Ammonium nitrate is unstable at best and becomes more unstable as it ages.
The Grandcamp was loaded with 2200 tons of ammonium nitrate when it detonated in Texas City (1947)
The guys hand loading 100 lb bags said it was so hot that it about burned their arms as they carried it. The humidity is thought to have caused the hot bags.
It began to burn and made a spectacular show and many of the towns people came down to watch the show and were killed when it detonated. Those fighting the fire vanished.
The Grandcamp caused the High Flyer docked nearby to catch fire which detonated some hours later. Everyone had been pulled back and the only fatality was a priest that was giving last rights.
It is still listed as the worst industrial accident in US history.
“At 8:12 am , small rolls of kelly green smoke and orange smoke are shooting up from the ship...” City on Fire by
Bill Minutaglio pg 77 Texas City explosion 1947
It’s my understanding that given the right enviroment and over time this stuff will self-ignite.
If you want a good book on the Texas City disaster, read “City On Fire” by Bill Minutaglio.
An epic retelling of what happened that day.