for making any of the pigments obtainable from the oxide itself, the only difference between the various raw materials being their degree of purity, so that it is not always so easy to obtain a certain desired shade from a given material in such beauty as is furnished by another material, the small quantities of impurities associated with the ferric oxide having, in many instances, an important influence on the colour.