From forging the workpiece so the workpiece very hot temperatures of up to 180-210° c and when the pieces were in place, the work piece work piece surface area contact. The heat exchange occurs, the cause of black point.
From hitting the wall molding. The specimen is very hot The high temperature of 180-210 degrees Celsius, and when the pieces are put in the work to make the surface area in contact with. The heat causes black spots.
From hitting molding. Specimens is very hot, with temperatures up to 180-210 degrees Celsius. And when the target is in the area of workpiece box surface contact. The heat exchange caused a black spot.