A gas bubble agitated mixing system for viscous fluids comprising: a container (1) in which a viscous fluid (2) may be held; one or more gas pipes (6), each gas pipe having an outlet (8) arranged to introduce gas bubbles (9) into the base region of the container (1), said gas bubbles (9) being of a size suitable to agitate the viscous fluid (2). The system also comprises a moveable carrier arm (4) upon which said one or more gas pipe outlets (8) are mounted; and wherein the movement of the carrier arm (4) causes the position at which each outlet (8) introduces gas bubbles (9) into the container (1) to vary over time.