Forensic Science: Trace Evidence
There are MANY different types of glass...
- Plexiglas is commonly seen as transparent, but in some cases, colors are added.
- This glass is much more durable than standard glass.
- Plexiglas conducts heat much more effectively than your typical glass.
- Plexiglass always remains in a static state.
- Pyrex glass is transparent in most forms, however color can be added to glass.
- As a fairly durable glass, Pyrex remains in a static state.
- Although this glass does not conduct electricity, it does conduct heat.
- Pyrex is commonly used in lab equipment and kitchenware, because it is known to be fairly heat resistant.
- On top of its heat resistance, Pyrex is also resistant to many chemicals.
- Although safety glass looks very similar to Pyrex glass safety glass is almost four times stronger.
- Safety Glass is the most durable form of glass.
- Conducting heat less than any other form of glass, safety glass allows for rooms to stay much cooler than rooms with standard glass windows.
- Safety glass always remains in a static state just like other forms of glass.
- Most commonly, safety glass is seen in car windows, mirrors, and more.
- The thin plastic layer in the middle of the glass keeps it from shattering, which reduces the danger of the glass when shattered.
The chemical properties of glass determine its resistance to attack by water, moisture, acids and alkalis.
Glass is a transparent compound which is made out of silica. It is hard and brittle. Chemical properties of glass include high energy of first ionization, very high boiling point and melting point.
First ionisation energy: The first ionisation energy is the energy required to remove the most loosely held electron from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+.
Example: H(g) yield H+(g) + e-
FORENSIC GLASS ANALYSIS
Why test the glass particles?
- With it's wide forms and compositions, there is a lot that the lab can do to test and to identify glass particles.
- Because tiny shards of glass can easily be transferred onto one's clothing, the presence of glass fragments on an individual can be extremely significant.
- If the properties of the glass found on the suspect do not share similar physical and mental properties and are not a match, then the suspect can be eliminated as the primary suspect.
- The main goal of the forensic analyses of glass particles is to determine whether the sample of glass from the scene and the sample of glass from the suspect's clothing could have originated from the same source.
Identifying Physical Properties
The primary steps that forensic scientists take when analyzing the physical properties of glass particles...
- Create a glass provenance (Hypothesis) "Is this glass from a lightbulb?"
- To determine whether or not the glass provenance made is true, analysts first look for class characteristics that could associate objects with another type of object, but not with one single object.
- Remember that, even though many objects may have a long list of identical properties, glass particles can originate from broken (not intact) objects.
- If the evidence is there, determine whether the two samples of glass originated from the same source. "Only physically matching two or more broken glass fragments allows for their association with each other to the exclusion of all other sources" (Scientific Working Group for Materials Analysis 2005). Finding one is good evidence, but you can't confirm that they're from the same source unless you find two or more matching samples.
Main Things to Look For...
- Color: Any differences in color represent a change in the chemistry of the glass, which can be used in order to differentiate glass fragments.
- Fluorescence: Upon being exposed to short/long waves of ultraviolet light, different types of glass will have different reactions. Having this throughout the body of the glass is often caused by the presence of certain elements, such as uranium, in the glass. If one sample proves to be fluorescent and one does not, it can be eliminated as a match to the source!
- Thickness: To determine thickness in the lab, forensic scientists make measurements with a micrometer or a caliper, when both glass shards have original surfaces. Thickness of glass is directly correlated to its viscosity, so again, this can be very influential in determining whether the samples are a match.
- Surface Features: Things such as mold, polish marks, mirrored backings, scratches, and decorative finishes like texturing, etching, or coatings can be HUGE indicators of whether two samples are a match or not. Mostly, these features will be viewed and compared in the lab under a microscope; often times, features such as these cannot be seen by the naked eye. X-Ray scattering, transmission electron microscopy, and infrared spectroscopy can also be used to identify and analyze glass coatings.
- Optical Properties: The refractive index is a measure of the speed of light in a transparent medium, such as glass. It is a function of the chemical composition and atomic arrangement in the glass. These things are determined and controlled by the chemical composition of the glass and the glass' cooling history.
- Density: Density can be defined as the mass per unit volume in a substance. This, like the optical properties, can be determined by the chemical composition and the atomic arrangement of the glass samples. In order to yield accurate results, the glass being tested must be extremely clean and must be larger than 2-3 millimeters in diameter. This causes density measurements to be performed less frequently than the refractive index tests.
Identifying Chemical Properties
- Often times, this can be the hard part of the forensic analysis of glass.
- Depending on its size, a sample of glass may or may not be able to be analyzed with reproducible effects.
- Even if its features are still present, the forensic lab cannot and should not analyze glass particles under a certain size due to the lack of results they will yield.
- Tests will not be conducted if no physical traits match between the original sample and the sample that is being compared to it.
- IE: If you're looking at thick, yellow glass, it's not logical to test a thin, transparent shard of glass from a suspect's clothing.
FAMOUS GLASS CASES
THE BALTIMORE RAPIST
- In Baltimore a few years ago, a serial rapist broke into the house of a young woman.
- Panicking after being unexpectedly confronted, the man dove through a window and only got a few blocks away before he was captured.
- Samples of glass silvers were taken from his clothing were taken and compared to the glass from the broken window in the house. After many tests, it was confirmed that he was linked to the crime, for the two glass samples had been a match.
- After discovering this, police went to several other houses with broken windows to find that, although they had been replaced, fragments were still present.
- By testing the density, refractive index, dispersion, and composition, the crime lab analyst was able to match several of the glass particles from the suspect's clothes to the samples of glass particles from the crime scene.
- Because of this, it was confirmed that the unidentified slivers that the suspect still inadvertently carried around were indeed a match to the slivers from the house.
- This evidence convicted him of three other rapes, and because of this, the court has given him a life sentence in a federal jail.
COPPELL CASE STUDY (July 12, 2012)
Thieves broke into 7 cars on July 12, 2012. (4 Andy Brown, 3 North Lake)
In all cases, purses and wallets were stolen from the car.
"Four vehicles were broken into," Coppell police Sgt. David Moore said. "In all four cases, a window was shattered and a purse was stolen from the inside of the vehicle."
Each of the cases occurred due to the breaking and smashing of car windows, which caused glass to go everywhere.
Skin fragments were found in the glass seen at the numerous crime scenes. Originally, this seemed as though it would catch the infamous robber(s).
However, after being tested for DNA, this evidence led to zero suspects, because the man or woman whose skin was found was not in system. On the contrary, this helped those working this case to conclude it was the same person every time at same place.
Thieves were reported to have been leaving in a black Xterra after being traced to Lewisville shopping centers.
One of the suspects is assumed to have been an amputee, based on what was seen in one of the security camera footages at a store in Lewisville.
- Unfortunately, there was never a case to take to court, because the police were never able to track down the thieves.
Deslich, Barbara, and John Funkhouser. Forensic Science for High School. Student Ed. Dubuque, Iowa: Kendall/Hunt, 2006. Print. "First Ionisation Energy." First Ionisation Energy. Web. 3 Feb. 2014. <http://www.chemguide.co.uk/atoms/properties/ies.html>. "Forensic Glass Comparison: Background Information Used in Data Interpretation." The Federal Bureau of Investigation. FBI, 28 June 2011. Web. 04 Feb. 2014. <http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/april2009/review/2009_04_review01.htm/#disablemobile>. "Physical Properties of Glass, Chemical Properties Glass." Physical Properties of Glass, Chemical Properties Glass. Web. 4 Feb. 2014. <http://www.glasscoasterstore.com/physicalpropertiesofglass.html>.
Deslich, Barbara, and John Funkhouser. Forensic Science for High School. Student Ed. Dubuque, Iowa: Kendall/Hunt, 2006. Print.
"First Ionisation Energy." First Ionisation Energy. Web. 3 Feb. 2014. <http://www.chemguide.co.uk/atoms/properties/ies.html>.
"Forensic Glass Comparison: Background Information Used in Data Interpretation." The Federal Bureau of Investigation. FBI, 28 June 2011. Web. 04 Feb. 2014. <http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/april2009/review/2009_04_review01.htm/#disablemobile>.
"Physical Properties of Glass, Chemical Properties Glass." Physical Properties of Glass, Chemical Properties Glass. Web. 4 Feb. 2014. <http://www.glasscoasterstore.com/physicalpropertiesofglass.html>."Purses Stolen From Parked Cars in Public Lots." NBC 5 Dallas-Fort Worth. Web. 4 Feb. 2014. <http://www.nbcdfw.com/news/local/Purses-Stolen-From-Parked-Cars-in-Public-Lots-162269755.html>.