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