The materials employed may be divided into assistants in the processes and components of the manufactured pigment. The assisting substances are those which are used in the manufacture of a colour without entering into its composition; from the component materials the colours are directly derived. For example, in the manufacture of Prussian blue, yellow prussiate of potash, an iron salt, water (in which the salts are dissolved) and nitric acid are used. In the blue obtained are contained portions of the iron salt and of the yellow prussiate, these are, therefore, component materials, whilst water and nitric acid are simply assistants, since they do not enter into the composition of the pigment.
In colour making a large number of assisting materials are employed, which comprise a considerable number of elements and compounds. Since these are of great importance for our purpose, we shall describe their properties, and, when necessary, briefly the method of preparation.
Among the component materials are to be reckoned a large number of salts of the alkaline earth and earth metals and of all the heavy metals. In addition, there are also the substances of animal or vegetable origin used in lake making.
In the description of the raw materials, if we were to overstep the line drawn here, we could include a great variety of compounds, those, for example, used in the manufacture of the so-called aniline dyes. These substances form, however, as we have stated, the object of a particular branch of manufacture, which forms a separate division of colour chemistry, but with which is not to be confounded what has been hitherto designated the manufacture of colours.
CHAPTER IV.
ASSISTANT MATERIALS.
Water, H₂O = 18.[1]—This substance plays a tremendous part in colour making; almost all the substances which are used in solution are dissolved in water; the removal from precipitates of admixed foreign bodies, the so-called washing, is always accomplished with water. The chemist does not understand by water quite that liquid which in general speech is so designated. We must consider the water which is at the disposal of the colour maker.
[1] We append the chemical formula and the molecular weight to the description of each compound.
Water, in the chemical meaning of the word, is a liquid composed only of hydrogen and oxygen, and leaving no residue when evaporated. Such water is not found in nature; it can only be obtained by distillation of well or river water. The water which falls in long continued rain, or is obtained by melting snow, is most nearly like distilled water; it contains only small quantities of dissolved substances, and generally such as would be without influence in colour making. Water of this description is available for but a limited use; the large quantities of water required in a colour works must be taken from springs or streams. These waters contain, however, more or less large amounts of dissolved salts, which act in a marked manner upon the substances dissolved in them.
In almost all spring and well waters is found carbonate of lime; such waters are called “hard”. River water contains generally little carbonate of lime; it is then called “soft water”. The influence of the carbonate of lime is especially evident when salts of lead, copper, iron and other heavy metals are dissolved in water; the carbonate of the particular metal gradually separates from solution, and the liquid becomes very turbid.
When only hard waters containing much lime are at the service of the manufacturer, turbid solutions are often obtained, which must be filtered before use. In many cases this can be avoided by adding milk of lime to the water in a large vessel; the free carbonic acid unites with the lime, and thus the carbonate of lime, which is only soluble in water containing free carbonic acid, separates as a fine precipitate. Water which has been treated in this way becomes clear after some time, through the deposition of the carbonate of lime; it is then soft water. In order to separate the carbonate of lime in this way, no more than the requisite quantity of milk of lime should be added, so that no lime remains in excess, since this would cause precipitates when salts of lead, copper, iron, etc., were dissolved. In many cases—for example, when lead or barium salts are dissolved—the lime contained in the water can be made harmless by slightly acidifying with acetic or nitric acid. Water which contains sulphate of lime (gypsum) is equally useless for many purposes, as, for example, the solution of lead and barium salts. These metals form insoluble compounds with the sulphuric acid, which render the solution turbid, and can be removed only with difficulty by filtering, on account of their great fineness. They are more easily removed by allowing to settle.