section_path: Granularity/Corning
section_name: Corning
paragraph_text: Corning
  
  For propellants to oxidize and burn rapidly and effectively, the combustible ingredients must be reduced to the smallest possible particle sizes, and be as thoroughly mixed as possible. Once mixed, however, for better results in a gun, makers discovered that the final product should be in the form of individual dense grains that spread the fire quickly from grain to grain, much as straw or twigs catch fire more quickly than a pile of sawdust.
  
  In late 14th century Europe and China, gunpowder was improved by wet grinding; liquid such as distilled spirits were added during the grinding-together of the ingredients and the moist paste dried afterwards. The principle of wet mixing to prevent the separation of dry ingredients, invented for gunpowder, is used today in the pharmaceutical industry. It was discovered that if the paste was rolled into balls before drying the resulting gunpowder absorbed less water from the air during storage and traveled better. The balls were then crushed in a mortar by the gunner immediately before use, with the old problem of uneven particle size and packing causing unpredictable results. If the right size particles were chosen, however, the result was a great improvement in power. Forming the damp paste into corn -sized clumps by hand or with the use of a sieve instead of larger balls produced a product after drying that loaded much better, as each tiny piece provided its own surrounding air space that allowed much more rapid combustion than a fine powder. This "corned" gunpowder was from 30% to 300% more powerful. An example is cited where 15 kilograms (34 lb) of serpentine was needed to shoot a 21-kilogram (47 lb) ball, but only 8.2 kilograms (18 lb) of corned powder.
  
  Because the dry powdered ingredients must be mixed and bonded together for extrusion and cut into grains to maintain the blend, size reduction and mixing is done while the ingredients are damp, usually with water. After 1800, instead of forming grains by hand or with sieves, the damp mill-cake was pressed in molds to increase its density and extract the liquid, forming press-cake. The pressing took varying amounts of time, depending on conditions such as atmospheric humidity. The hard, dense product was broken again into tiny pieces, which were separated with sieves to produce a uniform product for each purpose: coarse powders for cannons, finer grained powders for muskets, and the finest for small hand guns and priming. Inappropriately fine-grained powder often caused cannons to burst before the projectile could move down the barrel, due to the high initial spike in pressure. Mammoth powder with large grains, made for Rodman's 15-inch cannon, reduced the pressure to only 20 percent as high as ordinary cannon powder would have produced.
  
  In the mid-19th century, measurements were made determining that the burning rate within a grain of black powder (or a tightly packed mass) is about 6 cm/s (0.20 feet/s), while the rate of ignition propagation from grain to grain is around 9 m/s (30 feet/s), over two orders of magnitude faster.