Abstract: The device comprises a moving assembly (24) designed to be placed in an operating position in which said assembly (24) is over the opening (12e) of a receptacle (12) filled with a viscous material, first motor means (32) and a first guide system (30a, 34a, 42) serve to impart vertical translation movement to said assembly (24) relative to said receptacle (12), and variable-speed, second motor means (48) and a second guide system (54) serve to impart rotary motion to a moving subassembly (26) belonging to said assembly (24). Said subassembly (26) has a vertical shaft whose top portion (46a) is secured to said second motor means (48), and a rule (22) connected to the bottom portion (46b) of said shaft, the length of the rule being substantially equal to the radius of said opening (12e) and the rule having a bottom surface presenting a horizontal longitudinal portion designed to level the top surface (14a) of the viscous material.