50 BULLETIN 681, U. S. DEPARTMENT OF AGEICULTURE. 



A rather curious explosion took place in a flour mill in Nebraska 

 on September 22, 1914, in which the entire wall on one side above 

 the second story was blown out. One end showed a bad bulge, 

 but the roof remained intact, except that it sagged over the side 

 from which the wall was blown out. Apparently the force of the 

 explosion was exerted in one direction, or, rather, the wall on this 

 side offered less resistance than the others. It is believed that the 

 dust in one of the flour bins was ignited by a match struck by one 

 of the workmen. 



On December 11, 1916, a large cereal mill in Ontario, Canada, 

 was totally destroyed by an explosion and subsequent fire. Tlie 

 fire was started by an explosion in the feed-grinding building, and 

 was ascribed to the ignition of feed dust in one of the grinding ma- 

 chines. 



One of the authors witnessed several dust explosions in flour 

 mills. Although these explosions were of minor importanoe, be- 

 cause no serious consequences followed, the observations tend to 

 give an idea as to the explosibility of flour dust, the conditions under 

 which explosions may take place, and, in some cases, the cause of 

 the initial explosion. 



At a certain mill the settfings of the dust room, into wliich was 

 blown the dust drawn from the middlings purifiers and from the 

 exhaust off the millstones, were gathered periodically into a bin and 

 ground on a smaU stone. This mixture was composed of very fuie 

 pieces of bran, fiber, larger pieces of endosperm, and a fine, impal- 

 pable, starchy, flour dust. Often, when the spout became nearly 

 empty and the pile above started down, the dust descended with 

 sufficient force to spurt out of the opening in the spout over the 

 hopper, creating a dense cloud of dust. On one occasion an open 

 light was close at hand when this dust fell, and an explosion fol- 

 lowed. There were three distinct explosions or flashes of the burn- 

 ing dust, one following the other at intervals of a few seconds. These 

 explosions were succeeded by a faint crackling sound. Each suc- 

 cessive flash was wider in extent as the dust spread and diffused. 



The first enveloped only the vicinity of the spouts and millstone; 

 the second, a still larger area; and the third, almost the entire area 

 of the grinding floor. The first zone, composed of the heaviest and 

 largest dust particles, burned the most slowly, as if partially smoth- 

 ered; the next, more rapidly; and the last and largest zone, composed 

 of the very lightest of the starchy particles in the state of greatest 

 diffusion, burned most rapidly, and, consequently, with the greatest 

 heat. Tliis last explosion had more force than the others, and was 

 more of the nature of an explosion than the other two. 



On another occasion some purified middlings were shaken down 

 from the side of an empty bin, and, coming in contact with the flame 



