Once upon a time, police knew nothing about blood spatter and didn’t use science in their investigations
First, we will let you in on a little secret. When you or your brother or sister do something wrong, your parents have to figure out who did it. To parents, this is like a whodunit game, and in fact, they have a secret name for it that they have sworn never to reveal. It’s called the “so which kid do we smack this time” game. And no, they don’t roll dice or draw straws to figure out the winner. Parents have an eerie, almost superhero-like ability that allows them to scan human faces to determine the guilty party. It worked so well that a long time ago, policemen decided to use the technique to figure out which bad guy committed the crime. They would line all the bad guys against the wall and stared at them. This worked well until they ran across this guy.
After this, the police began using scientific principles to analyze crimes. Blood spatter analysis, aka bloodstain pattern analysis, not “blood splatter analysis,” is an example of a scientific principle police use to figure out whodunit.
How and why police use blood spatter analysis
Sophisticated police detectives and ingenious FBI agents (<- click this link to be added to the FBI watch list) use many different scientific techniques to figure out who the bad guy is. They dust for fingerprints, use special lights to look for blood, use chemicals to find out what mysterious substances are, and sample DNA to figure out whodunit. And in addition to figuring out who did it, they have to figure out how they did it. And to do that, they often use a technique called blood spatter (not “splatter”) analysis.
Blood is primarily made of water, and as such, it behaves much like water and, therefore, must obey the laws of motion and gravity. Particles in the blood are attracted to each other, and this cohesive property causes blood that falls onto a surface to leave behind droplets rather than just spreading out all over the place. The blood droplets (blood spatter) are pretty much round when traveling through the air but deform into various shapes and patterns when they land on a hard surface.
What the size of a blood splatter droplet tells detectives
The size and shape of these droplets tell detectives quite a bit about how the person lost the blood. From blood spatter, detectives can determine what type of weapon was used, how many times a person was hit with it, if the bad guy was right or left-handed, the position of the victim and how they moved during the attack, how long ago the crime was committed, and how long the person lived after their injuries.
The size of the bloodstain depends on how much blood was lost. If a lot of blood was lost, the blood droplets will be quite large. Detectives use this information when determining whether a person died or not. Of course, two fingers on the side of the neck work well too.
How police use the shape of a blood spatter stain to determine the direction the blood flew
The shape of the blood stain depends on the angle of the blood when it hits the surface and the speed at which the blood drop was traveling. A droplet that has fallen vertically will be rounder than a droplet that has hit the surface at an angle that will be oblongly shaped.
Drops that hit at an angle form a tail that typically points in the direction the drop traveled. If you measure the width and length of the drop, then divide the width by the length, you can determine at what angle the blood struck the surface. If this ratio comes out to be .5, the blood struck the surface at a 30-degree angle. If the ratio comes out to .25, the droplet struck the surface at a 14-degree angle. Here’s a table that will help you out with the angle calculation.
Width / length (for the geeks, the formula is the arcsin of width/length)
.1 = 6 degree angle
.2 = 11 degree angle
.3 = 17 degree angle
.4 = 24 degree angle
.5 = 30 degree angle
.6 = 37 degree angle
.7 = 45 degree angle
.8 = 53 degree angle
.9 = 64 degree angle
1 = 90 degree angle
How surface texture impact bloodstain pattern analysis
The texture of the surface that the blood lands on affects the shape of the blood pattern too and detectives have to take that into consideration when examining blood spatter. Glass, metal, and other smooth, hard surfaces tend to yield rounded blood stains. Hard rough surfaces like wood or paper tend to produce blood stains with irregular, jagged edges. Soft surfaces, like clothing or carpet, absorb the blood and make the edges spread more.
Here is what the blood spatter pattern would look like on a smooth surface.
And here is the pattern the same blood droplet would produce on a rough surface.
Dryness of blood splatter helps detectives determine the time the crime occurred
Detectives also look at how dry the blood is. Blood begins to dry and clot within 15 minutes (depending on factors such as heat and humidity). The outer edges of droplets tend to dry first, and a completely dry droplet may leave a dark ring. How dry the blood is gives the detectives an idea of how long ago the attack happened.
Detectives also look at other factors, such as whether the blood contains bone or tissue matter (which indicates a high-impact splatter). They look for voids or blank spots, which may indicate something else (a person maybe) absorbed the blood spatter and then left. They call this shadowing or ghosting.
The Blood Spatter Analysis Experiment
You must be careful with this one. You can hurt yourself with the hammer and screwdriver (which come to think of it, would add a bit of realism to the experiment).
- If stage blood is not available, you can make your own blood using cornstarch and food coloring. Mix 44 g of cornstarch with 80 mL of water. Add 160 mL of corn syrup and mix. Add 2-3 teaspoons of red food coloring and 2 drops of green food coloring. Don’t get it on your clothes no matter how funny you think it would be to douse yourself in it and sprawl out on the kitchen floor.
- Tape large sheets of paper on a wall. Make sure the paper reaches the floor and goes pretty high up.
- Lay paper on the floor underneath the papered wall.
- Pour two tablespoons of blood onto a damp sponge. The sponge should not be dripping wet but rather just moist.
- Lay the sponge on the floor about 2 feet from the wall.
- Strike the sponge with the hammer.
- Note the shape of the spatter, measure the length and width of each drop (don’t include the tail in your measurement), take pictures of the spatter pattern. The droplets will be ellipse shaped with tails pointing towards the sponge.
- Replace the paper on the floor and wall. Position it exactly like you did the first time.
- Pour two tablespoons of blood onto the damp sponge and place it back on the floor about 2 feet away from the wall (in the exact same position as you placed it before.
- Strike the sponge with the baseball bat.
- Repeat with a screwdriver by stabbing the screwdriver point into the sponge. Then set a mousetrap and apply blood to the metal and trip it with a pencil. Place a block of wood or an empty bottle between the sponge and wall and notice the void that is produced and splatter something again.
- Write your findings on an index card. Display your pictures on a poster board along with the index cards explaining your findings.
You may also want to try placing the sponge at various distances from the wall and take measurements.
Types of Blood Spatter (Patterns)
Detectives use the knowledge they gain from blood spatter analysis to figure out how the blood pattern was made and hence, how the person who bled was injured, where they were standing when injured, how long ago the crime was committed, and so on. They take pictures of the blood stains, write down lots of notes, and measure the blood stains they find. All this information is taken to a lab where it is studied so the detectives can reconstruct the crime. Below are the types of blood spatter patterns the detectives look for.
Low-velocity spatter – results from low-impact blows. Low-velocity blood spatter often occurs after the victim is injured, for instance, when they walk around after the attack and drip blood on the floor. Droplets are about .16 to .31 inches. They may be larger if they are tall or walking on a tightrope.
Medium-velocity spatter – results from attacks with a blunt object (like a bat) or a stabbing or when the injury is close to the surface of the skin and blood spurts out of the wound. Droplets are typically no more than 4 millimeters.
High-velocity spatter – occurs with gunshot wounds. The droplets look like a fine spray less than 1 millimeter in diameter.
Crazy-person spatter – rare spatter type but fairly obvious when you run across it. See example below.
We provide a table above to determine the blood spatter’s angle, but the mathematics to calculate it yourself is simple if you have a calculator handy. Below are the formulas that blood spatter analysts use to determine the angle of bloodstain patterns.
Angle of impact = Arcsin (c/a)
? = sin-1(c/a)
* ? = impact angle
* c = width of spatter (mm)
* a = length of spatter (mm)
Bloodstain pattern analysis experiment supplies
Supplies: Tape, Food dye, Corn starch, Sponge
Blood spatter and droplet pattern poster
Below is a blood spatter and droplet pattern poster that illustrates the various types and shapes of blood spatter.
Here is a downloadable PDF format of the same image.
In-Article Image CreditsFunny police mug shot photo via Unknown with usage type - Public Domain
Blood spatter pattern on a smooth surface via Unknown with usage type - Fair use (low res)
Calculating the area of origin of a blood splatter via Wikimedia Commons by Ashley Do with usage type - Creative Commons License. May 24, 2021
Examples of the type of blood spatter and droplet patterns that there could be in a crime scene via Wikimedia Commons by Kimberly Lopez with usage type - Creative Commons License. May 26, 2021
Featured Image CreditCalculating the area of origin of a blood splatter via Wikimedia Commons by Ashley Do with usage type - Creative Commons License. May 24, 2021