Artificial Ochres
Products very similar, both in chemical composition and colour, to the native ochres can also be very simply and cheaply made by artificial means. Their preparation may be particularly recommended to colour-makers who desire to turn out a wider range of iron pigments, but are not in a position to obtain natural ochres at a low price.
In the manufacture of artificial ochre, an endeavour is made to imitate the natural processes which have led to the formation of ochre, and, of course, to avoid anything likely to hinder the production of a suitable colour earth, for example the presence of sand or a considerable admixture of extraneous minerals.
As already mentioned, the chief impurities in natural ochres are clay and sand, both of which can be easily excluded during the manufacture of artificial ochre, or their amount controlled in such a manner that paler or darker products can be obtained at will, and the tone varied, in any desired manner, by calcination, as in the case of the native article.
The raw material for artificial ochre is always a ferrous salt, which can be purchased in large quantities and at very low prices, namely green vitriol, which, in the pure state, consists of ferrous sulphate, FeSO4 + 7H2O. This substance forms sea-green crystals, which are readily soluble in water and impart an objectionable inky flavour thereto. On exposure to the air, green vitriol turns an ugly brown colour, and is no longer completely soluble in water, passing gradually into the condition of basic ferrous sulphate. This is because ferrous oxide is a highly unstable substance, which attracts oxidation and changes into ferric oxide. This latter, however, requires for the production of soluble salts a larger quantity of acids than does ferrous oxide, and therefore the oxidation of ferrous sulphate in the air leads only to the formation of salts that are imperfectly saturated with acid, namely basic salts.
When a solution of green vitriol is left exposed to the air, basic ferric sulphate is also formed, which settles down to the bottom of the vessel as a rusty powder. If, however, a corresponding quantity of sulphuric acid be added to the solution at the outset, the resulting ferric sulphate remains in solution.
On treating the green vitriol solution with one of caustic potash, caustic soda or quick lime, the ferrous oxide is thrown down as the corresponding hydroxide, forming a voluminous greyish-green precipitate. This hydroxide still possesses a great affinity for oxygen, and when the precipitate is brought into contact with air, its colour rapidly changes to a rusty red, through the transformation of the ferrous hydroxide into ferric oxide. The ferrous hydroxide can also be precipitated by alkali carbonates, the deposits behaving in exactly the same manner as that thrown down by the caustic alkalis.
Various methods can be adopted in the preparation of artificial ochre, the selection depending on the properties desired in the finished product. To obtain an ochre with particularly good covering power, the method must be different from that employed to furnish a cheap product, in which low price is more important than covering power.
In the former case, the ferrous hydroxide is mixed with substances which, in themselves, possess fairly high covering power, such as chalk or white clay; in the second, gypsum, which is of low covering power, is used.
The preparation of the cheapest kinds of artificial ochre will be described first, followed by that of the higher grades which belong to the most valued artists’ colours.
For cheap artificial ochres, the ferrous hydroxide is thrown down by caustic lime from a solution of green vitriol. According as a lighter or darker shade is required, two to three parts of ferrous sulphate are dissolved in water, care being taken to select crystals of a pure green colour, since those that have a rusty look are only imperfectly soluble, because they contain basic ferric sulphate.
The solution will always be cloudy, owing to the partial precipitation of the hydroxide by the lime in the water; but this is immaterial. For the precipitation, a milk of lime is prepared by slaking one to two parts of quicklime (according to the quantity of ferrous sulphate to be treated) in water, and stirring this up in enough water to make a thin milk. Care must be taken to exclude any large particles of lime, since these would find their way into the finished product and make the colour uneven. On this account, the milk of lime should be carefully strained through a loosely woven cloth or fine sieve, into the precipitation vessel.
The ferrous sulphate solution is then poured in, the mixture being kept stirred, and an ugly, grey-green precipitate is produced, consisting of a mixture of ferrous hydroxide and calcium sulphate, the reaction being explained by the equation:—
FeSO4 + Ca(OH)2 = Fe(OH)2 + CaSO4.
The larger the amount of ferrous sulphate solution added to the milk of lime, the darker the resulting ochre. As soon as all the ferrous sulphate is in, the stirring is suspended, and the liquid is left until quite clear. The water is drawn off through tapholes in the side of the vessel, care being taken not to disturb the fine precipitate, and fresh water is added, in which the deposit is stirred up and again left to settle down. This operation, which is once or twice repeated, is to wash the precipitate.
When this object has been sufficiently accomplished, the mass is shovelled out of the vessel and spread thinly on boards, where it is left until the desired shade of colour has been attained, the colour changing quickly on exposure to air, owing to the oxidation of the ferrous hydroxide into ferric hydroxide. To ascertain whether oxidation is complete, a large lump of the mass is broken across; and if it is of a uniform yellow-brown colour throughout, without being darker on the outside than in the middle, all the ferrous hydroxide will have been transformed into the ferric state. The product can now be dried at once, and when ground will be ready for sale.
To obtain different varieties from the product, it is carefully heated (in a finely powdered condition) in shallow pans; but the operation needs caution, or the water in the gypsum present will be expelled, giving rise to drawbacks that are manifested when the colour is used.
For instance, in mixing such a colour with water, the gypsum would again absorb water and cause the whole mass to set as a useless solid lump. Since gypsum parts with its water at a comparatively low temperature, it is better not to heat these cheap ochres at all, but to obtain the various shades by modifying the proportion of ferrous sulphate employed.
Another defect of the ochres prepared by this method resides in the excess of lime present, it being impracticable to measure out the quantity of lime used with such accuracy that only just enough is taken to precipitate the ferrous hydroxide, there being always a slight excess. This lime is transformed into calcium carbonate on the mass being exposed to the air, just as in the preparation of Vienna white; but as the saturation with carbon dioxide takes a considerable time, some of the lime remains in the caustic state and is liable to affect other colours that may be mixed with the ochre.
An artificial ochre uniting in itself all the qualities of the natural product, and also capable of being shaded by burning, can be prepared in the following manner. An accurately weighed quantity of pure crystallised ferrous sulphate is dissolved in a definite amount of water, and the solution is treated with successive small portions of crude nitric acid, until all the ferrous oxide has been changed into the ferric state. The change can be detected by a very decisive test. If a liquid containing ferric oxide in solution is brought into contact with a solution of red prussiate of potash (potassium ferricyanide), no precipitate is formed in the absence of ferrous oxide, but only a brown coloration; whereas, if ferrous oxide is present, a beautiful blue precipitate is formed at once, the colour of which is so intense that very small quantities of ferrous oxide can be detected by this means.
For the purpose now under consideration, the presence of small amounts of ferrous oxide in the solution is immaterial, because they are soon changed into ferric oxide on exposure to the air. It might, therefore, be asked, why take the trouble to oxidise the ferrous oxide by means of an agent involving expense, which could be saved by allowing the oxidation to take place in the air?
The advantage, however, of the direct employment of a solution of ferric oxide is that it gives at once a colour that can be dried straight away; whilst at the same time the colour undergoes no change in drying, whereas it does when ferrous oxide solution is used.
The method of producing ochres from this ferric solution varies according as the product is to be used without any further treatment than drying, or is to be modified by firing.
In the former event, caustic lime is again used as the precipitant, but in only just sufficient quantity to throw down all the ferric oxide in the solution. This amount can be calculated exactly, 36·84 parts by weight of pure burnt lime being required for every 100 parts of pure ferrous sulphate taken. The actual quantity, whether larger or smaller, will depend on the relative purity of the sulphate and lime; and this can readily be ascertained by a simple trial.
The lime is used in the form of milk of lime, as already described. If lime alone is employed, the precipitate will consist of pure ferric hydroxide and the calcium sulphate thrown down at the same time. The resulting colour, when dried, will be an intensely brown mass, which can be used in place of the very dark natural ochres.
In order to obviate entirely the disadvantages resulting from the presence of a large amount of caustic lime in the precipitate, fine levigated chalk or white clay is added in the preparation of the lighter shades of ochre, the addition being made as soon as the two ingredients have been brought into contact; and the mixture is thoroughly stirred, to ensure uniform admixture with the ferric hydroxide. The colour of the settled deposit will be lighter or darker in proportion to the amount of chalk or clay employed; and in this way the whole range of shades from pale yellow to bright brown can be obtained without the application of heat.
Ochre that has been made with chalk is unsuitable for toning by heat, because this treatment would causticise the lime, and the ochre could not be mixed with other colours, since these would be affected by that substance. On the other hand, when white clay is used in preparing the ochre, the latter can be more easily toned by firing, provided care be exercised in the process. The ochre must be dried completely in the air, and either spread out in thin layers on iron plates, for the burning process, or else put into a drum, of the kind already described, in which the mass is moved onward by a worm.
The clay remains unaltered in firing, but the gypsum parts with its water of crystallisation. In order to restore the latter, the ochre issuing from the drum is discharged direct into a vessel of water, in which it can be kept in constant motion by a stirrer. The water is soon warmed by the heat of the mass, and absorption by the gypsum proceeds at a rapid rate. When the whole charge has been fired and collected in the vessel of water, the stirrer is stopped and the precipitate dried, being then ready for use.
In certain circumstances, ochre can be made by other methods. In large towns, ammonium salts are sometimes obtainable at a moderate price, being manufactured in large quantities as a by-product in gasworks. For our purpose, crude gas liquor might be used, since it contains ammonia for the precipitation of the ferric hydroxide. In most cases, however, this gas liquor contains only very small quantities of ammonia, and, therefore, in a works of any size, very large vessels would be needed for the production of a comparatively small quantity of ochre. On this account, preference is given to crude carbonate of ammonia, which is also obtainable at low prices.
On bringing a solution of this salt into contact with one of ferric oxide, ferric hydroxide is precipitated, and the sulphate of ammonia resulting from the reaction remains in solution. By stirring white clay into the liquid at the same time, the ochre can be correspondingly lightened in shade.
The precipitates obtained in this way can be dried at once, and converted into any shade obtainable with natural ochre, from brown to red, by strong firing. The sulphate of ammonia still remaining in the air-dried product is completely volatilised by the heat, and the resulting ochres are even superior to the natural varieties in beauty and permanence.