April 4, 1913. _______________________________________________________________________________________________________ THE COLLIERY GUARDIAN. 711 THE TESTING OF SAFETY EXPLOSIVES.* By Prof. Vivian B. Lewes, F.I.C., F.C.S., &c. Thirty years ago the so-called safety explosive was practically unknown, and gunpowder was the chief agent used by the miner both for coal and quarry work, whilst where greater power was needed dynamite was employed, but by 1890 the list of safety explosives had attained considerable dimensions. In 1896, the require- ments of the Coal Mines Regulation Act led to the erection of a testing gallery at Woolwich, copied from one that had been fitted up by the Institution of Mining Engineers in Northumberland, in which the liability of the explosives to ignite mixtures of coal gas and air could be determined, whilst their relative strength could be ascertained by firing into the ballistic pendulum. As far as I know, at the time the testing gallery at Woolwich was introduced, the only Continental installation of the kind was the gallery at Gelsenkirchen, where, under the auspices of Dr. Beyling, much good work was done, but later several other private testing galleries were set up, and the French, Austrian, Belgian, and American testing stations were erected. There were, however, great differences between the methods of procedure adopted, the chief being that in the Continental practice the explosives were fired unstemmed into the test mixtures, or even suspended in them, whilst at Woolwich stemming was used. For some time prior to 1912, vague rumours had been afloat amongst the manufacturers of safety explosives that alterations of a far-reaching character were about to be introduced in the official testing of explosives for inclusion in the Permitted List, that the testing station was no longer to be at Woolwich, but was to be moved to the coalfields, and that the tests were to be made so severe that only the best on the existing list would be likely to survive them. These rumours were confirmed by the issue, in May 1912, of a Home Office Memorandum, revising the test of explosives for inclusion in the Permitted List after March 31 of this year. The cause of this alteration is a praiseworthy desire on the part of the Home Office authorities, not only to bring our method of testing into line with that adopted by other countries, but also to render the explosives permitted for use so absolutely safe as to eliminate practically all danger from them in gassy and dusty mines, and as blown-out shots have proved the most frequent cause of ignition of gas or dust, they have adopted the procedure for the Rotherham tests of doing away with the tamping used in the Woolwich mortar, and of firing the charge direct into an explosive mixture of lighting-gas and air, and also into finely-divided coaldust suspended in air, the largest charge which fails to ignite either, mixture over a series of five shots being known as the “ maximum charge,” and this must not be less than 8 ozs. (226*8 grammes). There is not the least doubt but that this is a test so Chemistry at Berlin, that as every explosive when fired untamped into the test mixture of gas and air would cause ignition if the charge were only large enough, the largest charge which in a given number of shots failed to ignite the mixture should be called the charge limite. This idea met with general approval, as it afforded a method for arranging the explosives in order of danger, and although it by no means followed that when used in practice, that is to say, tamped, an explosive which failed to pass the test would lead to trouble in the mine, yet it did seem certain that one that passed the test with a high charge limite would not only be safer, but that if the charge limite was not exceeded even a blown- out shot in a gassy mine would not be likely to cause a disaster. In nearly all the early tests made to establish the charge limite on the Continent, coaldust was used suspended in the gaseous text mixture, but it was soon found that, contrary to expectation, it did not increase the sensitiveness of the mixture to explosion, and that a mixture of coaldust and air alone was often more sensitive than when gas was present. This led to the introduction of the method of using two test mixtures— the one an explosive mixture of pit-gas, coal-gas, arti- ficial methane or benzene vapour with air, and the second a mixture of coaldust and air, the charge limite of an explosive being the largest charge that in a definite number of shots failed to ignite either mixture. This procedure was adopted at Frameries ; and at the Inter- national Congress of Applied Chemistry, held in London in 1909, M. Watteyne communicated the results obtained by M. Bolle in re-testing the explosives already on the Belgian Permitted List for gassy mines. Attractive as was the idea of being able to fix a definite ratio of danger in the explosives, it was impossible to introduce the charge limite into England with the apparatus in use at Woolwich, as the gallery being only 2 ft. 6 in. in diameter the explosives could not be fired unstemmed, as a few grammes would have ignited the mixture, whilst they could not be fired in sufficient quantities if stemmed without destroying the test cannon. When, however, circumstances arose that necessitated the removal from Woolwich and remodelling of the test apparatus, it was decided that the Continental practice should be adopted, and explosives for the Permitted List should pass the tests with a “ maximum charge ” of not less than 8 oz. When this method of testing became general on the Continent, it was at once found that, although the results given at each individual gallery were fairly concordant, no two galleries agreed in the weight of the maximum charge assigned to various explosives, and that although the order of safety—or, if it be preferred, the degree of danger—was much the same, yet the conditions at some testing stations were so much more sensitive than at others that the charge limite at one might vary by 50 per cent, from that fixed at another- Area, square metre... Class. Wetter dynamite ... Carbonite ........ * Firedamp mixture. 0’28 0 95 2 Relative charge- limite. 0’46 0 46 1 072 0 94 1 See footnote. the nature of the products must play a very important part in the effect produced, the influence of the size of the gallery will depend largely upon the kind of explosive used. MM. Watteyne and Bolle, at Frameries, tried the effect of the area of the gallery, and found that the maximum charge was reduced very largely with some explosives, and but little affected with others. They used galleries having roughly the area of two square metres, one square metre, and quarter square metre. They experimented with several representatives of the chief classes of safety explosives, and found that a “ wetter dynamite ” showed no reduction in the maximum charge in dust with decrease in the size of the gallery, whilst under the same conditions a carbonite gave the greatest reduction. On the other hand, the wetter dynamite had its maximum charge reduced to one-half in gas, whilst the carbonite was reduced only by 28 per cent, in the smallest gallery. The figures obtained are of considerable interest:— Coaldust mixture. ... 0*28 095 2 Relative charge- limite. ... 1 1 1 ... 0 06 0 28 1 The difference is caused by the fact that the products of combustion of the wetter dynamite contain 45 per cent, of water vapour, which is very effectual in prevent- ing the ignition of coaldust, whilst the products of the carbonite contain only 7 per cent, of water vapour, giving probably less than any other explosive. It is clear from these experiments that the check on the free expansion of the products of combustion by diminution in the area of the gallery does not affect all classes of explosives alike, and although in erecting the Rotherham gallery it was a pity for the sake of uni- formity that the 2 square metres area was not adhered to, yet even if the old idea as to the effect of area had been correct, it would have been only 10 per cent, more sensitive than the Frameries gallery. At Rotherham the bore of the cannon is 4 ft. long by 12 in. in diameter, and the cartridges supplied have to be f in. in diameter, with an allowance of in.—that is to say, the cartridge will occupy half the sectional area of the bore. It is clear that an unstemmed shot under these conditions in no way represents a blown-out shot in practice. Beyling has pointed out that with a mortar having a bore of 2|in., i.e., -^in. larger than the Rother- ham cannon, an explosive of the class that gives off carbon monoxide on explosion, done up in 1g in. cart- ridges, would pass the test of firing unstemmed with a charge limite of 1,000 grammes, because the products expanded in the bore are cooled to such an extent that . when discharged into the mixtures neither gas nor dust is ignited, but that when the cartridges were fired in a narrow-bore cannon (1*57 in.), which they nearly fitted, severe that very few of the explosives up to the present on the Permitted List will pass it, and inasmuch as some have proved absolutely safe in practical working, it will be well to examine how far the new procedure really does attain the object in view, and whether in attempting to gain perfect safety the authorities are not running the risk of opening the door to still greater danger than now exists. At the outset it will be of interest to recall the inception of the “ unstemmed ” test, and to note that it originated, not with Government officials desirous of shelving responsibility for any accidents due to permitted explosives, but with the mineowners them- selves. The law in Germany is that only “ safety explosives ” shall be used in mines liable to gas or dust, and leaves the onus of finding out what a “safety explosive ” is to the mineowner ; in this way not only ridding themselves of responsibility, but ensuring the best expert knowledge being applied to solving the problem. The outcome of this was the erection of the Gelsenkirchen testing station, and at a later date the one at Grube Maria, both of which are under the auspices of the Berggewerkschaftskasse. It was soon realised that safety with an explosive is purely a relative term, and that, as it was impossible to reproduce in testing all the varied conditions of practical work in a mine, it was better to apply as stringent a test as possible so as to select the safest explosives from amongst the host that had sprung up, and so, at the iniative of the mineowners, the German practice arose of firing the explosive untamped into the gas mixture, whilst in the Austrian tests it was simply fired suspended in the mixture. In 1903, M. Watteyne, who was in charge of the then newly-erected Belgian official testing station, Frameries, suggested, at the International Congress of Applied * From a paper read before the Royal Society of Arts, April 2, 1913. Experiments soon showed that these variations were due to a large number of factors, some of which may not be known even yet, and that the shape and section of the gallery, the dimensions of the bore of the gun, the percentage and character of the gas used for making the mixture, the warming of the gallery by the sun, the composition, size, and freshness of the coaldust used, and even the atmospheric conditions, were all factors, apart from the explosive itself, that influenced the results obtained. The testing station at Frameries is under exactly the same official conditions as the new station at Rotherham, explosives being submitted for test by the manufacturers, and the composition, strength and charge limite of those that pass the test being published in the official reports, and it will be well to see in what respects the Rotherham procedure differs from the Frameries tests, and how far the results obtained from the latter can be taken as an indication of the probable results found at Rotherham. The dimensions of the testing gallery and bore of the cannon at the two stations are as follow;— Gallery. Sectional area .... Length ................... Material .......... Cannon. Length of bore.... Diameter........... Frameries. 2 sq. metres .... 30 metres ...... Wood.......... 46 cm. = 15 ft.... 55 cm. = 2 in. ... Much valuable work has been done Rotherham. 18. eq. metres (5 ft. diam.) 15 2 metres (50 ft.) Iron 4 ft. = 120 cm. 2 in. = 5 5 cm. on the effect of the diameter of the gallery, and its sectional area on the sensitiveness ot explosive mixtures, and the general view at first taken was that approximately it varied inversely with the sectional area—that is, a gallery of one square metre section was twice as sensitive as one of two square metres section. It is clear, however, that although this might be so, if the prevention of ignition of the text mixtures depended entirely upon the cooling of the products of combustion by expansion, yet that as 735 grammes ignited gas and 130 grammes ignited dust, because the carbon monoxide is blown out at a tempera- ture high enough to ignite on coming in contact with air, and then ignites the dust. That is to say, an explosive which would possibly pass the highest class at Rotherham might, with a blown-out shot in practice, with cartridges fitting the bore, give an explosion in a dusty mine with a charge of 130 grammes. It will be noticed that the bore of the Rotherham cannon is much longer than of those employed at Frameries, and it seems probable that this must give rise to differences, as it has been shown that increase in the length of the bore increases the length of the flash from the cannon, and at * Prof. Lewes, at this point in his paper, observed that the suggestion was that the smaller gallery served to check the expansion of the products of combustion by banking them up, thus retarding their cooling effect. There were other points, however, which he thought might be tested at Eskmeals. Thus, in certain of the photographic tests carried out in Austria by Col. Hess and Herr Alfred Siersch in 1895 (Trans. Inst.Min.Eng., vol. xi., p. 2), in which two similar explosive cartridges were suspended in the open on a copper wire, and simultaneously detonated, it was found that, using charges of wetter dynamite, a flash of light was formed midway between the cart- ridges. This was due, it was thought, to the products of combustion meeting and giving light and heat under compression, and as the products of combustion of wetter dynamite are rich in carbon monoxide, it was believed that the flash was due to the combustion of this gas. Similar tests, however, were carried out with roburite, an explosive that gives off no carbon monoxide, when the same luminous zone was reproduced on the photographic plate. He had come to the conclusion that this luminous zone was a physical rather than a chemical effect. In the case of both these explosives the rapidity of detonation is less than nitro-glycerine, and when two cartridges of gelatin- dynamit No. 1 were subjected to the same test, no trace was found of any intermediate zone of luminosity. One possible explanation was that the detonation was so rapid that it was impossible to overcome the weight of the atmosphere, so that the compression was thrown back upon the products of combustion, and the heating effects were confined to the locality of the explosion. In other words, the atmosphere played the part of tamping.