Lead Brown.—When red lead is treated with nitric acid, lead monoxide is dissolved, whilst deep brown lead peroxide remains. When the action of the nitric acid is finished the residue is well washed and dried. Lead peroxide is now extensively used for lucifer matches; it readily gives up oxygen on heating, and thus accelerates the ignition of the composition. Brown match-heads owe their colour to lead peroxide.
Manganese Brown consists of manganic oxide; it occurs in nature, but seldom in a state of sufficient purity to be used as a pigment; it is, therefore, generally made artificially. The process is very simple. A solution of manganese sulphate is precipitated by caustic soda solution, the manganous hydroxide produced is quickly changed by the action of the air into manganic hydroxide. The colour is most rapidly developed when the precipitate is spread out in thin layers; it is well washed after it has changed to brown.
Pyrolusite Brown is manganese peroxide in a state of fine division. It can be made from the residual liquors of the preparation of chlorine by adding sodium hypochlorite. A brown precipitate results, which is kept in contact with the liquid until it no longer changes in colour. It is then washed with water slightly acidified with sulphuric acid, and finally with pure water. When finely powdered, manganese peroxide has a very fine brown colour. It is quite unaltered by the air and thus, in consideration of its cheapness, deserves a much greater use than it has yet found.
Prussian Brown.—When Prussian blue is heated in air it soon begins to glow, and is converted into a brown mass, the shade of which depends on the amount of impurity in the Prussian blue. It is desirable to use Prussian blue quite free from impurities, so that a brown of the same shade is always obtained. The brown obtained can be shaded by additions of indifferent substances.
Iron Brown.—By igniting a mixture of 100 parts of finely levigated natural yellow ochre with 5 parts of common salt a brown product is obtained, which is pale or deep according to the temperature used. It is a cheap and durable colour.
Copper Brown.—According to Cartheuser solutions of 2 parts of copper sulphate and 1 part of Epsom salts are mixed, and a strong solution of potassium carbonate gradually added so long as a precipitate forms. The precipitate is washed, dried and ignited, when copper brown is obtained as a brown powder of great permanence in air.
According to Schreger solutions of 2 parts of copper sulphate, 2 parts of alum and 0·5 to 1 part of ferrous sulphate are mixed, potassium carbonate solution added, the precipitate dried and ignited. The redness of the shade is proportional to the quantity of ferrous sulphate.
Hatchett Brown is copper potassium ferrocyanide. It is obtained by precipitating a soluble copper salt with potassium ferrocyanide. Copper salts behave towards potassium ferrocyanide in the same manner as iron salts; different compounds result according as the ferrocyanide or the copper salt is in excess. Hatchett brown is somewhat largely used for painting wood.
Chrome Brown.—When solutions of potassium chromate and a copper salt are mixed a precipitate of the composition CuCrO₄.2CuO.2H₂O is obtained. On drying it acquires a fine brown colour; it forms a very stable pigment.
Chrome brown can also be made by dissolving 10 parts of potassium bichromate in 20 parts of water, heating to boiling, adding 13·5 parts of solid copper chloride and then gradually a boiling solution of 10 parts of soda in 20 parts of water, until effervescence no longer takes place. On cooling, chrome brown separates as a soft brown precipitate.
Cobalt Brown is a very durable compound pigment of a pleasing shade. It is obtained in various hues by adding ferric oxide to the mixture of alumina and a cobalt salt used for making cobalt blue. The process may also be carried out by igniting ammonia alum with cobalt sulphate and ferrous sulphate. The temperature required in this case to obtain a bright colour is very high, and must be maintained for a long time to decompose the whole of the ferrous sulphate.
The pigment can be obtained at a far lower temperature when ferric chloride is used in place of ferrous sulphate. The mixture is made by grinding together 5 parts of cobalt hydroxide and 25 parts of ammonia alum, then adding a solution of ferric chloride, rapidly drying the whole to a powder, and whilst still hot filling it into the crucible in which the ignition is performed. When small quantities of ferric chloride are used chocolate brown shades inclining to violet are produced; the larger the quantity of the iron compound the more pure is the brown.
It has been stated that it is important, in the preparation of cobalt pigments, to prevent the entry of fire gases to the heated mixture; the reducing action of the gases would materially damage the beauty of the colour. It has been proposed to place a small quantity of mercuric oxide at the bottom of the crucible. This would decompose on heating, and create an atmosphere of pure oxygen in the crucible. Apart from the cost of the mercuric oxide, which is considerable, it would only be effective at the commencement of the operation, since it is completely decomposed at a low red heat. The author has found an addition of pyrolusite considerably more effective. This may be applied by spreading a small quantity, about 5 per cent. of the mixture to be heated, on the bottom of the crucible and covering it with powdered glass, upon which the colour mixture is placed. An alternative course is to place the crucible in a second, filling the space between the crucibles with powdered pyrolusite. At a strong red heat oxygen is slowly evolved from the pyrolusite; the entry of fire gases into the crucible is thus prevented. In using the first method the crucible is frequently spoilt, whilst in the second method the surrounding pyrolusite protects it, so that it can be used again.