Understanding Cast-off Bloodstains
When blood droplets are released from an object in motion it is referred to as “Cast-off”. Technically there are two types of cast-off classifications; swing cast-off and cessation cast-off. Swing cast off is the most easily recognized at crime scenes. It occurs when a bloody object is swung and the centrifugal force of that motion overcomes the surface tension between the blood droplet and the surface. When that occurs the blood droplets will release from the object and travel until stopped. In a classic visual imagine blood droplets flung from a bloodstained crowbar as it is swung by the offender. These stain patterns are typically linear in appearance and that “line” of bloodstains is oriented along the path of the swing.
So if your suspect is swinging the weapon up and down the “line” of blood droplets will be oriented up and down as well. If they are swinging the weapon side to side then the droplets will follow a side to side path. A careful examination of the droplet ellipse will indicate the directionality. This may help answer the question of whether the swing was coming down or going up (for example). Obviously, in cases where there are multiple blows struck you may see a combination of swing paths (as the suspect repeatedly raises the weapon and then brings it down again on the victim for example). The more paths that overlap the more difficult it may be to determine an accurate count as to the number of blows. Analysts also have to remember that the first blow will not cause blood spatter so when counting the number of swing paths we always add 1 and describe the number as a minimum number of blows.
Try this experiment. Get a bowl of water and go outside to your driveway (if it is cement). Get your hand wet and swing it at the ground like you are swinging a weapon. Notice how the droplets create a “line”. Now, water is a poor substitute for blood but you’ll get a basic idea of how this process works. Now consider this. Imagine you were at a crime scene and you saw seven linear cast-off patterns on a wall over the dead victim. What would be the minimum number of blows you’d expect to find on the victim? Here is a simple way to reconstruct it. Use up and down arrow symbols to indicate each swing. So seven paths would be represented by four down arrows separated by three up arrows (assuming you have four downward directional “lines”). Each down arrow is representative of a connecting blow (+1 blow to start the flow) so you would expect to see five injuries from that weapon.
Now you may not see that many injuries. The suspect could swing down and miss or swing down and hit a part of the body that doesn’t cause blood flow or leave a distinctive mark (like striking a thick wallet or other object in the way). It’s not always definitive but examining the patterns can give you an idea of how many blows were delivered at a particular spot. Again, the greater the number of blows the more difficult it becomes to separate each “line” because they begin to overlap. In the close-up photo above notice the directionality of the blood drops. The ones on the right are going downward and the ones to the left are generally going upward.
Another thing we can sometimes tell is the general position of the attacker. Try this. Pretend you are standing in a room swinging a hammer from over your head to the “victim” on the ground next to a wall. As you swing your arm there will be a point at which your arm is perpendicular (90 degrees) to the ceiling and again to the wall. At those two points (in theory) the cast off blood droplets will be striking at a perpendicular angle and will look more circular than elliptical. By lining up those two points you get an idea of the location of the swing fulcrum (suspect).
The second type of cast-off is called cessation. This occurs when the bloodstained object abruptly comes to a rest after striking an object. Most of the time the weapon strikes the body and you won’t see any droplets released because they are in contact with the body as the weapon strikes. But if the weapon were to strike another surface (like a balcony hand rail) then the abrupt stopping of the weapon would release blood droplets traveling in the direction of the blow. You might see them impacting on the ground below.
Consider putting these stain patterns in your crime scene to demonstrate a beating or even a rage attack. They can be quite memorable when found in abundance.
Posted on October 8, 2011, in The Crime Scene and tagged Bloodstain Pattern Analysis, BSPA, cast-off, cessation cast-off, crime, Crime Scene, csi, detective, fiction, forensics, murder, mystery, police, swing cast-off, thriller, tom adair. Bookmark the permalink. Leave a comment.