The manufacture of earth colours

by Josef Bersch

Graphite seems to be of frequent occurrence all over the world, though only few deposits are known which yield a product that is suitable for all the purposes to which graphite is applied.

In European countries, Austria is particularly rich in graphite; and very large deposits of this mineral are found in Bohemia. Considerable deposits also occur in Bavaria, where they have long been worked. English graphite is celebrated for its excellent quality. All these European deposits, however, are surpassed, both in extent and in the quality of their products, by those discovered in Siberia, the largest being that producing the aforesaid Alibert graphite and situated, near the Chinese frontier, in eastern Siberia. At one time, America imported all her blacklead pencils from Europe, having, at that period, no known graphite deposits furnishing a suitable product. At present, however, deposits of this kind have been found in California, and there can be little doubt but that many others of this valuable mineral remain to be discovered in that enormous continent, the geological investigation of which is still far from being complete.

The graphite of some deposits is so highly contaminated by extraneous minerals that it cannot be utilised, since the cost of purification would exceed the value of the product. On the other hand, the purer kinds, when suitably refined, yield a graphite that is fully adapted to all requirements.

The refining process may be either chemical or mechanical, the choice of methods depending entirely on the character of the associated minerals. If these mainly consist of coarse, stony fragments, preference should be given to mechanical treatment; but if they are of such a character that they cannot be eliminated in this way, chemical methods must be employed. Sometimes the two systems are combined, by first subjecting the graphite to a rough mechanical purification, and then completing the operation with chemical reagents.

The mechanical treatment consists in first removing as many of the impurities as possible by hand-picking, and grinding the remainder in edge-runner mills, along with water. The turbid liquid, containing the powdered graphite and extraneous minerals in suspension, is led through long launders, the sides of which are notched at intervals to allow the water to overflow into large pits. The graphite settling in the first of these pits contains numerous particles of the heavy associated minerals; but that remaining suspended in the water and carried on to the further pits constitutes the bulk. The water is left to clarify completely in the pits, and is then drawn off, the pasty residue being shaped into prisms, which are compressed under heavy pressure, to increase their density, when partially dry.

Although levigation will remove most of the accompanying extraneous minerals, it cannot eliminate the ash constituents of the graphite. Experiments made in this direction have demonstrated that the ash content of the levigated graphite is exactly the same as that of the crude material. Whilst these ash constituents do not affect the quality of graphite for certain of its uses, they nevertheless impair its beautiful black colour to a considerable degree. The chemical treatment necessary to eliminate these constituents is attended with many difficulties, the chief of which resides in the fact that the ferric oxide present is in a form that is not readily accessible to the action of chemicals. For this reason, attempts to purify graphite with crude hydrochloric acid are hardly likely to prove successful, since both the ferric oxide and the accompanying silicates obstinately resist the action of this acid.

In order to obtain graphite of a high state of purity, the attempt must be made to bring this ferric oxide and the silicates into a soluble condition. This can be accomplished in various ways, and the choice of the method will depend on the purpose for which the graphite is intended. For example, the operations may either be confined to purification, or else include the attainment of a maximum condition of subdivision. When foliaceous graphite has to be treated—and this kind of graphite cannot, in its original condition, be used for making lead pencils—it is preferable to employ a method which will produce both the above results. The purification may consist in crushing the graphite to powder, and fusing this with a mixture of sulphur and carbonate of soda, whereby the silicates present are converted into soluble compounds, and the ferric oxide into ferric sulphide. On extracting the melt with water, a portion of the contained salts pass into solution and is carried off. The residue is then treated with dilute hydrochloric acid, which dissolves out the ferric sulphide, with liberation of sulphuretted hydrogen, and leaves the graphite in a very pure condition after washing.

In order to render foliaceous graphite suitable for lead pencils, a different method is pursued, but should only be employed in special circumstances, on account of the expense entailed.

According to the process recommended by Brodie, the graphite, ground to coarse powder, is mixed with about one-fourteenth of its own weight of chlorate of potash, this mixture being heated, with two parts by weight of sulphuric to each part of graphite, in a water bath so long as fumes of hypochlorous acid continue to be disengaged. The heating must be performed in stoneware or porcelain vessels, those made of any other materials being strongly corroded by the chlorine compounds formed.

When the evolution of fumes ceases, the mass is allowed to cool, and is carefully washed with a large volume of water, the residue being then dried and heated to redness. During this calcination the graphite undergoes a peculiar change, increasing considerably in bulk and forming an exceedingly soft powder which, after another washing, consists almost entirely of chemically pure carbon.