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?