SHOT-FIRING IN COAL-MINES: AN IMPROVED METHOD.

Shot-firing or blasting in coal-mines is a subject which has for many years engaged the attention of mining experts and scientists, in consequence of the disastrous explosions which have so frequently resulted therefrom; but the discovering of an agent or the devising of a method by which the operation would be attended with perfect safety, has hitherto remained a problem too difficult to solve. At a very remote date in mining history, the use of explosives for blasting purposes was altogether unknown, and the various minerals, &c., were obtained from the bowels of the earth by means of hammer and wedge. Large quantities of these products were not then required, and the laborious and primitive method adopted for procuring them was fully equal to supplying the demand. But as time rolled on, mining produce became in much greater request, and means had to be devised which would enable mine-owners to meet the growing requirements of commerce and civilisation. Gunpowder was consequently utilised for this purpose, being first employed on the continent in 1620; and in the same year it was introduced into England as a blasting agent by some German miners brought over by Prince Rupert, and who employed it at the copper mine at Ecton in Staffordshire. Gradually it came into general use as a means of rapidly developing the mineral resources of the earth; and by its use, the output of our coal-mines has been increased by more than fifty per cent.

To its employment for obtaining coal, however, there were some great objections, both from a pecuniary and hygienic point of view. Large quantities of coal were converted into ‘slack,’ or a semi-pulverised state, in some cases to the extent of twenty-five per cent., and therefore great loss was sustained by the colliery proprietor, the marketable value of slack being very small. Again, the explosion of gunpowder is always attended with the formation of immense volumes of sulphuretted hydrogen, carbonic anhydride, and other gases, which are so deleterious to health, that, for a considerable space of time after a charge has been fired, the miners cannot work in that vicinity. Where large quantities of this substance are daily used, these noxious gases contaminate the air passing through the mine to such an extent that in the course of time they exercise an injurious effect on the health of the workmen.

Under these circumstances it was very desirable that other agents should be employed; but it is only within the last thirty years that other explosive substances have been submitted to mine-owners. The first of these was gun-cotton, which was invented by Professor Schönbein in 1846. It was not, however, until some years after its discovery that it came into use as a mining agent, such serious explosions attending its manufacture and storing, immediately after its introduction to the world, that no one would have more to do with so deadly an explosive. Eventually, however, it was ascertained how to render it safer, and it came into extensive use as a mining agent. Though it burns harmlessly away when simply ignited, yet, when fired by means of a detonator, as is done for mining purposes, it possesses some six times the explosive power of gunpowder; and its combustion in this way is so complete that no noxious gases are given off. It can be used either in the form of yarn or in a compressed block. When used in the former state, it is the opinion of many that its combustion is too rapid, and that it is thereby prevented doing its full amount of effective work. It bursts the minerals asunder with great force; but it lacks the cutting property which is essential to the performance of good work. The compressed cotton is free from these defects. It possesses all the force of yarn cotton; and in consequence of its slower combustion, it cuts in such a way as to make the block of mineral ready for the next charge. This latter is a great advantage to the workman, and hence the gun-cotton used for mining purposes is generally in a compressed state. By the use of this agent, mining of all descriptions was immensely facilitated, and the dangerous operation of ‘tamping,’ or filling the shot-hole with brick or coal dust rammed hard, was rendered unnecessary.

At a somewhat later period, nitro-glycerine attracted much attention, the first to attempt its use as an explosive agent being Alfred Nobel, a Swedish engineer, in 1864. So far as explosive power was concerned, it was all that could be desired, possessing ten times the force of gunpowder, and therefore being of nearly double the strength of gun-cotton. On the other hand, it was open to most serious objections. The danger of its exploding from concussion was very great, and many dreadful accidents have thus been caused by it. The liquid also, when poured into a shot-hole, has frequently run into some unknown crevice, and when fired, has produced an explosion under the very feet of the miners. To obviate this in some degree, cartridges have been employed; but in whatever light it is viewed, nitro-glycerine is a most perilous explosive.

To remove many of the dangers associated with the use of nitro-glycerine, particularly those of concussion, Mr Nobel invented dynamite, which was tried and approved as a mining agent at Merstham in 1868. When properly prepared, it constitutes one of the safest, most convenient, and most powerful explosives applicable to industrial purposes. It burns without explosion when placed in a fire or brought into contact with a lighted match. If struck with a hammer on an anvil, the portion struck takes fire without igniting the dynamite around it; and if packed with moderate care, it may be transported by road, railway, or canal with little danger of an explosion either from heat, sparks, friction, concussion, or collision. Such conditions of safety, however, entirely depend upon dynamite being properly made. If the Kieselguhr or porous infusorial earth, of which it contains about twenty-five per cent., be not properly dried and prepared, so as not only to absorb but to permanently hold in absorption the nitro-glycerine mixed with it, exudation is apt to take place; and if this only occurs to the extent of a thin greasy layer over the surface, there are present all the dangers of nitro-glycerine pure and simple. It is of a pasty consistence, and thus possesses the advantage that, whilst being very little less powerful as an explosive than nitro-glycerine, bore-holes can be filled with it without the dangers attending that liquid, and no cartridge case is required.

Since the introduction of dynamite, several other nitro-compounds have been brought forward as blasting agents, such, for instance, as dualine, lithofracteur, blasting gelatine, and gelatine-dynamite. With the exception of the two last named, however, they have not found much favour as mining agents in this country, and their use is mainly confined to the continent.

Whilst all the explosives mentioned in this article are more or less suited to blasting in mines, so far as their propulsive force is concerned, yet the use of each and all is attended with great danger in a coal-mine, and for the following reason: coal, being of vegetable or organic origin, is constantly giving off numerous gases, the most dangerous of which, under ordinary circumstances, is methylic hydride or marsh-gas, known in mining districts as fire-damp. It is of an inflammable nature; and when it becomes mixed with from seven to ten times its volume of air, it is highly explosive. It was the presence of this gas in coal-mines that gave rise to the researches of Humphry Davy and George Stephenson, and which resulted in the production of two kinds of safety-lamp, differing but little from each other in construction. As a mark of distinction for his invention, the first-named gentleman received the honour of knighthood. Explosive as is methylic hydride when mixed with air in the proportions stated, it becomes infinitely more so when the air contains a proportion of coal-dust. A very small percentage of fire-damp when mixed with air and coal-dust is sufficient to cause a disastrous explosion. In all dry coal-mines there is a considerable quantity of coal-dust (coal in a state of impalpable powder) lying about, and a certain proportion of it is always floating in the air through the workings of the mine. Now, when explosives are used, no matter how they are ignited, their combustion is always attended with the formation of a mass of flame, and consequently there is always great danger of an explosion of fire-damp taking place. Especially is this the case with gunpowder, which, requiring to be used in large quantities to produce the desired effect, is accompanied with much flame at the moment of its ignition. Gun-cotton being a much more powerful explosive than powder, can be used in far smaller proportions, and therefore to a certain extent possesses an advantage over it, inasmuch as its combustion is not attended with so great a mass of flame; thus to some extent, though only very slightly, reducing the danger of an explosion of fire-damp. In addition to showing flame at the moment of its ignition, dynamite possesses the drawback, that the Kieselguhr is liable to become incandescent, and whilst in this state, to be blown about by the force of the explosion of the blasting charge, and so fire any gas or mixture of gas and coal-dust which may be in the vicinity.

But great as is the danger always attending blasting in coal-mines, it becomes immeasurably greater in the case of a blown-out shot—that is, a shot which blows out the tamping, and does not bring down the coal—for the flame then issues unobstructed from the bore-hole, and extending for some distance, is free to ignite any inflammable mixture with which it may come in contact. To blown-out shots or charges is due the majority of colliery explosions. Before a shot is fired in a seam of coal, a portion of the latter is hewn away at the top to a depth of four or five feet, and is continued down one side, near the bore-hole, so as to decrease the resistance to be overcome by the explosive. If the shot-hole has been properly drilled, the blasting agent does its work; but if the hole has been drilled into the ‘fast’—that is, if it has been bored farther into the seam than the cavity produced by hewing out a portion of the coal extends—a blown-out shot is the result; for the charge of explosive is in such a case placed in the solid bed of coal, and the resistance, consequently, being too great to be overcome, the ramming with which the shot has been fixed in its place is forced out, an outlet being thus formed, through which the propulsive power of the explosive issues without bringing down any of the coal.

From what has been said, it will be seen that the great desideratum of mine-owners has been the discovery of an agent whose propulsive power could be utilised without any attendant flame, or the devising of a method by which the ordinary explosives could be rendered harmless in this respect—that is, that their flame could be extinguished at the moment of its formation. Mining experts, scientists, and others have for years been endeavouring to solve this problem, but without success. At last, however, an invention has been brought forward which leaves but little doubt that all difficulties have now been overcome, and that so soon as the appliance is in general use, colliery explosions resulting from shot-firing will be at an end, and the dreadful loss of life and limb with which they are too frequently attended will be a thing of the past.

The invention, which has been patented, is introduced by Mr Miles Settle, managing director of the Madeley Coal and Iron Company, Staffordshire. The explosive used is gelatine-dynamite (a chemical combination of gun-wood and nitro-glycerine), three ounces of which are equal in explosive power to a pound of gunpowder. It is of a straw colour, and about the consistence of soap. The design of the patent is to inclose the charge of gelatine-dynamite in a tin case or any other material, not necessarily waterproof, and to insert this in a larger case of oiled paper, india-rubber, tin, or anything that is waterproof. Projections from the sides and ends of the inner case keep it in such a position that when the outer vessel is filled with water, the cartridge case is completely surrounded with fluid. A detonator is fixed to the explosive, and this is in turn connected with a magneto-electric machine. When the outer case has been so secured as to prevent the escape of the water, the whole is inserted in the shot-hole, and is fixed there by ramming, as for an ordinary powder shot. The operator then retires to a safe distance and fires the charge by electricity. No flame accompanies its explosion, as at the moment of its formation it is extinguished by the water surrounding the cartridge. In addition to this, the water causes the gelatine-dynamite to exert its power equally in all directions, and it also absorbs the gases formed by the combustion of the explosive, so rendering it possible for men to commence working at the coal immediately after the discharging of the shot. Moreover, the coal dislodged by this method contains a minimum of slack, and there is therefore a great saving to the colliery proprietor in this respect.

The cartridge has recently been put to some very severe tests in some of the most fiery coal-mines in North Staffordshire; in fact, shots have been fired with this explosive in mines which are so gaseous that blasting is strictly prohibited in them, and the coal has to be obtained by the expensive and ancient method of hammer and wedge. In some of these fiery mines, blown-out shots have actually occurred; and all the experts who were present at the time expressed a unanimous opinion that had such a circumstance happened in the ordinary method of blasting, a disastrous explosion would inevitably have been the result. To prove the safety with which one of these cartridges can be fired, they have been exploded in bags of coal-dust, and not the slightest vestige of flame has attended their combustion. Gunpowder has been exploded under similar circumstances, with the result that the coal-dust instantly became ignited, and shot into the air for several yards like one sheet of flame.

All the experts who have witnessed the experiments, both on the surface and down in the mine, have expressed their perfect satisfaction with the invention in every way, and have stated their belief that it can be used with entire safety in the most fiery mines. The government Inspector of Mines for North Staffordshire, who has been present at some of the experiments, has announced that he is prepared to report to the Home Office that the appliance possesses the element of safety which is claimed for it.

A magneto-electric machine is used to fire the shot in preference to an electric battery, as the former is considered much the safer of the two. With a magneto-electric machine, the current, as is well known, is generated by friction, and it can therefore be broken simultaneously with the firing of the shot; whilst in an electric battery it is generated for the most part by means of strong acids, and cannot be broken without disconnecting one of the wires from the battery. It is just possible, therefore, that as the current is continuous in the last-named machine, the two wires might still remain so close together after the discharging of a shot as to allow a spark to pass between them, which in a very fiery mine would certainly cause an explosion.

Looking at the construction of Mr Settle’s patent and at the very severe tests to which it has been subjected, there seems every reason to believe that at last has been solved the difficult problem of shot-firing with safety in coal-mines, and that henceforth explosions arising from this cause will be unknown. Such disasters are among the most dreadful calamities which can overtake a community; and only those who have been eye-witnesses of the widespread sorrow and suffering they entail—whole villages and districts being in a moment plunged into mourning, and dozens of children rendered fatherless or orphans—can form an adequate idea of the boon which the ‘water-cartridge’ promises to be to the mining population. That the highest expectations concerning it may be fully realised, is the devout wish of all who are connected with the management or working of our collieries, and who are so frequently called upon to witness some of the saddest and most heartrending spectacles that it is possible for humanity to gaze upon.