Tempering apparatus having disc-free stirring tool (8) with plate-shaped arms (12, 13). The upper side (14) of each arm (12) has a different geometrical configuration than the lower side (19) thereof. Each neighbouring arm (13) has the opposite configuration of the upper and lower sides (23 and 24). The mass is thereby given an intermittent up- and downwards movement in the mass treatment chamber (2) and the tempering capacity of the apparatus is improved.