Forensics for Writers: Blood Spatter and Ballistics
First thing, before the gory details: it’s blood spatter, no L. Not splatter, even if that is descriptive of how it got there, and there, and ew… there. The technical term is spatter, not splatter. Now that I have assuaged my inner editor…
As a writer the mystery of a crime scene can be worth revisiting, just as an investigator must do to try and recreate what happened there, from the evidence left behind. In the case of a violent crime, that evidence can speak volumes from the blood. The human body only contains 5-6 liters of blood (roughly) but it is under pressure as the heart beats. To an investigator, blood reveals much. It can reveal whether the bleeding person stood still – a passive droplet is completely round, but there are differences in the shape dependent on the surface it fell on, which can lead to ‘satellite’ splashes as the blood strikes a hard surface. The blood will show the direction a person was moving – you look at the ‘tail’ of a drop, and it is pointing in the same direction the drop was moving when it touched down, like a tiny accusatory arrow.
Blood that is coughed up, or expirated, will have tiny bubbles in it, a distinctive indicator of internal trauma that can remain long after the injured person moved on. From a deep wound, arterial spray shows gouts of blood, and if it was directed against a vertical surface (as I have seen a photo of someone trying to make it down a stairwell) you will see peaks in the wave-like pattern, from the beat of the heart, but the peaks get lower, and finally there is the last big smear as the injured person fell against the wall and too the floor…. You can prove death without a body in the case of spilling so much blood the body could not have sustained a heartbeat and would surely have been dead.
When an object strikes the body with sufficient force, blood leaves the body. The level of force, and roughly the size of the object, can be determined from this. A gunshot wound will show signs of high-velocity blood spatter, which is defined as less than 1 mm in diameter. This never travels very far, so you are most likely to find it on the victim, the suspect if they were within a half meter of the wound when it was made, or objects that were close. As the bullet strikes the body, the blood is projected outward in the opposite direction, but this isn’t blowback – that is the powder residue of that gun which will land on the shooter’s hand and clothing.
The size of an object makes a difference in the size of the blood spatter. An object with a smaller diameter, say, a length of rebar, will cause smaller droplets than a baseball bat. In class we speculated about what kind of spatter a cast-iron skillet would cause – I’m going to guess fairly large, low-velocity spatter, more than 4 mm in diameter. Messy… Medium-velocity spatter falls in between, from 2-4 mm in diameter. All those are rough numbers, but they allow the investigator to see patterns, like castoff from an attack where the victim was struck more than once, and the bloody implement slung droplets off as it was brought down again. Long, thin trails of castoff with small medium-velocity droplets indicates a sharp blade like a knife or a sword. These cast-off marks can show what hand the suspect preferred, useful if you need to know you are looking for a sinister rather than a dextral.
Currently, most crime scene analysts (ignoring the TV shows, please) carefully measure and plot the directionality of the droplets, to get an idea of where the victim was, where the assailant was, any movement during or after the attack – this can be told by altered marks, which can show smearing and retain prints of friction ridges, palm prints being as useful as finger prints. Movement after a certain time leads to what are called skeletonized droplets, which dry from the outside in. When touched, the still liquid interior smears, but the dry outline, or skeleton, remains. This drying begins at roughly (depending on atmospheric conditions) fifty seconds. This allows a timeline of the crime to be developed. There is a formula for determining the angle at which a droplet hit the surface, Sin (impact)=width/length. And if something or someone was at the scene, and then removed, that absence causes missing blood, which is called a void.
Related to the determination of the angle at which blood struck a surface is the determination of what angle a bullet was traveling when it struck a person. Ballistics is only this – it is not marks on the bullet itself or the firearm, those are properly referred to as toolmarks. “Ballistics (from Greek βάλλειν ballein, “to throw”) is the science of mechanics that deals with the launching, flight, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.” You can find a cool calculator here.
Using ballistics, your investigator can discover where the projectile came from, how far it was likely to have traveled, based on what sort of bullet it was… pistols and shotguns don’t have a terribly long range, but that rifle bullet could have traveled hundreds of meters, and a sniper round even further. Determining the sort of bullet falls under next week’s post on toolmarks, fingerprints, and other ways criminals leave their signatures at a scene.
You can find week one here, the Introduction, then Crime Scene, and Evidence and Investigation. I am planning two more installments, one on marks and another on toxicology, and perhaps one last post wrapping it all up with a list of the links and resources for people. As always, ask questions in the comments and I’ll answer there, or in the posts where they fit. Hopefully this is being helpful to some!