The cause of the accident was the blasting with dynamite of a solidified mass of ammonium sulfate nitrate compound fertiliser (a double salt of ammonium sulfate and nitrate in a mole ratio of 1:2). The stockpile was stored outside and solidified through the action of rainfall. The solid mass was loosened for shipping by blasting. This process had been practiced under supervision for many years, and about 30,000 blasting procedures had been carried out without major incident until the Oppau disaster. It is estimated that on that day about 4,500 tons of fertiliser was in the stockpile of which, it was estimated, only about 450 tons exploded. It is assumed that the last cartridge exploded in a region of the mass where the density was lower than usual, because of recent changes in the production, and where the ammonium nitrate concentration in the mixed salt was higher than usual. The important influence of these changes on the sensitivity of the product had not been realised. It was shown later that the sensitivity of the mixed salt to an explosion trigger increased rapidly with increasing ammonium nitrate concentration. This fact also explains why only part of the stockpile exploded.