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The mixture is affected by humidity because ammonium nitrate is highly hygroscopic. Ammonal's ease of detonation depends on fuel and oxidizer ratios, 95:5 ammonium nitrate and aluminum being fairly sensitive, however not very oxygen balanced. Even copper metal traces are known to sensitize bulk amounts of ammonium nitrate and further increase danger of spontaneous detonation during a fire, most likely due to the formation of tetramines. More oxygen balanced mixtures are not easily detonated, requiring a fairly substantial shock, though it remains more sensitive than trinitrotoluene and C-4.
The detonation velocity of ammonal is approximately 4,400 metres per second or 9,842 miles per hour.
From early 1916, the British Army employed ammonal for their mines during World War I, starting with the Hawthorn Ridge mine during the Battle of the Somme, and reaching a zenith in the mines in the Battle of Messines which were exploded on 7 June 1917 at the start of the Third Battle of Ypres (also known as the Battle of Passchendaele). Several of the mines in the Battle of Messines contained 30,000 lbs (over 13.6 tonnes) of ammonal, and others contained 20,000 lbs (over 9 tonnes). The joint explosion of the ammonal mines beneath the German lines at Messines created 19 large craters, killing 10,000 German soldiers in one of the largest non-nuclear explosions in history. Not all of the mines laid by the British Army at Messines were detonated, however. Two mines were not ignited in 1917 because they had been abandoned before the battle, and four were outside the area of the offensive. On 17 July 1955, a lightning strike set off one of these four latter mines. There were no human casualties, but one cow was killed. Another of the unused mines is believed to have been found in a location beneath a farmhouse, but no attempt has been made to remove it.
Ammonal used for military mining purposes was generally contained within metal cans or rubberised bags to prevent moisture ingress problems. The composition of ammonal used at Messines was 65% ammonium nitrate, 17% aluminium, 15% trinitrotoluene (TNT), and 3% charcoal. Ammonal remains in use as an industrial explosive. Typically, it is used for quarrying or mining purposes.
- ammonium nitrate 58.6%
- aluminum powder 21%
- charcoal 2.4%
- TNT 18%
- Tweedie, N. (2004-01-12). "Farmer who is sitting on a bomb". The Daily Telegraph. Retrieved 2010-05-17.
- Brown, G. I. (1998). The Big Bang: A History of Explosives. Sutton Publishing. p. 163. ISBN 0-7509-1878-0.
- Chamberlain, J. S. (1921). A Textbook of Organic Chemistry. Philadelphia: P. Blankiston's Son & Co. p. 534. OCLC 245530036. Retrieved 2010-05-17.
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