An explosion occurred inside a pyrotechnic unit devoted to producing fireworks while a subcontractor was engaged in renovation work. The vibrating bowl to be modified had been cleaned earlier by the site operator. The company assigned the job was performing arc welding at the time of the deflagration. A blast was felt along with the projection of material debris. One person sustained 2nd-degree burns on the thorax, hands and face, in addition to slight injuries to the eardrums. The facility was not damaged extensively. Following an investigation, it appeared that only the upper helicoid had undergone decontamination. The lower one, which proved less accessible, had not been cleaned. Pyrotechnic experts have recalled that in general, it is helpful to: systematically soak in water or an appropriate solvent any device submitted to an aggressive intervention; and ensure that all devices not permitting full disassembly for control and cleaning be devoid of traps capable of accumulating explosive products. When it becomes impossible to avoid hollow parts, such parts must be filled by an inert product (polymers).