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.
Vogel’s Iron Red.—This preparation, which is particularly brilliant and therefore highly suitable for an artists’ colour, is made by adding a saturated solution of oxalic acid to a boiling solution of green vitriol. The greenish yellow precipitate of ferrous oxalate is collected on a filter and well washed with water. After drying, the precipitate is heated in a shallow iron dish to a temperature of 200° C., at which the ferrous oxalate decomposes and is converted into a soft fiery red powder consisting of pure ferric oxide. By igniting this powder in covered crucibles the different shades of ferric oxide can be obtained.
Macay’s English Red.—Seven hundred and four parts of ferrous sulphate, 1,000 parts of copper chloride, and 1,678 parts of common salt are dissolved, the solution boiled and the precipitate ignited.
In the manufacture of fuming or Nordhausen sulphuric acid, ferric oxide is obtained as a residue. It is then known under the names of English red, caput mortuum, colcothar, rouge and Indian red as a very cheap pigment. It is also used as a polishing material. Fuming sulphuric acid is made by heating green vitriol at a white heat in retorts placed in furnaces; sulphur dioxide and trioxide are evolved, whilst in the retorts there remains a residue of almost pure ferric oxide, containing small quantities of basic ferric sulphate, which can only be decomposed by long continued violent heating. The vapours of sulphur trioxide are caught in receivers containing oil of vitriol, in which they dissolve and produce fuming sulphuric acid.
The residue in the retorts, which has only a low commercial value, can be converted into a good pigment without the expenditure of much money or labour. It is ground in mills as finely as possible, and, if necessary, afterwards levigated. The fine powder is mixed with varying quantities of common salt, the object of which is to prevent the temperature from rising too high in the calcining process. Common salt is volatile at a temperature approaching a strong red heat; when the temperature has once risen so far, a further rise is prevented by the heat taken up in volatilising the salt. In order to make ferric oxide of a yellow tinge, 2 per cent. of salt are added, and the mixture heated with a moderate fire for 1 hour. To obtain the deeper shades, the addition of common salt is increased even to 6 per cent. For a brownish-red oxide, 4 per cent. of salt are added and the mixture heated for 4 hours; for a dark violet oxide, 6 per cent. of salt are used, and the mixture is heated for 6 hours with the fiercest fire.
It has been observed that the shade of the product is finer the more completely air is excluded from the glowing mass and the more quickly the product, after sufficient heating, is cooled down to the ordinary temperature.
Fire-clay tubes are used for heating the ferric oxide, which are similar to gas retorts, and are built one above the other in furnaces. The number in one furnace may reach 60. Each retort is closed by a well-fitting lid, which is luted with clay after the retorts are filled, a small opening being left through which the heated air may escape. If the retorts were closed completely air-tight they would burst on heating.
Ferric oxide pigments are made in very large quantity by several works, which bring up to 20 different shades, varying from reddish yellow to dark violet, into the market.
On account of their great resistance to the action of the atmosphere and of chemical agents, the ferric oxide pigments are particularly suitable for painting iron and other metals which are exposed to air or water. They are also suited to fresco work.
Lower qualities of iron reds are made by calcining ochres, large deposits of which occur in nature, and also from the residues of basic ferric sulphate obtained in the alum manufacture. These varieties are at the best usable for ordinary painting, but never for artists’ pigments.
According to the method of Steinau, iron pigments can be made from wrought-iron scraps, turnings, etc., by causing them to rust through alternate contact with air and water. The resulting ferric hydrate is either at once used as a pigment or converted into other shades by calcining. By heating in the air a red pigment is formed; by heating with coal in the absence of air a black; and from mixtures of red and black different shades of brown. If the iron turnings can be obtained cheap, this process should be well adapted to the manufacture of fine iron pigments.
Indian Red consists of ferric oxide. It was originally obtained from a very pure hæmatite, occurring in India, by grinding and levigating. It can, however, also be artificially made, in shades varying between bright red and dark brownish-red, by heating pure ferric oxide. This valuable pigment is extremely brilliant and durable.
The darkest brownish-red shades can be obtained from Indian red by mixing it with varying quantities of litharge, and heating the mixture very strongly in a covered crucible.