Cast-off Blood

A moving source of blood, such as a bleeding victim or a blood-stained weapon, can give rise to cast-off blood—that is, droplets of blood flung from the object so as to make a trail of blood where it lands. Such bloodstain patterns can be very informative about the nature of an attack. Forensic scientists distinguish two types of cast-off blood dependent on the kind of movement producing it. In swing cast-off, the blood droplets come from an arcing motion of a weapon like a piece of wood or maybe the bloody hands of the attacker or victim. Cessation cast-off arises when the motion of a source of blood is suddenly arrested; the impact of a weapon with the victim is a typical example.

The laws of physics account for the phenomenon of cast-off blood. Wet blood tends to move to the end of an object, where it pools. Depending on the shape of the object, there may be one or more pools of blood ready to be cast off. If the momentum—product of mass and velocity—of the object can overcome the surface tension making the pool of blood cling to the surface, then the blood is cast off in a series of spherical droplets.

Swing cast-off is, in theory, more likely to occur during a forward swing because more momentum is gathered during this phase of motion than in the backward swing. But the situation is a bit more complex than this; in practice, more cast-off is seen in a backward swing, simply because more blood is available. Once the forward swing begins, most of the blood has already been cast-off. In cessation cast-off, the deceleration of the arresting impact transfers force to the blood. It is then cast-off in a pattern not unlike that seen in impact spatter, which is caused by a force such as kicking or beating applied directly to wet blood.

The size of the droplets of cast-off blood depends upon many factors such as the shape and surface of the object, its velocity, and the amount of blood it is carrying. Long, relatively light weapons create more cast-off staining than shorter, heavier weapons. The blood is usually flung away from the arc of movement, which means that it tends to land on nearby surfaces, rather than on the attacker. The resulting cast-off trails may help the investigator to deduce how the attack took place.