Instabilities during clay extrusion are investigated experimentally using transparent tubular dies of various lengths and diameters. The measurements include extrusion pressure and velocity (flow curves) supplemented by high speed flow visualization. Opposite to extrusion of molten plastics the clay is not sticking to the die's wall already at very low velocities, but is continuously slipping. Above a certain pressure the known decrease in the slope of the flow curve takes place ("spurt") and there is a strong evidence that it is caused by a detachment of the material from the die's wall. This detachment first reveals itself in small bubbles, which separate the product from the wall near the die exit. These bubbles rise in number and area until complete detachment of the product from the die surface is taking place. In this case periodical distortions could appear, caused by wall friction forces, leading to pendulum-like or helical movements of the product. At still higher flow rate the detachment seems to trigger another type of severe surface defect ("gross fracture"). In the detached area, small crumbs of material become visible, which are rolling between the detached product and the die surface in a chaotic way, printing furrows into the product surface. With further rise in flow rate the number and size of these crumbs increase and the product surface is getting severely damaged. Nozzle-like shaping and heating of the die lips could suppress the surface instabilities to a large extent within the investigated range of flow rates.