On Saturday, September 4, 2016, in the German town of Nettetal, near Düsseldorf, during a neighborhood party, in the middle of a tent erected for the event, a petanque boule spontanously exploded. The explosion ripped a hole in the ceiling of the tent and left a small crater in the ground, but nobody was injured. A Düsseldorf bomb squad removed the remaining seven boules in the set and safely detonated them.
The investigation of this incident is still ongoing, but it recalls a similar incident that occurred in Switzerland in 2009. In that incident, the Swiss Federal Laboratory for Materials Science and Technology (EMPA) analyzed the remaining boules in the set in an effort to determine the cause of the explosion. The boules in question were cheap leisure boules manufactured by Dutch company Nebus BV. They had thin metal walls and were filled with a sand-like mixture (French news reports referred to it as “mortar”) to bring them up to proper weight. The sand was damp and contaminated with iron filings.
The moisture in the sand corroded the iron filings. Inside the sealed boules, in the absence of oxygen, that chemical reaction produced hydrogen gas, which caused high pressure inside the boules. The cheap boules had poorly welded seams. Under the pressure of the hydrogen gas, the seam of one of the boules failed and the boule exploded. It literally blew its top.
In the 2009 incident the set of boules was sitting on a shelf in its original packaging. In that incident, the Swiss department store chain Coop, which sold the sets, immediately stopped selling them and launched an aggressive recall campaign.
On the Wednesday following the explosion in Nettetal, the German federal government issued a press release warning the public of the danger of cheap boules. The German Petanque Federation (DPA) recommended buying certified boules and shopping for boules in specialty stores rather than in supermarkets.
For an interesting short video showing EMPA testing cheap boules, see THIS.
For EMPA diagrams comparing cheap leisure boules to competition boules, see THIS.
The EMPA engineering failure analysis, “Investigation into the mechanisms leading to explosion of pétanque balls”, is available HERE.