Writing on the Wall: Reading Blood Splatters

There’s gore all over the room, the TV police officer comes in and apparently reads the whole murder from the blood splatters. Isn’t it all just a random mess all over the walls? Or is there really information that can be found when analyzing the blood patterns to solve a crime?

In a previous post, I talked about the way finding the smallest drop of blood can help to catch a murderer (https://mozartcultures.com/en/what-stain-is-this/ ). But there is also a lot that can be learned from large amounts of blood sprayed everywhere, and the voids where there are no splatters.  Using Blood pattern analysis (BPA) to solve crime is a science, not an art and as such it is described using mathematics.

Types of Splatter

For those of you who like your science seasoned, let’s talk about patterns first.

Image 1: Blood splatter patterns (‘Principles’, 2013)

Bloodstains come in three main types. They are classified as passive, transfer or impact.

Passive

Passive stains are those that fall with low velocity such as a single drop of blood. A drop falling from a 90° angle is most likely to form a sphere, as seen in the left hand picture in image 1. Pooling is another passive form of bloodstain, however a pool can cover more useful blood patterns. Passive bloodstains are blood leaking from the body and responding to gravity (‘7 Things, 2015).

Transfer

Transfers marks include hand and footprints, and also drag marks. They are best used to help read how objects have been moved around the scene (‘Blood Pattern Analysis’).

Impact

These are the actual blood spatters, as seen in the picture on the right in image 1.The raindrop-shaped bloodstains help determine the location of the crime within the scene. These come from gunshots or arterial spurts. Another form of splatter is made as Cast-off: when the instrument used to wound is drawn back and blood flies off. In cast-off, the pattern forms in a line, either curved or straight, depending on the angle of the draw-back (‘Blood Pattern Analysis). If there is more than one swing, then a Crime Scene Investigator (CSI)  will see more than one line when reading the pattern.

Landing Surface

The surface that the stain lands on can also affect the reading. If the surface is absorbent then the stain is less likely to form a helpful pattern. If the stain lands on vinyl wallpaper then the stain will look very like the pictures in image 1 (‘7 Things’, 2015).

Void

A void happens when an object, such as a person, is in the way of the spray of blood and it leaves a blank area in the patterns in the room. It allows the investigators to see where people were during the incident (‘Principles’, 2013).

With blood splatters, the important point to remember is that that tail points away from the direction of flight, and that’s where mathematics comes in.

Mathematics

Now on to the part I enjoy. Without mathematics, BPA would be based on the subjective viewpoint of the officer in charge. I agree that humans instinctively calculate how to jump or throw a ball (Peery, 2003). But by using mathematics,  it is possible to read the action in the room from the bloodstains and the voids and have evidence that stands up in court.

The shape of the bloodstain depends on  the angle of impact. As I said above, at a 90° angle the bloodstain is likely to be a sphere.  Once the angle is below 75°, then the pattern elongates into the classic teardrop shape( National Forensic Science Technology Center). The angle of impact is vital to learning where the droplet originated. This can be learned by dividing the width of the droplet by the length as shown in image 2, below.

Image 2 Calculating the angle of impact (Gomez, 2021)

By taking the inverse sin of the result, we get the angle of impact, as show in the formula below.

Image 3: formula for calculating the angle of impact (Gomez, 2021)

As I mentioned, the tail of the droplet is always away from the direction of travel. Once the angle of impact and direction of each droplet is determined, then the CSI can use this information to attach strings to the room. Where the strings cross is where the injury happened.  If you are interested in this process here is a video showing stringing. https://www.youtube.com/watch?v=vS6aMoczicA from 1.51, but the earlier part shows how to do the calculations.

History

In 1895, Dr. Eduard Piotrowski of the University of Kraków was the first to describe the usefulness of blood pattern analysis at a crime scene in his paper ‘On the formation, form, direction and spreading of blood stains after blunt trauma to the head’. But BPA was not used in the US until the 1950s (Leverhulme, 2018).

Finally

Image 4: Definition of Forensic Science

The purpose of Forensic investigation is to have evidence-based convictions. Therefore, along with using personal experience to read blood patterns, it is vital to include mathematics to back up your evidence.

Should you wish to know more, then check out the articles and books in the references.

References

‘7 Things You Didn’t Know About Blood Splatter Analysis’. (2015, May 6). Forensics Colleges. https://www.forensicscolleges.com/blog/resources/7-things-about-blood-spatter-analysis

‘Blood Pattern Analysis’. The Forensics Library. https://aboutforensics.co.uk/bloodstain-pattern-analysis/

‘Principles of Bloodstain analysis’. (2013) Forensic Science Simplified. http://www.forensicsciencesimplified.org/blood/principles.html

Peery, N.R., (2003) Instincts: The Hidden Spirits That Drive You. iUniverse, inc. https://books.google.co.uk/books?id=4LgkOcUroiYC&pg=PT102&lpg=PT102&dq=instinctive+mathematics+of+throwing+a+ball&source=bl&ots=_IOBh5NtcT&sig=ACfU3U1V854No_dU4i9la-x8GsKHWfACng&hl=en&sa=X&ved=2ahUKEwjI55POv5j2AhUOY8AKHbEhCMwQ6AF6BAgXEAM#v=onepage&q=instinctive%20mathematics%20of%20throwing%20a%20ball&f=false

National Forensic Science Technology Center. A Simplified Guide to Bloodstain Pattern Analysis. Bureau of Justice Assistance.  http://www.forensicsciencesimplified.org/blood/BloodstainPatterns.pdf

Gomez, M. (2021, November 22). ‘Solving Crimes with Math: Blood Pattern Analysis’. +Plus Magazine: Living with Mathematics. https://plus.maths.org/content/solving-crimes-maths

Leverhulme Research Centre for Forensic Science, (2018, October 26). ‘#ForensicFactFriday, Blood Pattern, Splatter, Splatter Analysis’. University of Dundee. https://www.dundee.ac.uk/leverhulme/news/2018/article/forensicfactfriday-blood-pattern-splatter-spatter-analysis.php