Ammonium Nitrate has the chemical formula NH₄NO₃. Produced as small porous pellets, or “prills”, it’s one of the world’s most widely used fertilizers. It is also the main component in many types of rifle targets, where it’s mixed with fine aluminum and detonated by being hit with a high velocity round. Ammonium nitrate does not burn on its own. Instead, it acts as a source of oxygen that can accelerate the combustion (burning) of other materials.
Large masses of ammonium nitrate have been known to explode on some occasions although it is very insensitive. Smaller quantities are less likely to detonate. The risk of detonation increases when it is molten or mixed with fuels such as metal powders or organic substances. It should never be mixed with chlorates as this may result in ammonium chlorate formation, possibly leading to spontaneous ignition. Mixtures of metal powders and ammonium nitrate are likely to heat up spontaneously and may ignite, especially when moist.
It is very hygroscopic and therefore not used very often in fireworks. It finds some use in composite propellants, but performance is not as good as perchlorate-based propellants.