The plant began producing ammonium sulfate in 1911, but during World War I when Germany was unable to obtain the necessary sulfur, it began to produce ammonium nitrate as well. Ammonia could be produced without overseas resources, using the Haber process. Compared to ammonium sulfate, ammonium nitrate is strongly hygroscopic, so the mixture of ammonium sulfate and nitrate clogged together under the pressure of its own weight, turning it into a plaster-like substance in the 20 m high silo. The workers needed to use pickaxes to get it out, a problematic situation because they could not enter the silo and risk being buried in collapsing fertilizer. To ease their work, small charges of dynamite were used to loosen the mixture. This apparently suicidal procedure was in fact common practice. It was well known that ammonium nitrate was explosive – it had been used extensively as such during World War I – but tests conducted in 1919 had seemed to indicate that mixtures of ammonium sulfate and nitrate containing less than 60% nitrate were unlikely to explode. On such grounds, the material handled by the plant, nominally a 50/50 mixture, was considered stable enough to be stored in 50,000-tonne lots – more than ten times the amount involved in the disaster. Indeed, nothing extraordinary happened during an estimated 20,000 firings, until the fateful explosion on September 21. As all involved died in the explosion, the causes are not clear. However, it is known from work more recent than the above-mentioned 1919 tests that the "less than 60% nitrate = safe" criterion is inaccurate; in mixtures containing 50% nitrate, any explosion of the mixture is confined to a small volume around the initiating charge, but increasing the proportion of nitrate to 55–60% significantly enhances the explosive properties and creates a mixture whose detonation is sufficiently powerful to initiate detonation in a surrounding mixture of a lower nitrate concentration which would normally be considered minimally explosive. Changes in humidity and density also significantly affect the explosive properties. A few months before the incident, the manufacturing process had been changed in such a way as to lower the humidity level of the mixture from 3-4% to 2%, and also to lower the apparent density. Both these factors rendered it more likely to explode. There is also evidence that the lot of mixture in question was not of uniform composition and may have contained pockets of up to several dozen tonnes of mixture enriched in ammonium nitrate. The explanation is therefore proposed that one of the charges was by chance placed in such a pocket, which exploded with sufficient violence to set off some of the surrounding lower-nitrate mixture. Two months earlier, at Kriewald, then part of Germany, 19 people had died when 30 tonnes of ammonium nitrate were detonated by people doing the same thing. It is not clear why this warning was not heeded.
Scale of the explosion
Two explosions, half a second apart, occurred at 7:32 am on September 21, 1921, at Silo 110 of the plant, forming a crater 90 m by 125 m wide and 19 m deep. The explosions were heard as two loud bangs in north-eastern France and in Munich, more than 300 km away, and are estimated to have contained an energy of 1-2 kilotonnesTNT equivalent. The damage to property was valued in 1922 at 321 million Marks, estimated by The New York Times at the time to be equivalent to 7 million US dollars . About 80 percent of all buildings in Oppau were destroyed, leaving 6,500 homeless. The pressure wave caused great damage in Mannheim, located just across the Rhine, ripped roofs off up to 25 km away and destroyed windows farther away, including all the medieval stained-glass windows of Wormscathedral, 15 km to the north. In Heidelberg, traffic was stopped by the mass of broken glass on the streets, a tramway was derailed and there were even some roofs ripped off. Five hundred bodies were recovered within the first 48 hours, with the final death toll recorded being in excess of 560 people. The funeral was attended by German President Friedrich Ebert and Prime MinisterHugo Lerchenfeld, and saw crowds of 70,000 people at the cemetery in Ludwigshafen. According to some descriptions, only 10% of the 4,500 tonnes of fertilizer stored in the silo detonated in the incident.
