Resolving Ambiguity

There are two types of flour bomb:

• The relatively innocuous use of flour in a fragile container, thrown at a person or object to produce an inconvenient stain.
• The dangerous explosion that can occur when an air suspension of a fine organic, often starchy, substance is ignited.

Prank or protest

Flour in a fragile container thrown at an object or person of derision is a classic protest method, along the throwing of eggs and/or overripe tomatoes. A recent example was the use of purple-dyed flour in condoms thrown at the British Prime Minister in the chamber of the House of Commons by protestors campaigning for improved access rights to their children. The event highlighted the poor security methods employed in and around the Houses of Parliament.

The effect of flour bombs is made worse by the inclusion of eggs, or continers of other liquid, making the removal of the resultant mixture difficult.

Explosion Risk

The classic high school flour bomb demonstration shows the explosive power of flammable powders under the right circumstances. Given a large enough suspension of combustible flour or grain dust in the air, a significant explosion can occur. For example, the 1998 explosion of the DeBruce grain elevator in Wichita, Kansas which killed 7 people. Photos here http://www.wichita.gov/CityOffices/Fire/Photos/Gallery1.htm .

Almost any finely divided organic substance will produce an explosive mixture in an air suspension. For miltary use, so-called hyperbaric or thermobaric fuel-air bombs have been produced that work by initially creating a mist of jet-fuel (kerosene) or diesel, then igniting the mist. The resulting explosion not only destroys structures, but depletes the atmospheric oxygen in the immediate vicinity as this type of bomb does not use an explosive that contains its own oxidising agent. Hyperbaric bombs are designed to produce maximal overpressure, and hence structural damage; whereas thermobaric bombs are designed to produce a very high temperature in the zone of explosion/combustion.

Very fine flour is dangerous in air suspension as it too can explode. As the flour does contain a significant amount of oxygen in its organic make-up, the effect can be greater than a simple fuel-air explosion. This is a significant risk when milling grain to produce flour, so mills go to great lengths to remove sources of sparks. These include carefully sifting the grain before it is milled or ground to remove stones which could strike sparks from the millstones, and the use of magnets to remove metallic debris which can also strike sparks.

The earliest recorded flour explosion took place in an Italian mill in 1785, but there have been many since. The following two references give numbers of recorded flour and dust explosions in the USA in 1994 and 1997. In the ten year period up to and including 1997, there were 129 explosions.

http://www.oznet.ksu.edu/historicpublications/Pubs/1994Dust_Explosions.pdf

http://www.oznet.ksu.edu/historicpublications/Pubs/1997Dust_Explosions.pdf

The following reference gives a good overview of the science of dust explosions:

http://www.chemeng.ed.ac.uk/~emju49/SP2001/webpage/intro/intro.html