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Ohinemuri Regional History Journal 5, May 1966

By G.H. WORTH

Before the introduction of the present easily handled explosives the hard rock miners and others breaking rock in those days must have found the explosives and methods used very slow and tedious.

My history is not good enough to tell me when gunpowder was first introduced, but it was many centuries ago and must have been one of the first explosives used. It was not a great step from gunpowder to blasting powder, a much more effective explosive still much in use. Much milder and slower in action than the present day Nitro-Glycerine explosive it is mostly used in the splitting of timber or anywhere else a milder explosive is needed to do the job. It is a contact explosive, that is, it explodes when contacted by a spark or flame of any kind. How it was fired before the invention of Safety Fuse I do not know, but once Safety Fuse was used it became a simple matter to fire a hole. The fuse was simply put down the hole until it contacted the powder, made airtight with clay and lit and as soon as the fuse burned down to the powder the explosion occurred.

NITRO-GLYCERINE EXPLOSIVES

It was not until Nitro-Glycerine came into use as an explosive that hard rock men found something reasonably quick to use that would break rock effectively and cheaply. I think the first Nitro-Glycerine explosive was Dynamite. This was superseded later by Gelignite, much the same as Dynamite but with less fumes and after effects. The three main explosives using Nitro-Glycerine as a base are Gelignite, Gelatine-Dynamite and Gelatine. Gelignite contains sixty percent Nitro-Glycerine and is suitable for ninety-five per cent of rock met with in mining or tunnelling. If a stronger explosive is required then Gelatine-Dynamite is used, this being 65 per cent Nitro-Glycerine. In still harder rock and in reefs containing Rhodenite then Gelatine is used. This is a 75 per cent Nitro-Glycerine product and is the strongest explosive used except pure Nitro-Glycerine. In Gelignite, etc., the Nitro-Glycerine is combined with other materials to make a stick or plug as it is generally called, of a firm yet pliable substance easily handled. It is generally made in two sizes, one a plug about 6 inches long by ½ an inch diameter for hand steel holes and the other about 8 or 9 inches long by an inch and one-eighth in diameter for machine holes. Now, Nitro-Glycerine explosives are not contact explosives and are exploded by shock or force; actually a small explosion to explode a much bigger explosion. This force is called a detonating force and the agent responsible for it is a high explosive called Fulminate of Mercury. This is contained in a copper or aluminium capsule, generally called a detonator or "cap". This cap is about one inch and a quarter long by ¼ inch diameter and contains about ½ inch of Fulminate of Mercury. The balance of the cap is where the safety fuse is placed and crimped on to hold it firm. It is now ready to be fired. A hole is made in the end of a plug and the cap inserted.

METHODS OF FIRING

The miner or tunneller having bored the number of holes required now proceeds to load them. Gelignite or other explosive being used is pushed up to the end of the hole. The number of plugs being at the discretion of the man loading the hole as miners are very good judges as a rule of what is required to break the rock. This loading of holes is all done with wooden tamping rods; the use of metal rods is illegal as likely to cause a spark and a premature explosion. The last plug to go up the bore hole contains the detonator with fuse attached.

It is pushed up until it meets the charge and is then made airtight, with clay tamping. The hole is now ready for firing. The length of fuse used is governed by three things: Firstly, the depth of the hole, secondly, the number of holes, and thirdly, the time required by the men to reach a point of safety. As safety fuse burns at about thirty seconds to the foot a reasonable time can generally be estimated to govern these three conditions. The fuses are generally lit so the holes can be rotated and counted. The other method of firing a round of holes is by electricity. The detonator in this case is practically the same in principle as a fuse detonator except that it is airtight with two wires attached which connect with the Fulminate of Mercury. The procedure here is to load the holes as in any other operation, but when the holes have been loaded, each hole is connected to the next to form an electric circuit. This circuit is not completed until connected to an Electric battery. When the plunger of the battery is pushed down this completes the circuit and an instantaneous explosion takes place. Firing cannot be rotated when a battery is being used to explode the charges. This method of firing charges is especially useful in shaft-sinking and in wet places.

OTHER EXPLOSIVES

There are a number of low powered explosives on the market. These are generally for use in places where a very light charge is required. My only experience of any of these was at Arapuni in the sinking of the valve shaft for the tunnel gates. This was a huge shaft forty-one feet long by 9 feet wide. It was in very unstable ground and the management asked us to try out one or two of these explosives to reduce vibration to a minimum. We found it unsatisfactory for breaking ground, and it also left a very heavy concentration of fumes and it was not long before we had to revert to the use of Gelignite.

SAFE HANDLING OF EXPLOSIVES

Handled with the care and respect that is due to them all Nitro-Glycerine explosives are safe. They are at their most dangerous at the two extremes of heat and cold. Heated until they are volatile or nearly so they are extremely dangerous and there have been numerous cases of explosions taking place under these conditions. Mrs. Innis gave an example of what could happen in a recent Journal. One of the worst of these explosions happened at Waiorongomai. The miners, I believe, were in the habit of warming the Gelignite in the barrel of water used by the blacksmith to cool steel. The result was that the water became impregnated with Nitro-Glycerine and, one day when the blacksmith plunged a hot steel in the barrel, there was a terrific explosion, demolishing the smithy and killing several men. I have had no experience of frozen Gelignite but I have always understood that it is very dangerous to handle it, and a plug can be exploded by merely snapping it in half.

METHODS OF STORING EXPLOSIVES

Explosives are generally stored in a brick or concrete building called a magazine. The Waihi Company used huge quantities of explosives and stored them in a magazine in the east end of the town with a hill at the back so, in the case of an explosion the town was more or less immune. They also had a smaller magazine near No. 4 shaft which held 3 or 4 day's supply. From here it was distributed to the different shafts and levels where required. Once underground it was taken by the contractors to private magazines. These were generally situated in the end of a disused drive or crosscut, where the conditions were dry and the temperature more or less even, so the explosive was generally in a perfect condition for using.