Spark erosion has been used to produce flat compression plates with different surface finishes. By the compression of ring-specimens, the coefficient of friction has been found to depend in an approximately linear manner on surface finish, as expressed by the centre-line average (CLA), a proprietary zinc phosphate-soap system being used for lubrication. Spark-eroded dies give rise to an increase in extrusion pressures over those required for ground dies, the increases being greater as the CLA of the surface increases. Accompanying variations in metal flow as a result of increased friction are detectable from the external appearance of the extruded product.