Section III.—Appliances for Firing Blasting Charges.
In the foregoing sections, the machines and tools used in rock boring have been treated of. It now remains to describe those which are employed in firing the charges after they have been placed in the bore-holes. In this direction, too, great progress has been made in recent times. With the introduction of new explosive agents, arose the necessity for improved means of firing them. Attention being thus directed to the subject, its requirements were investigated and its conditions observed, the outcome being some important modifications of the old appliances and the introduction of others altogether new. Some of the improvements effected are scarcely less remarkable than the substitution of machine for hand boring.
Fig. 28.
Fig. 29.
The means by which the charge of explosive matter placed in the bore-hole is fired constitute a very important part of the set of appliances used in blasting. The conditions which any such means must fulfil are: (1) that it shall fire the charge with certainty; (2) that it shall allow the person whose duty it is to explode the charge to be at a safe distance away when the explosion takes place; (3) that it shall be practically suitable, and applicable to all situations; and (4) that it shall be obtainable at a low cost. To fulfil the second and most essential of these conditions, the means must be either slow in operation, or capable of being acted upon at a distance. The only known means possessing the latter quality is electricity. The application of electricity to this purpose is of recent date, and in consequence of the great advantages which it offers, its use is rapidly extending. The other means in common use are those which are slow in operation, and which allow thereby sufficient time to elapse between their ignition and the explosion of the charge for a person to retire to a safe distance. These means consist generally of a train of gunpowder so placed that the ignition of the particles must necessarily be gradual and slow. The old, and in some parts still employed, mode of constructing this train was as follows: An iron rod of small diameter and terminating in a point, called a “pricker,” was inserted into the charge and left in the bore-hole while the tamping was being rammed down. When this operation was completed, the pricker was withdrawn, leaving a hole through the tamping down to the charge. Into this hole, a straw, rush, quill, or some other like hollow substance filled with gunpowder, was inserted. A piece of slow-match was then attached to the upper end of this train, and lighted.
The combustion of the powder confined in the straw fired the charge, the time allowed by the slow burning of the match being sufficient to enable the man who ignited it to retire to a place of safety. This method of forming the train does not, however, satisfy all the conditions mentioned above. It is not readily applicable to all situations. Moreover, the use of the iron pricker may be a source of danger; the friction of this instrument against silicious substances in the sides of the bore-hole or in the tamping has in some instances occasioned accidental explosions. This danger is, however, very greatly lessened by the employment of copper or phosphor-bronze instead of iron for the prickers. But the method is defective in some other respects. With many kinds of tamping, there is a difficulty in keeping the hole open after the pricker is withdrawn till the straw can be inserted. When the holes are inclined upwards, besides this difficulty, another is occasioned by the liability of the powder constituting the train to run out on being ignited. And in wet situations, special provision has to be made to protect the trains. Moreover, the manufacture of these trains by the workmen is always a source of danger. Many of these defects in the system may, however, be removed by the employment of properly constructed trains. One of these trains or “squibs” is shown full size in [Fig. 28].
Safety Fuse.
—Many of the defects pertaining to the system were removed by the introduction of the fuse invented by W. Bickford, and known as “safety fuse.” The merits of this fuse, which is shown full size in [Fig. 29], are such as to render it one of the most perfect of the slow-action means that have yet been devised. The train of gunpowder is retained in this fuse, but the details of its arrangement are changed so as to fairly satisfy the conditions previously laid down as necessary. It consists of a flexible cord composed of a central core of fine gunpowder, surrounded by hempen yarns twisted up into a tube, and called the countering. An outer casing is made of different materials, according to the circumstances under which it is intended to be used. A central touch thread, or in some cases two threads, passes through the core of gunpowder. This fuse, which in external appearance resembles a piece of plain cord, is tolerably certain in its action; it may be used with equal facility in holes bored in any direction; it is capable of resisting considerable pressure without injury; it may be used without special means of protection in wet ground; and it may be transported from place to place without risk of damage.
In the safety fuse, the conditions of slow burning are fully satisfied, and certainty is in some measure provided for by the touch thread through the centre of the core. As the combustion of the core leaves, in the small space occupied by it, a carbonaceous residue, there is little or no passage left through the tamping by which the gases of the exploding charge may escape, as in the case of the squibs. Hence results an economy of force. Another advantage offered by the safety fuse is, that it may be made to carry the fire into the centre of the bursting charge if it be desired to produce rapid ignition. This fuse can be also very conveniently used for firing charges of compounds other than gunpowder, by fixing a detonating charge at the end of it, and dropping the latter into the charge of the compound. This means is usually adopted in firing the nitro-glycerine compounds, the detonating charge in such cases being generally contained within a metallic cap. In using this fuse, a sufficient length is cut off to reach from the charge to a distance of about an inch, or farther if necessary, beyond the mouth of the hole. One end is then untwisted to a height of about a quarter of an inch, and placed to that depth in the charge. The fuse being placed against the side of the bore-hole with the other end projecting beyond it, the tamping is put in, and the projecting end of the fuse slightly untwisted. The match may then be applied directly to this part. The rate of burning is about two and a half feet a minute.
Safety fuse is sold in coils of 24 feet in length. The price varies according to the quality, and the degree of protection afforded to the train.
Electric Fuses.
—The employment of electricity to fire the charge in blasting rock offers numerous and great advantages. The most important, perhaps, is the greatly increased effect of the explosions when the charges are fired simultaneously. But another advantage, of no small moment, lies in the security from accident which this means of firing gives. When electricity is used, not only may the charge be fired at the moment desired, after the workmen have retired to a place of safety, but the danger due to a misfire is altogether avoided. Further, the facility afforded by electricity for firing charges under water is a feature in this agent of very great practical importance. It would therefore seem, when all these advantages are taken into account, that electricity is destined to become of general application to blasting purposes in this country, as it is already in Germany and in America.
An electric fuse consists of a charge of an explosive compound suitably placed in the circuit of an electric current, which compound is of a character to be acted upon by the current in a manner and in a degree sufficient to produce explosion. The mode in which the current is made to act depends upon the nature of the source of the electricity. That which is generated by a machine is of high tension, but small in quantity; while that which is generated by a battery is, on the contrary, of low tension, but is large in quantity. Electricity of high tension is capable of leaping across a narrow break in the circuit, and advantage is taken of this property to place in the break an explosive compound sufficiently sensitive to be decomposed by the passage of the current. The electricity generated in a battery, though incapable of leaping across a break in the circuit, is in sufficient quantity to develop a high degree of heat. Advantage is taken of this property to fire an explosive compound by reducing the sectional area of the wire composing a portion of the circuit at a certain point, and surrounding this wire with the compound. It is obvious that any explosive compound may be fired in this way; but for the purpose of increasing the efficiency of the battery, preference is given to those compounds which ignite at a low temperature. Hence it will be observed that there are two kinds of electric fuses, namely, those which may be fired by means of a machine, and which are called “tension” fuses, and those which require a battery, and which are known as “quantity” fuses.
In the tension, or machine fuses, the circuit is interrupted within the fuse case, and the priming, as before remarked, is interposed in the break; the current, in leaping across the interval, passes through the priming. In the quantity or battery fuses, the reduction of the sectional area is effected by severing the conducting wire within the fuse case, and again joining the severed ends of the wire by soldering to them a short piece of very fine wire. Platinum wire, on account of its high resistance and low specific heat, is usually employed for this purpose. The priming composition is placed around this fine wire, which is heated to redness by the current as soon as the circuit is closed.
The advantages of high tension lie chiefly in the convenient form and ready action of the machines employed to excite the electricity. Being of small dimensions and weight, simple in construction, and not liable to get quickly out of order, these sources of electricity are particularly suitable for use in mining operations, especially when these operations are entrusted, as they usually are, to men of no scientific knowledge.
Another advantage of high tension is the small effect of line resistance upon the current, a consequence of which is that mines may be fired at long distances from the machine, and through iron wire of very small section. A disadvantage of high tension is the necessity for a perfect insulation of the wires.
When electricity of low tension is employed, the insulation of the wires needs not to be perfect, so that leakages arising from injury to the coating of the wire are not of great importance. In many cases, bare wires may be used. Other advantages of low tension are the ability to test the fuse at any moment by means of a weak current, and an almost absolute certainty of action. For this reason, it is usually preferred for torpedoes and important submarine work. On the other hand, the copper wires used must be of comparatively large section, and the influence of line resistance is so considerable that only a small number of shots can be fired simultaneously when the distance is great.
Fig. 30.
Fig. 31.
Fig. 32.
In [Fig. 30] is shown an external view of an electric tension fuse. It consists of a metal cap containing a detonating composition, upon the top of which is placed the priming to be ignited by the electric spark. The ends of two insulated wires project into this priming, which is fired by the passage of the spark from one of these wires to the other. The insulated wires are sufficiently long to reach a few inches beyond the bore-hole.
Sometimes the fuse is attached to the end of a stick, and the wires are affixed to the latter in the manner shown in [Fig. 31]. The rigidity of the stick allows the fuse to be readily pushed into the bore-hole. When the ground is not very wet, bare wires are, for cheapness, used, and the stick is in that case covered with oiled paper, or some other substance capable of resisting moisture. These “blasting sticks,” as they are called, are extensively used in Germany. When heavy tamping is employed, the stick is not suitable, by reason of the space which it occupies in the bore-hole.
A mode of insulating the wires, less expensive than the guttapercha shown in [Fig. 30], is illustrated in [Fig. 32]. In this case, the wires are cemented between strips of paper, and the whole is dipped into some resinous substance to protect it from water. These “ribbon” wires may be used in ground that is not very wet. They occupy little or no space in the bore-hole, and therefore are suitable for use with tamping.
To connect the fuses with the machine or the battery, two sets of wires are required when a single shot is fired, and three sets may be needed when two or more shots are fired simultaneously. Of these several sets of wires, the first consists of those which are attached to the fuses, and which, by reason of their being placed in the shot-hole, are called the “shot-hole wires.” Two shot-hole wires must be attached to each fuse, and they must be of such a length that, when the fuse has been placed in its proper position in the charge, the ends may project a few inches from the hole. These wires must also be “insulated,” that is, covered with a substance capable of preventing the escape of electricity.
The second set of wires consists of those which are employed to connect the charges one with another, and which, for this reason, are called “connecting wires.” In connecting the charges in single circuit, the end of one of the shot-hole wires of the first charge is left free, and the other wire is connected, by means of a piece of this connecting wire, to one of the shot-hole wires in the second hole; the other wire in this second hole is then connected, in the same manner, to one of the wires in the third hole; and so on till the last hole is reached, one shot-hole wire of which is left free, as in the first. Whenever the connecting wires can be kept from touching the rock, and also from coming into contact one with another—and in most cases this may be done—bare wire may be used, the cost of which is very little. But when this condition cannot be complied with, and, of course, when blasting in water, the connecting wires, like the shot-hole wires, must be insulated. When guttapercha shot-hole wires are used, it is best to have them sufficiently long to allow the ends projecting from one hole to reach those projecting from the next hole. This renders connecting wire unnecessary, and moreover saves one joint for each shot.
Fig. 33.
Fig. 34.
Cables.
—The third set of wires required consists of those used to connect the charges with the machine or the battery. These wires, which are called the “cables,” consist each of three or more strands of copper wire well insulated with guttapercha, or better, indiarubber, the coating of these materials being protected from injury by a sheathing of tape or of galvanized iron wire underlaid with hemp. Two cables are needed to complete the circuit; the one which is attached to the positive pole of the machine, that is, the pole through which the electric current passes out, is distinguished as the “leading cable,” and the other, which is attached to the negative pole, that is, the pole through which the current returns to the machine, is described as the return cable. Sometimes both the leading and the return cables are contained within one covering. When a cable having a metallic sheathing is used, the sheathing may be made to serve as a return cable, care being taken to make good metallic contact with the wires that connect the sheathing to the fuses and to the terminal of the machine. The best kind of unprotected cable consists of a three-strand tinned copper wire, each 0·035 inch in diameter, insulated with three layers of indiarubber to 0·22 inch diameter, and taped with indiarubber-saturated cotton to 0·24 inch diameter, as shown in [Fig. 33]. The best protected cable consists of a similar strand of copper wire, covered with guttapercha and tarred jute, and sheathed with fifteen galvanized iron wires of 0·08 inch diameter each, to a total diameter of 0·48 inch, as shown in [Fig. 34].
Detonators.
—The new explosives of the nitro-cotton and nitro-glycerine class cannot be effectively fired by means of safety or other fuse alone. To bring about their instantaneous decomposition, it is necessary to produce in their midst the explosion of some other substance. The force of this initial explosion causes the charge of gun-cotton, or dynamite, as the case may be, to detonate. It has been found that the explosion of the fulminate of mercury brings about this result most effectively and with the greatest certainty; and this substance is therefore generally used for the purpose. The charge of fulminate is contained in a copper capsule about a quarter of an inch in diameter, and from 1 inch to 11⁄4 inch in length. These caps, with their charge of fulminate, which are now well known to users of the nitro-compounds, are called “detonators.” It is of the highest importance that these detonators should contain a sufficiently strong charge to produce detonation, for if too weak, not only is the whole force of the explosive not developed, but a large quantity of noxious gas is generated. Gun-cotton requires a much stronger charge of fulminate than dynamite.
Fig. 35.
In the electric fuses [illustrated], the metal case shown is the detonator, the fuse being placed inside above the fulminate. When safety fuse is used, the end is cut off clean and inserted into the cap, which is then pressed tightly upon the fuse by means of a pair of nippers, as shown in [Fig. 35]. When water tamping is used, and when, with ordinary tamping, the hole is very wet, a little white-lead or grease must be put round the edge of the cap as a protection. The electric fuses are always made waterproof; consequently, they are ready for use under all circumstances. When the safety fuse burns down into the cap, or when, in the other case, the priming of the electric fuse is fired, the fulminate explodes and causes the detonation of the charge in which it is placed.
Firing Machines and Batteries.
—The electrical machines used for firing tension fuses are of two kinds. In one kind, the electricity is excited by friction, and stored in a condenser to be afterwards discharged by suitable means provided for the purpose. In the other kind, the electricity is excited by the motion of an armature before the poles of a magnet. The former kind are called “frictional electric” exploders; the latter kind are known as “magneto-electric” exploders. When a magneto-electric machine contains an electro-magnet instead of a permanent magnet, it is described as a “dynamo-electric exploder.”
Frictional machines act very well as exploders so long as they are kept in a proper state. But as they are injuriously affected by a moist atmosphere, and weaken rapidly with use by reason of the wearing away of the rubbers, it is necessary to take care that they be in good electrical condition before using them for firing. Unless this care be taken, the quantity of electricity excited by a given number of revolutions of the plate will be very variable, and vexatious failures will ensue. If, however, the proper precautions be observed, very certain and satisfactory results may be obtained. In Germany and in America, frictional exploders are generally used.
Magneto-electric machines possess the very valuable quality of constancy. They are unaffected, in any appreciable degree, by atmospheric changes, and they are not subject to wear. These qualities are of inestimable worth in an exploder used for ordinary blasting operations. Moreover, as they give electricity of a lower tension than the frictional machines, defects of insulation are less important. Of these machines, only the dynamo variety are suitable for industrial blasting. It is of primary importance that an exploder should possess great power. The mistake of using weak machines has done more than anything else to hinder the adoption of electrical firing in this country.
Fig. 36.
The machine most used in Germany is Bornhardt’s frictional exploder, shown in [Fig. 36]. This machine is contained in a wooden case 20 inches in length, 7 inches in breadth, and 14 inches in depth, outside measurement. The weight is about 20 lb.
To fire the charges by means of this exploder, the leading wire is attached to the upper terminal B, and the return to the lower terminal C, the other ends of these wires being connected to the fuses. The handle is then turned briskly from fifteen to thirty times, according to the number of the fuses and the state of the machine, to excite the electricity. The knob A is then pressed suddenly in, and the discharge takes place. To ascertain the condition of the machine, a scale of fifteen brass-headed nails is provided on the outside, which scale may be put in communication with the poles B and C by means of brass chains, as shown in the drawing. If after twelve or fourteen turns, the spark leaps the scale when the knob is pushed in, the machine is in a sufficiently good working condition. To give security to the men engaged, the handle is designed to be taken off when the machine is not in actual use; and the end of the machine into which the cable wires are led is made to close with a lid and lock, the key of which should be always in the possession of the man in charge of the firing operations.
Fig. 37.
Fig. 38.
In America, there are two frictional exploders in common use. One, shown in [Fig. 37], is the invention of H. Julian Smith. The apparatus is enclosed in a wooden case about 1 foot square and 6 inches in depth. The handle is on the top of the case, and is turned horizontally. This handle is removable, as in Bornhardt’s machine. The cable wires having been attached to the terminals, the handle is turned forward a certain number of times to excite the electricity, and then turned a quarter of a revolution backward to discharge the condenser and to fire the blast. By this device, the necessity for a second aperture of communication with the inside is avoided, an important point in frictional machines, which are so readily affected by moisture. The aperture through which the axis of the plate passes, upon which axis the handle is fixed, is tightly closed by a stuffing-box. A leathern strap on one end of the case allows the machine to be easily carried. The weight of this exploder is under 10 lb.
The other exploder used is that designed by G. Mowbray. This machine, which is shown in [Fig. 38], is contained in a wooden barrel-shaped case, and is known as the “powder-keg” exploder, the form and dimensions of the case being those of a powder-keg. The action is similar to that of the machine last described. The cable wires having been attached to the terminals at one end of the keg, the handle at the other end is turned forward to excite the electricity, and the condenser is discharged by making a quarter turn backward, as in Smith’s machine. The handle is in this case also removable. The weight of the powder-keg exploder is about 26 lb.
Both of these machines are very extensively used, and good results are obtained from them. They stand well in a damp atmosphere, and do not quickly get out of order from the wearing of the rubbers. They are also, especially the former, very easily portable.
Fig. 39.
The machine commonly used in England is the dynamo-electric exploder of the Messrs. Siemens. This machine, which is the best of its kind yet introduced for blasting purposes, is not more than half the size of Bornhardt’s frictional exploder; but it greatly exceeds the latter in weight, that of Siemens’ being about 55 lb. The apparatus, which is contained within the casing shown in [Fig. 39], consists of an ordinary Siemens’ armature, which is made, by turning the handle, to revolve between the poles of an electro-magnet. The coils of the electro-magnet are in circuit with the wire of the armature; the residual magnetism of the electro-magnet cores excites, at first, weak currents; these pass into the coils, thereby increasing the magnetism of the cores, and inducing still stronger currents in the armature wire, to the limit of magnetic saturation of the iron cores of the electro-magnets. By the automatic action of the machine, this powerful current is, at every second turn of the handle, sent into the cables leading to the fuses.
To fire this machine, the handle is turned gently till a click is heard from the inside, indicating that the handle is in the right position to start from. The cable wires are then attached to the terminals, and the handle is turned quickly, but steadily. At the completion of the second revolution, the current is sent off into line, as it is termed, that is, the current passes out through the cables and the fuses. As in the case of the frictional machines, the handle is, for safety, made removable. This exploder is practically unaffected by moisture, and it is not liable to get out of order from wear.
Induction coils have been used to fire tension fuses; but it is surprising that they have not been more extensively applied to that purpose. A coil designed for the work required of it is a very effective instrument. If constructed to give a spark not exceeding three inches in length, with comparatively thick wire for quantity, it makes a very powerful exploder. An objection to its use is the necessity for a battery. But a few bichromate of potash cells, provided with spiral springs to hold the zincs out of the liquid, and designed to be set in action by simply pressing down the zincs, give but little trouble, so that the objection is not a serious one. The writer has used an induction exploder in ordinary mining operations without experiencing any difficulty or inconvenience. It is cheap, easily portable, and constant in its action.
Batteries are used to fire what are known as “quantity” or “low tension” fuses. Any cells may be applied to this purpose; but they are not all equally suitable. A firing battery should require but little attention, and should remain in working order for a long time. These conditions are satisfactorily fulfilled by only two cells, namely, the Léclanché and the Bichromate of Potash. The latter is the more powerful, and generally the more suitable. The Léclanché is much used in this country for firing purposes, under the form known as the “Silvertown Firing Battery.” This battery consists of a rectangular teak box, containing ten cells. Two, or more, of these may be joined up together when great power is required. In France, the battery used generally for firing is the Bichromate. This battery is much more powerful than the Léclanché, and as no action goes on when the zincs are lifted out of the liquid, it is equally durable. It is moreover much cheaper. At the suggestion of the writer, Mr. Apps, of the Strand, London, has constructed a bichromate firing battery of very great power. It is contained in a box of smaller dimension than the 10-cell Silvertown. The firing is effected by simply lowering the zincs, which rise again automatically out of the liquid, so that there is no danger of the battery exhausting itself by continuous action in case of neglect. Externally, this battery, like the Silvertown, appears a simple rectangular box, so that no illustration is needed. With either of these, the usual objections urged against the employment of batteries, on the ground of the trouble involved in keeping them in order, and their liability to be injured by ignorant or careless handling, do not apply, or at least apply in only a very unimportant degree.
To guard against misfires, the machine or the battery used should be constructed to give a very powerful current. If this precaution be observed, and the number of fuses in circuit be limited to one-half that which the machine is capable of firing with a fair degree of certainty, perfectly satisfactory results may be obtained. The employment of weak machines and batteries leads inevitably to failure. In the minds of those who have hitherto tried electrical blasting in this country, there seems to be no notion of any relation existing between the work to be done and the force employed to do it. The electrical exploder is regarded as a sort of magic box that needs only to be set in action to produce any required result. Whenever failure ensues, the cause is unhesitatingly attributed to the fuses.