GUNPOWDER, an explosive substance formed by mixing saltpeter, charcoal and sul phur together. The mixture mayin com position between quite wide limits yetpos sess explosive properties; but the proportions adopted by the United States governmental au thorities are saltpeter, 75 per cent; charcoal, 15 per cent, and sulphur, 10 per cent. The salt peter used is the India saltpeter or nitre, which is known to chemists as potassium nitrate, and although found native as an incrustation on the soil in India it is to-day largely made from Chile saltpeter, or sodium nitrate, by reacting on the latter with potassium nitrate. It is care fully purified, finely ground and thoroughly dried for use, in the manufacture of gunpowder. The charcoal most suitable for gunpowder is that variety which is mostly readily ignited, most quickly burned and gives the least quan tity of ash. Such charcoal is produced from dogwood, willow or alder, by heating the air dried woods in closed iron cylinders or retorts out of contact with air so that they undergo destructive distillation and leave the charcoal as a residue, this method of manufacture hav ing been invented in England by Bishop Land lotf and adopted in that country in 1797. The dogwood, which is really the alder-buckthorn, Rhainnus Frangula, is cut when one inch in diameter; the willow and alder when four inches; though these dimensions vary in prac tice. The wood is cut in the spring when in full sap, stripped of its bark and seasoned by an exposure of two to three years; the dog wood being stacked under shelter, but the Other woods in the open so that the rain may wash the sap from the wood and the sun's rays and the air may destroy the spiral cells. The char ring is effected by fires outside the retorts or by passing superheated steam or hot carbon dioxide gas through the retorts. The char acter and yield of the charcoal produced varies with the temperature to which the wood is exposed and the time of exposure. When the wood is heated to 290° C. red charcoal is formed; when heated to 350° C. or above, black charcoal is produced. When the heating is done quickly the yield of charcoal is much larger than when the heating is slow. Red charcoal is much more easily ignited and burns faster than black charcoal. Charcoal for the manufacture of gunpowder is ground to a fine powder by rotation in a drum with a quantity of brass or bronze balls. Sulphur of commerce is purified for use in this manufacture by fusion and dis tillation; being eventually obtained in the form of roll brimstone, which is then crushed to a fine powder by heavy rollers. It must be free from sulphuric and sulphurous acids, as well as solid impurities, and should consist entirely of that modification of sulphur which is com pletely soluble in carbon disulphide.
The dry, finely ground and sifted saltpeter, charcoal and sulphur are weighed into the mix ing machine, which consists of a gun-metal drum arranged to make 40 revolutions a minute and provided with hollow bearings which a shaft is passed which carries 44 arms or fliers of such length as to just clear the interior surface of the drum. This shaft re volves in an opposite direction to and with twice the speed of the drum. After the ingre dients are put in the drum the mixing is car ried on for five minutes and then the mixture goes to the incorporating or wheel mill. The
process of incorporation is of the greatest im portance in this manufacture. It consists in the long continued grinding together of the ingre dients in order to mix them so intimately that the product appears to the naked eye as a homogeneous mass, for, unless this be done, complete reaction between the components of the powder by combustion cannot be expected. The finished gunpowder depends tor its excel lence largely upon the completeness and thor oughness of the incorporation. The incorpo rating mill consists of a circular bed of iron or stone on which the mixture is placed. A ver tical shaft rising through the centre of this bed carries a horizontal one, on the two ends of which heavy stone or iron wheels, called edge runners, are hung. These wheels rotate about the horizontal shaft and, as the vertical shaft revolves, they travel at the same time in a circle around the bed so that, at the points on the bed where the edge runners touch, the motions of rotation and translation are con verted into a twisting motion, like that of a muller, and the material beneath is thus over turned and very intimately mixed. The edge runners weight from three to seven tons, are from four to seven feet in diameter and are so movable on the spindle that they can accom modate themselves to varying thicknesses of powder on the bed. One of the edge runners is a little nearer the vertical shaft than the other, so that they travel in different paths and they are followed by a scraper which throws toward the centre of the bed the material that has been forced to the exterior by the edge runners. To incorporate, 50 pounds of the mixture are spread out on the mill-bed and slightly moistened and the wheels are set in motion. If the wheels are of stone weighing 31/2 tons and making 71/2 revolutions per minute, the incorporation is completed in hours. If the wheels are of iron weighing four tons and making eight revolutions per minute, 21/2 hours are required for cannon powder. The operator does not remain constantly in the mill, but goes in occasionally to wet the charge, from 2 to 10 pints of water being used in accordance with the weather. The chief dan ger from accidental explosions during the man ufacture of gunpowder is found in the incor porating mills; fortunately there is less ex plosive material here at any time than there is at any other part of the works. To render the damage done by an explosion as slight as pos sible, the buildings in which these operations are conducted are built with a strong frame work covered with light boards, or else with three sides of stone and the fourth and roof of light wood, so that when an explosion oc curs the framework or the stone walls remain. These mills are usually built in groups, and to prevent an explosion in one being communi cated to the others, each is provided with a drenching apparatus which automatically wets and protects the charges in the mills adjacent to the one which is blown up. The communi cation of fire or explosion is also arrested by means of barricades built about the mills which consist of masonry filled with earth, or simple earth mounds or sometimes wooden structures built in the shape of a letter A.