Gunpowder
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If one refers to the word chemistry, images of little men in white labcoats poring over pages and pages of notes while a test tubes bubbles away, then suddenly an idea pops into their mind. This is the biased image of chemistry. In the outside world, it is so much more than simply test tubes and beakers. Chemistry IS our world. Take for instance gunpowder. It is used frequently, yet people do not realise that there is a practical application of chemistry involved throughout the extraction, processing, refining and detonation of it.
The first recorded explosive was “black powder”, which consists of a mixture of saltpeter (potassium nitrate), sulfur, and charcoal. When prepared in roughly the correct proportions (75 percent saltpeter, 14 percent charcoal, and 11 percent sulfur), it burns rapidly when ignited and produces approximately 40 percent gaseous and 60 percent solid products, the latter mostly appearing as whitish smoke. The saltpeter was originally extracted from compost piles and animal wastes .
Deposits found in India provided a source for this explosive substance for many years. During the 1850s tremendous quantities of sodium nitrate were discovered in Chile, and saltpeter was formed by reaction with potassium chloride, of which there was a plentiful supply:
Chilean nitrate was not at first considered satisfactory for the manufacture of black powder because it too readily absorbed moisture. Lammot du Pont, an American industrialist, solved this problem and started making sodium nitrate powder in 1858. It became popular in a short time because, although it did not produce as high a quality explosive as potassium nitrate, it was suitable for most mining and construction applications and was much less expensive. To distinguish between them, the potassium nitrate and sodium nitrate versions came to be known as A and B blasting powder respectively. The “A” powder continued in use for special purposes that required its higher quality, principally for firearms, military devices, and safety fuses .
In a confined space such as the breech of a gun, the aforementioned pent-up gas can be used for propelling a missile such as a bullet or artillery shell. Black powder is relatively insensitive to shock and friction and must be ignited by flame or heat. Though it has largely been supplanted by smokeless powder as a propellant for ammunition in guns, black powder is still widely used for ignition charges, primers, fuses, and blank-fire charges in military ammunition. With varied proportions of ingredients, it is also used in fireworks, time fuses, signals, squibs, and spatting charges for practice bombs.
Black powder is thought to have originated in China, where it was used in fireworks and signals by the 10th century. It is possible that the Chinese also used black powder in bombs for military purposes, and there is written record that in the mid-13th century they put it in bamboo tubes to propel stone projectiles. There is, however, some evidence that the Arabs were the first to develop black powder. By 1304 the Arabs had produced the first gun, a bamboo tube reinforced with iron that used a charge of black powder to shoot an arrow. Black powder was adopted for use in firearms in Europe from the 14th century but was not employed for peaceful purposes, such as mining and road building, until the late 17th century. There is a doubtful claim that it was used in mining operations in Germany in 1613 and fairly authentic evidence that it was employed in the mines of Schemnitz, Hungary (modern Banskб Stiavnica, Czechoslovakia), in 1627. For various reasons, such as high cost, lack of suitable boring implements, and fear of roof collapse, the use of black powder in mining did not spread rapidly, though it was widely accepted by 1700. The first application in civil engineering was in the Malpas Tunnel of the Canal du Midi in France in 1679. It remained a useful explosive for breaking up coal and rock deposits until the early 20th century, when it was gradually replaced by dynamite for most mining purposes .
A strong case can also be made that black powder was discovered by the English medieval scholar Roger Bacon, who wrote explicit instructions for its preparation in 1242, in the strange form of a Latin anagram, difficult to decipher. But Bacon read Arabic, and it is possible that he got his knowledge from Arabic sources.
The preparation of black powder from solid ingredients requires uniform mixing and blending of the saltpeter, charcoal, and sulfur. The earliest manufacturing processes used hand methods; the ingredients were simply ground together into a powder using a mortar and pestle. Beginning in the 15th century, water-driven crushing devices of wood, called wooden stamps, came into use to grind the ingredients, and power-driven metallic crushing devices replaced the wooden stamp mills in the 19th century. The following balanced equation shows the reaction of gunpowder when it is struck:
KNO3 + S + 3C —-> K2S + N2 + 3CO2
In the modern process, charcoal and sulfur are placed in a hollow drum along with heavy steel balls. As the drum rotates, the steel balls pulverize the contents; this device is called a ball mill. Heavy steel rollers crush the saltpeter separately. Next, a mixture of several hundred pounds of saltpeter, charcoal, and sulfur is placed in a heavy iron device shaped like a cooking pan. There it is continuously turned over by devices called plows, then ground and mixed by two rotating iron wheels, which weigh from 10 to 12 tons each. The process takes several hours; water is added periodically to keep the mixture moist.
The product of the mills is next put through wooden rolls to break up the larger lumps and is then formed into cakes under high pressure–namely, from about 210 to 280 kilograms per square centimetre (3,000 to 4,000 pounds per square inch) of pressure. Coarse-toothed rolls crack the cakes into manageable pieces and the corning mill, which contains rolls of several different dimensions, reduces them to the sizes desired.
Glazing (the next operation) consists of tumbling the grains for several hours in large wooden cylinders, during which friction rounds off the corners, and, aided by forced air circulation, brings the powder to a specified moisture content. The term glazing derives from the fact that graphite is added during this process, forming a thin film over the individual powder grains. Glazed powder flows more readily than unglazed powder and is more moisture resistant. After glazing, the powder is graded by sieves into different sizes and packaged, usually in kegs.
Because the burning of black powder is a surface phenomenon, a fine granulation burns faster than a coarse one.