Black Lead.

The Living Age 960, 25.10.1862

Since the failure of the blacklead mines at Borrowdale, Cumberland, the discovery of a new source of this valuable mineral has been a matter of considerable importance. The International Exhibition contains several magnificent specimens; the Siberian blacklead trophy recently erected in the nave, introduces an entirely new locality for the supply of graphite. The mineral in the trophy is carved and polished into a variety of shapes, so that it is somewhat difficult to judge of its quality from a mere inspection; but in the Siberian Court several blocks of the graphite may be seen just as they came from the mine, and may be thoroughly examined by those who take an interest in the subject. The mineral seems tolerably good, and occurs in considerable masses and veins a foot or eighteen inches in thickness. We are not acquainted with an exact analysis of it, but we believe it contains a somewhat large quantity of oxide of iron, which would materially diminish its commercial value. Whilst this Siberian graphite, owing to its prominent position, has been the subject of much comment, the equally fine specimens exhibited in the Canadian department have been passed over almost unnoticed. The quality of this, as far as can be judged from a mere inspection, appears to be very good, - it has a foliated texture, the laminae being flexible. The masses are very large, quite equal to those from Siberia, and altogether we think that these mines will prove a valuable addition to the already known sources of black lead. Several good specimens of graphite are also exhibited from Ceylon, India, and other places, but none equal the Siberian or Canadian mineral in magnitude and beauty. A fine collection of specimens of plumbago, from most of the known localities, is also shown by the Plumbago Crucible Company.

A most interesting and instructive series of specimens illustrating a new mode of treating and purifying graphite, is exhibited by the discoverer of the process, Dr. Brodie, professor of Chemistry in the University of Oxford. By this mode of treatment the commonest variety of graphite, which can be obtained plentifully, but is of very little value, can be converted into cakes equal to the best native varieties of the mineral. The coarse lumps, containing a large pro portion of oxide of iron, silica, and other gritty materials, are first finely powdered, and boiled in hydrochloric acid, to remove lime, part of the iron, and similar impurities. The next operation consists in beating the dried powder with a mixture of diluted sulphuric acid and chlorate of potash. This mixture has the property of evolving a considerable quantity of oxygen gas when it is heated, and the graphite enters into some sort of combination with this gas and the acid, the nature of which is, however, not very well understood. Professor Brodie shows specimens of this sulphuric acid compound; in appearance it is very similar to the coarsely powdered graphite, the lustre, however, being somewhat different. When this is heated in the dry state a remarkable change takes place; the gas which is intimately combined with the graphite is suddenly evolved, and tears the particles of the mineral asunder, swelling it up to twenty or thirty times its original volume, and reducing it to a most intimate state of division. The operation being almost parallel to that brought out some years ago by Claussen for treating flax, the fibres of which were blown out and disintegrated in a similar manner by the sudden liberation of carbonic acid in the pores, reducing it to a material similar to cotton. The disintegrated graphite is then shaken up with water, and the coarser particles, consisting of gritty matter, etc., quickly fall to the bottom of the liquid, the graphite remaining suspended. This is then poured off from the heavier particles, and the suspended graphite separated from it by filtration, or other means, and dried. In this form it presents the appearance of shrivelled up leaves, not unlike some of Dr. Hassell's tea. It has the color of black lead, but is quite devoid of lustre, and is excessively light, so much so that it is almost impossible to remove the cover from the jar without sending a cloud of the powder into the air. The original appearance of the graphite can, however, be restored to this light powder by pressure a portion squeezed between the thumb and finger immediately flakes into one mass, and the slightest friction communicates to it a brilliant lustre. The last of the series of bottles exhibited by the professor contains several solid lumps of graphite produced by squeezing the powder together under immense pressure. We should imagine from the appearance of them that they are not such favorable specimens as could be produced by forcing the particles of the powder together in some of the hydraulic presses specially constructed for this purpose, the air being at the same time exhausted from its pores. By this mode of treatment we have no doubt that blocks superior in quality to the finest native black lead could be obtained. The product may be considered as chemically pure carbon, and leaves no appreciable amount of ash on incineration. Professor Brodie's process has now been before the scientific world for some years, but we are not aware that it has yet been taken up commercially by any firm; this apathy on the part of our manufacturers is rather surprising, as the process seems to offer no practical difficulties, whilst the expense of converting an almost waste product into a very valuable substance is but trifling.