Broken Glass Analysis Links Suspects to a Crime Scene

I walked through Glasgow City Center on a Monday morning, heading to class, when I spotted fragments of glass all over the pavement. Broken glass is nothing new in Glasgow after a weekend. Luckily, Forensic Science can use the analysis of broken glass to help link potential suspects with crimes scenes as an aid to solving crime.

But all glass looks the same

Pick up any two pieces of clear glass and it would be nearly impossible to detect any difference with the naked eye.

Image 1: A sandy beach

The main component of glass is silica, a component of sand. Sand has a real problem, because, like many other resources, the world is running out of sand as a raw material (Vaughan, 2022). The composition of sand varies. From your own experience, individual beaches contain different colors of sand. The differing combination of these minerals should make it easy to match up where your glass was made (Castro, 2013).

There are tests to perform on glass to find out if our hypothetical two pieces of broken glass come from the same source.

Why bother?

Glass is considered trace evidence in Forensic Science. Often, glass fragments can have fingermarks on them, see my previous post on fingerprints, here. While breaking the glass, a suspect may cut themselves, leaving traces of blood, see my post regarding blood spot location, here. Also, identifying glass is useful in crimes such as vehicle accidents and burglary (Cellmark). Small bits of broken glass get caught up in the clothing of someone who breaks a window, when committing a crime. If the glass on the suspected criminal can be matched to the glass at the crime scene, it provides one more piece of evidence to link a person to a crime scene (Cellmark).

Mechanics of glass transfer

When glass breaks, fragments travel through the air. Larger shards drop almost immediately to the ground, but smaller particles can fly through the air up to 3 meters away from the breaking point (Jones). Anyone standing close to the breaking glass will get slivers of glass in their clothing (Heinichen).

How long does the glass stay on a person?

This is an arena that has been well studied (Rupert, 2018). Larger pieces of glass will fall off clothing very quickly, but smaller fragments work their way inside the weave of the fabric. A woolly knitted cardigan has large gaps in the fabric to retain the glass, a closely-woven polyester anorak will shed evidence quickly. If the suspected criminal has turn-up on the cuffs of their trousers or shirt then that will hold trace evidence better than clothes without (Carlyse-Davies, 2019). If a criminal walks away from a crime scene, the evidence stays on them longer than if the person runs, as movement shakes the evidence off.

Recovery

The naked eye is not the tool required for this stage. Examining clothing under a low power microscope allows an examiner to pick out any embedded evidence with a pair of forceps. When finding broken glass at the scene of the crime, Crime Scene Investigators (CSIs) must collect control samples.This allows an examiner to determine if a similarity exists between recovered glass and the crime scene. Collect of glass on tape is possible, but the sticky residue can interfere with analysis (CastleView Forensics, 2021).

How do we analyse glass?

A set pattern exists in Forensic Science, used for all trace evidence analysis.

Observe > Non-destructive tests > Destructive tests

Observe

Describe the sample received in the laboratory. Weigh it. What color is the sample? Does it appear to have coatings or wire to strengthen it? All these things are vital to making a match. If you have multiple pieces do they fit together like a jigsaw puzzle? (Carlyse-Davies. 2019)

Image 2:File:Smithsonian-archives-broken-glass-negative.JPG

The pattern of the glass fall

When  an object strikes a glass pane, most of the glass will fall away from the force. However, some will fall on the same side of the impact. Therefore, when finding a broken window at the crime scene, the CSIs can determine whether the blow came from the inside or the outside.

You can watch how broken glass falls in this video.

Glass fracture patterns

Glass breaks in a very specific way. From the impact, fractures radiated out – called radials. Then, concentric circles form. The force of the impact leaves rib marks on the edge of glass pieces. For radials, the rib marks curve towards the force of impact. On the concentric circles, the rib marks curve away from the impact. If you know where the piece of glass comes from, by putting together a jigsaw puzzle, then you can determine which side of the glass the impact hits.

You can watch the way glass fractures in this video.

Non-destructive tests

Let’s say you have a small piece of glass. The size makes it difficult to determine its origin. Did it come from a curved article like a bottle, or a flat article like a window pane? Using an interferometer will show dark and light bands across the glass in a pattern. With flat glass the lines appear straight. Glass from a bottle curves the light patterns. (TelescopeOptics).

Image 3: Interference patterns on flat glass vs curved glass credit: https://www.telescope-optics.net/testing_optical_quality.htm

Destructive tests

If you have to, only destroy a portion of your sample because it may be necessary to repeat the tests.

GRIM

The main destructive test for glass is Refractive Index (RI). A portion of the sample is crushed into oil and the RI describes how the glass bends light it passes through the glass. The Glass Reflective Index Measurement (GRIM) from different sources have different RIs. This is, unfortunately, less useful with modern glass. Most window glass is mass manufactured in large factories allowing less local variation(Jones).

Element Analysis

This proves is good for analyzing specialist glassware, as these glass types contain distinctive elements. Unfortunately, the process evaporates a portion of your sample. The resultant gas is analysed using Mass Spectrometry (Almirall, 2012).

Conclusion

It is important to analyse all the available evidence. Trace Evidence such as glass is helpful in the investigation of volume crime, such as burglary. Using all of the gathered evidence, it is possible to obtain a great deal of information. All of which, must stand up in court, to hopefully establish a link between a suspected criminal and the scene of a crime.

References

Almirall, J, Naes, B, Cahoon, E, And Trejos, T. (2012, December). Elemental Analysis of Glass by SEM-EDS, μXRF, LIBS and LA-ICP-MS. US Department of Justice. https://www.ojp.gov/pdffiles1/nij/grants/240592.pdf

Carlysle- Davis, F. (2019, November). Glass And Documents. Course Lecture at the University of Strathclyde Centre for Forensic Science.

CastleView Forensics. (2021). Recovering Evidence at a Crime Scene. https://www.castleviewuk.com/ch1-recovering.html

Castro, J. (2013, May 28). What is Sand? Live Science. https://www.livescience.com/34748-what-is-sand-beach-sand.html

Cellmark Forensic Sevices. Glass. https://www.cellmarkforensics.co.uk/services/forensic-casework/glass/

Heinichen, I. [Video Transcript]. Glass As Forensic Evidence: Purpose, Collection & Preservation. https://study.com/academy/lesson/glass-as-forensic-evidence-purpose-collection-preservation.html

Jones&Bartlett Learning PLC. Forensic Analysis of Glass. Ascend Learning Co. http://samples.jbpub.com/9781284142617/9781284142617_CH05_Girard_SECURE.pdf

Rupert, K & Ho, M & Trejos, T. (2018). Study of Transfer and Persistence of Glass in a Mock Kidnapping Case. 8. 16-33.  https://astee.s3.amazonaws.com/359148_JASTEE_2018_8-1_Rupert.pdf

TelescopeOptics.net Tests for Optical Surface Accuracy. https://www.telescope-optics.net/testing_optical_quality.htm

Vaughan, A. (2022, March 24). We Are Running Out of Sand and Global Demand Could Soar 45% by 2060. New Scientist. https://www.newscientist.com/article/2313170-we-are-running-out-of-sand-and-global-demand-could-soar-45-by-2060/