Debris mortar

An air mortar (or debris mortar) is a device used to create movie special effects, consisting of pressurized air or gas forced through a tube to propel material (dirt, glass, fake blood) to create a bursting or splattering effect.

A cinema journal from 1941 describes the air mortar as "inaugurated" in the film Caught in the Draft: "this new development utilizes air blown under pressure through pipes, as well as powder and pyrotechnics, to gain unusual effects."