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.