In spite of these favourable properties antimony vermilion has so far been little used. Considering the small cost of its preparation, especially when calcium hyposulphite, which gives an equally good result, is used in place of the sodium salt, its use is to be recommended in the place of mercury vermilion. It appears as if the high price demanded for this substance by several manufacturers has prevented its general employment.
Appendix. Antimony Blue.—This fine blue pigment can be prepared by the addition of a dilute solution of potassium ferrocyanide to a clear solution of antimony in aqua regia. According to Krauss, it contains no antimony as colouring principle, but is a Prussian blue obtained from the ferrocyanide, which is decomposed by the strong acid, with evolution of hydrocyanic acid.
CHAPTER XIX.
FERRIC OXIDE PIGMENTS.
The pigments composed of ferric oxide are used in enormous quantity. They are distinguished by a high degree of permanence. Large deposits of ferric oxide occur in nature, and in places where it is found in considerable quantity iron is manufactured from it on the spot. Several varieties of natural ferric oxide are distinguished: specular iron ore forms crystalline masses of brilliant lustre; another variety in small crystals is called iron glance; micaceous iron ore consists of shining scales; red hæmatite has the appearance of bundles of fibres; an earthy variety of hæmatite is also common.
The compound of ferric oxide with water, ferric hydroxide, is still more abundant than hæmatite; brown hæmatite, limonite and other iron ores consist essentially of this compound. The pigment known as ochre is also ferric hydrate.
Very pure red hæmatite has so fine a red colour that it may often be used as a pigment after grinding or levigating. The famous red of Pompeii, which has been found on the ruined walls of the town, still shows, after eighteen centuries’ exposure to damp, the brightest shade, certainly a striking proof of the extraordinary permanence of this pigment. Considering its great permanence, its easy preparation, and its low price, it is surprising that ferric oxide is not more extensively used by artists than it is at present. It is used, however, extensively in colouring earthenware, for which purpose its stability at high temperatures makes it suitable. Principally on account of its cheapness it is largely used in ordinary painting, but for artistic purposes it is not used to the extent it deserves.
Every colour maker well knows that artists justly complain that the pigments offered to them have generally but a small degree of permanence. They are surprised that the paintings of the old masters show now, after the lapse of centuries, their colours unaltered, whilst the pigments manufactured at the present day, instead of corresponding to the high standard of chemical knowledge, are often discoloured within a few months after use. But it was just the restricted knowledge of chemistry which the ancients possessed which compelled them to make extensive use of the permanent earth pigments, to which class of permanent colours ferric oxide belongs. The advances of science have succeeded in preparing ferric oxide, not only as a red pigment, but in the different shades of red, from yellow to brown and deep violet, all consisting entirely of pure ferric oxide. Ferric oxide has the property of altering its molecular condition on protracted heating; this change is accompanied by an alteration in colour. If ferric oxide is heated for a very long time at the highest temperatures its colour changes at last to black.
Ferric oxide can be prepared by different methods for artists’ purposes. The process by which it is made is not unimportant. Either ferrous or ferric salts can be employed as the raw material. With the latter, pure ferric oxide is at once formed, or hydroxide, which is converted into oxide by heating. The ferrous salts are generally cheaper than ferric salts; they are therefore commonly used for the preparation of ferric oxide, as well as of the other iron pigments. Even in combination with the strongest acids, ferrous oxide has but little stability; when separated from its salts as ferrous hydroxide, the greatest precautions must be taken to obtain it pure; in contact with the air it at once takes up oxygen and changes to ferric hydroxide. Ferrous carbonate shows this same degree of instability; the natural substance, occurring in large crystals as spathic ironstone, is no exception: on exposure to the air it is gradually changed to ferric oxide.
There is another reason for the advisability of using ferrous salts to prepare ferric oxide. When ferric hydroxide is made by precipitating the solution of a ferric salt with ammonia or caustic potash, the least excess of the precipitant unites with the hydroxide to form a compound which is only decomposed by long washing with water. The precipitate is, however, gelatinous, and consequently very difficult to wash.
In order to prepare ferric oxide suitable for an artists’ pigment the following process may be used: 17 parts of soda are dissolved in 68 parts of water; the solution is boiled in an iron pan, and 10 parts of crystallised ferrous sulphate are added in small quantities with continual stirring. The boiling and stirring are continued until the green vitriol has completely dissolved, when the greenish white precipitate is allowed to settle, washed several times with water, and then exposed to the air in thin layers. The precipitate, which begins to turn yellow during washing, becomes in a short time ochre yellow in the air, being changed into ferric hydroxide. After drying and calcining, a fine red powder of pure ferric oxide is formed. The shade depends on the temperature at which the substance is calcined: the higher the temperature and the longer the heating is continued, the darker is the product.