During direct extrusion, metal flow and forces required are affected by the friction between the work piece and the chamber walls. Particularly in hot working, oxide scale build up on the outer surfaces of the work piece can negatively influence the operation. For these reasons, it is common manufacturing practice to place a dummy block ahead of the ram. The dummy block is of slightly smaller diameter than the chamber and work piece. As the metal extrusion proceeds, the outermost surface of the work is not extruded and remains in the chamber. This material will form a thin shell, (called skull), that will latter be removed. Much of the skull will be comprised of the surface layer of oxidized scale from the work metal.