The limit of temperature is 50° C. or thereabouts. Beyond this the jelly should never be allowed to go, and to 50° only under exceptional circumstances.
The tank in which the jelly is made is double-lined, in order to allow of the passage of hot water between its inner and outer linings. A series of such tanks are generally built in a wooden framework, and the double linings are made to communicate, so that the hot water can flow from one to the other consecutively. The temperature of the water should be about 60° C. if it is intended to gelatinise at 45° C., and about 80° if at 50° C.; but this point must, of course, be found by experiment for the particular plant used. An arrangement should be made to enable the workman to at once cut off the supply of hot water and pass cold water through the tanks in case the explosive becomes too hot.
[Illustration: FIG. 31.—MR M'ROBERTS' MIXER FOR GELATINE EXPLOSIVES.]
The best way to keep the temperature of the water constant is to have a large tank of water raised upon a platform, some 5 or 6 feet high, outside the building, which is automatically supplied with water, and into which steam is turned. A thermometer stuck through a piece of cork and floated upon the surface of the tank will give the means of regulating the temperature.
When the jelly in the tanks has become semi-transparent and the cotton has entirely dissolved, the mixture should be transferred to the mixing machine. The mixing machines are specially designed for this work, and are built in iron, with steel or bronze kneading- and mixing-blades, according to requirements.
A suitable machine for the purpose is that known as the Nito-Universal Incorporator, shown in Fig. 30, which has been specially constructed by Messrs Werner, Pfleiderer, & Perkins, Ltd., after many years' experience in the mixing of explosive materials, and is now almost exclusively adopted in both Government and private factories. Mr George M'Roberts'[A] mixing machine, however, which is shown in Fig. 31, is still used in some factories for dynamite jelly.
[Footnote A: See Jour. Soc. Chem. Ind., 1890, 267.]
If it is intended to make gelignite, or gelatine dynamite, it is at this point that the proper proportions of wood-pulp[A] and potassium nitrate should be added, and the whole well mixed for at least half an hour, until the various ingredients are thoroughly incorporated.
[Footnote A: Most of the wood-pulp used in England is obtained from pine-trees, but poplar, lime, birch, and beech wood are also used. It is chiefly imported as wood-pulp. The pulp is prepared as follows:—The bark and roots are first removed, and the logs then sawn into boards, from which the knots are removed. The pieces of wood are afterwards put through a machine which breaks them up into small pieces about an inch long, which are then crushed between rollers. These fragments are finally boiled with a solution of sodium bisulphite, under a pressure of about 90 lbs. per square inch, the duration of the boiling being from ten to twelve hours. Sulphurous acid has also been used. Pine-wood yields about 45 per cent. and birch about 40 per cent. of pulp when treated by this process. The pulp is afterwards bleached and washed, &c.
Birch. Beech. Lime. Pine. Poplar.
Cellulose 55.52 45.47 53.09 56.99 62.77 per cent.
Resin 1.14 0.41 3.93 0.97 1.37 "
Aqueous extract 2.65 2.47 3.56 1.26 2.88 "
Water 12.48 12.57 10.10 13.87 12.10 "
Lignine 28.21 39.14 29.32 26.91 20.88 ">[