| 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.