Only two minerals are known that can be used as black earth colours, namely black chalk or shale black, and blacklead or graphite. Whereas the former of these is of merely subordinate importance, most of the black chalks being prepared artificially, graphite is all the more so because it is employed, not only as the sole material for lead pencils, but also for making graphite crucibles, as blacklead stove polish, as a lubricant, etc. One of its numerous applications is in connection with the electro deposition of metals, its high electrical conductivity causing it to be used for coating the interior of the moulds in which this deposition is effected.
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.