Graphite
Graphite, also known as plumbago or blacklead, consists of carbon. It is usually spoken of as pure carbon, but from a very large number of carefully conducted analyses, it would appear that native graphite is never quite pure, even the finest grades of the mineral containing 96·8% of carbon at the most. The accompanying substances—which in some cases form nearly 50% of the whole—are of divergent composition and consist of iron, silica, lime, magnesia and alkalis. Even the combustible constituent of graphite is not pure carbon, but always contains a certain—though small—proportion of volatile substances. These slight traces of volatile matter are of considerable importance in connection with the hypothesis on the origin of the mineral.
Contrary to the old idea, it is now almost universally considered that, instead of being of volcanic origin, graphite consists of the remains of long-dead organisms, and in this respect is closely related to coal. This hypothesis, however, fails to explain one point, namely the crystalline nature of graphite; for even anthracites, which form the oldest coals known to have had their origin in the decomposition of organic substances, do not reveal the faintest traces of crystalline structure. The upholders of the theory that graphite was formed by the action of plutonic forces adduce, in support, the fact that graphite can actually be produced, in certain chemical processes, at high temperature. Molten cast-iron in cooling causes the separation of carbon in the form of graphite; and the same substance is also formed, in large quantities, in gas retorts, through the decomposition of certain carbonaceous compounds when brought into contact with the glowing walls of the retorts. Recent investigations, however, have shown that the temperature necessary for the transformation of non-crystalline carbon into crystalline graphite is by no means so high as was formerly supposed; and it is now known that the change takes place at as low as red heat. Possibly the two theories could be reconciled by the assumption of a very old coal—such as is found, for instance, as anthracite in many parts of the world—being so strongly heated, by plutonic action, as to change into graphite.
Native graphite crystallises in the form of hexagons, mostly tabular; but really well-developed crystals are of extremely rare occurrence, and by far the greatest quantities of this mineral are found in the condition of dense lumps, in which only the crystalline structure, and not any decided crystals, can be discerned. The hardness of the mineral fluctuates within fairly wide limits, ranging from 0·5 to 1·0. The sp. gr. averages 1·8018–1·844, but, in the case of impure lumps may increase to 1·9–2·2.
The following analyses will give some idea of the considerable divergence existing between graphites from different deposits:—
| Siberian Graphite | ||
| 1 | 2 | |
| Carbon | 94·28 | 40·55 |
| Ash | 5·72 | 56·56 |
| Water | — | 2·80 |
| Portuguese Graphite | ||
| Carbon | 42·69 | |
| Water (chemically combined) | 3·96 | |
| Ash | 53·35 | |
| Bohemian Graphite | ||
| 1 | 2 | |
| Carbon | 61·01 | 69·04 |
| Alumina | 7·80 | 6·86 |
| Silica | 17·34 | 14·18 |
| Magnesia | 1·03 | 0·53 |
| Lime | 2·56 | 0·80 |
| Ferric oxide | 5·54 | 4·00 |
| Potash | 0·87 | 0·91 |
| Water and volatiles | 3·24 | 2·89 |
| Sulphur | 0·51 | 0·62 |
| Graphites from Upper Styria | |||
| 1 | 2 | 3 | |
| Carbon | 85·00 | 87·16 | 82·21 |
| Ash | 14·89 | 12·66 | 17·92 |
| 4 | 5 | 6 | |
| Carbon | 82·40 | 81·10 | 55·50 |
| Silica | 12·38 | 11·61 | 21·00 |
| Alumina | 3·90 | 5·60 | 14·56 |
| Ferric oxide | 0·53 | ||
| Manganese protoperoxide | 0·62 | 2·00 | 4·84 |
| Lime | 0·02 | 2·00 | 4·84 |
| Alkalis | Trace | Trace | 0·62 |
| Sulphur | — | — | 0·30 |
| Loss on incineration | — | — | 2·43 |
Of these Styrian specimens, Nos. 1–4 are crude kinds, of sp. gr. 2·1443; No. 5 was levigated in the laboratory, and No. 6 was levigated from an inferior quality at the mine.
According to the character of the crystalline structure, the colour of graphite varies, but is mostly deep black. Very pure specimens, such as the beautiful graphite blocks (from the renowned Alibert graphite mines in Siberia) which, as a rule, are only to be seen in exhibitions and mineralogical collections, have the appearance of unpolished steel or white pig iron (spiegeleisen). The most important property of native graphite is its low hardness and cohesion, in consequence of which it leaves a streak when drawn over the surface of paper.
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.
Graphite purified in this way can be used for any purpose for which this material is employed, and may be made into the finest lead pencils. However, as already mentioned, this process is usually too expensive for general application.
The use of graphite for writing is more ancient than is usually supposed, having been tentatively employed between 1540 and 1560. It was during this period that the graphite mines in Cumberland were discovered; and the extremely pure graphite found there soon began to be used as a writing material.
Up to the close of the eighteenth century, lead pencils were made by selecting pure lumps of graphite and sawing them into thin rods, which were then encased in wooden sticks. Apart from their high price, these pencils exhibited various defects, one of the chief being that a stick of such pencil was seldom of uniform hardness throughout its length, most of them being so soft in parts as to make a deep black, smeary mark, whilst other parts would hardly give any mark at all.
The defects inherent in native graphite are completely removed by the method now generally employed in making lead pencils; and on this account the old process of sawing the lumps has been abandoned.
Graphite with a fine earthy texture alone is suitable for lead pencils, scaly varieties being useless for this purpose, unless specially prepared, since they will not give a solid black streak. By means of the Brodie process, however, even the most highly crystalline kinds can be rendered suitable for this purpose. Siberian graphite is distinguished by extremely high covering power, and is specially preferred for the manufacture of pencils. Excellent varieties for this purpose are also found in many parts of Europe; and indeed, a large proportion of all the lead pencils used throughout the world are made from Bohemian, Styrian and Bavarian graphite.
At present, all pencils are made from ground graphite, the extremely finely ground and levigated material being kneaded into a paste with clay. This operation fulfils a twofold purpose, the plasticity of the clay increasing the cohesion of the individual particles of graphite, whilst the amount of clay used determines the hardness of the pencil.
The larger the proportion of clay, the harder the pencil when baked, and therefore the paler the mark the pencil will make on paper. In the pencil factories, the clay is incorporated in special machines; and the operation requires extreme care, since only a perfectly uniform mixture will give a composition of regular character in all cases.
The intimately mixed material is formed into thin rods, which are dried and then baked, the heat driving out the water in the clay and transforming it into a solid mass.
An addition to this main application of graphite, the mineral is also used for making crucibles, chiefly for melting the noble metals. Crucibles of this kind are largely manufactured near Passau, Bavaria, and similar crucibles are made in England from Ceylon graphite.
Another important use for graphite is as a coating for iron articles to protect them from rust. For this purpose, however, only the inferior kinds are employed; and these can also be made up into excellent cements capable, in particular, of offering considerable resistance to the action of heat and chemicals.
To complete the tale of the applications of graphite, its employment as a lubricating agent for machinery, especially for reducing friction in machines made of wood, may be mentioned. Latterly also, the finest levigated graphite has come into use, in admixture with solid fats or mineral oils, for lubricating large engines, for which purpose it is excellently adapted.