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.
CHAPTER XX.
OTHER RED MINERAL PIGMENTS.
Chrome Red or Chrome Vermilion.—Under the most varied names, chrome orange, Persian red, Derby red, Chinese red, Indian red, chrome vermilion, etc., there occur in commerce numerous pigments, orange to dark red in colour, which can all be made from neutral lead chromate. When this salt, i.e., ordinary chrome yellow, is treated with smaller or larger quantities of a strong base, such as caustic potash or caustic soda, “basic” lead chromates are formed, which exhibit a more intense red shade the more lead they contain. These compounds can be made either by adding caustic potash to the solution of potassium chromate used for making the chrome, or the precipitated chrome yellow may be treated with caustic potash. The latter process readily yields good results, so that it is generally followed.
Recent researches have shown that differences in the shade of chrome red are due to the varying size of the single microscopic crystals of which it consists. When chrome reds of the most different shades are ground the powder has almost the same colour. The larger are the crystals the deeper is the shade of the red, so that the art of making deep chrome red lies in working so as to produce large crystals.
As was said in describing the preparation of chrome yellow, the lead liquors used in its manufacture contain varying quantities of acetic acid, which affect the shade of the product. The quantities of caustic potash necessary to turn a certain quantity of chrome yellow into chrome red of a certain shade can, therefore, only be estimated from time to time in an empirical manner. This estimation may be performed by the method of Habich with great certainty. The precipitate of chrome yellow is made in the ordinary way, and well washed 6 to 8 times. Equal portions of the paste are brought into vessels of equal height and diameter. To the first of these portions caustic soda solution of a certain strength is added; to the second rather stronger caustic soda solution; in each succeeding vessel the strength of the solution is increased to a known extent. After the addition of caustic soda the vessels are energetically shaken. They are then set aside for the precipitate to settle in a place completely free from vibration. In one of the vessels, an orange or red shade will be seen very similar to that it is desired to make. Now, it is known how much caustic soda solution of a certain strength has been added to this quantity of colour, so that the quantity necessary for the whole of the chrome yellow precipitate may be found by a simple calculation. In working on the large scale, the process is exactly the same as in the preliminary test; the calculated amount of caustic soda solution is added to the precipitate in the tub, it is well stirred, and left at rest for some hours. When the proper shade appears, the liquid above the precipitate, which contains sodium chromate in proportion to the strength of the caustic soda solution used, can be drawn off and used to precipitate fresh lead acetate solution. The precipitate is washed first in the tub and then on the strainers.
Since, as has been said, the depth of colour depends upon the size of the single crystals, care should be taken in washing the precipitate not to agitate it too vigorously, or the pigment will lose in depth by the breakage of a large number of crystals.
Special formulæ for the manufacture of chrome red are abundant, but if the above process be followed they are superfluous, and it is only for the sake of completeness that one or two are given. Deep chrome red is made by the action of 25 parts of caustic soda on 100 parts of pure chrome yellow.
A noteworthy process is due to Liebig. Saltpetre is melted in a crucible, and heated to a temperature below that at which oxygen is evolved. Dry chrome yellow is introduced so long as effervescence occurs; the fused mass appears, whilst liquid, deep black. As soon as it is in quiet fusion it is poured on a cold plate. In consequence of the rapid cooling it becomes brittle and can be easily powdered. The hot mass is broken up and boiled with water, potassium chromate going into solution. The chrome red made by this process is little inferior in brilliance to vermilion.
According to Prinveault a handsome chrome red is obtained by treating 25 grammes of neutral lead carbonate with 10 grammes of yellow potassium chromate dissolved in 5 litres of water, boiling for half an hour, washing the violet precipitate, and finally treating with 1 gramme of sulphuric acid diluted with 100 grammes of water.