Abstract: A dump bailer is disclosed herein comprising a tool body defining a chamber for containing a material to be deposited and an outlet in the tool body through which the material can be deposited. The dump bailer also comprises a piston assembly slideably mounted in the chamber and comprising a swabbing piston, a supply of pressurized fluid, and a valve for releasing the pressurized fluid to act on the swabbing piston to drive it along the chamber to expel material contained therein through the outlet.