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Crime Scene Forensics – Analyzing Properties of Glass Found at a Crime Scene

Suppose a person became the victim of a hit-and-run accident in which the perpetrator drove off without stopping to check if anybody got hurt yet leaving clues of broken glass at the scene of the crime.

How do crime scene investigators (CSI’s) go about studying the shards of glass? In this article, I will attempt to explain how glass is analyzed by studying its inherent properties.

What is glass?

Heating a combination of silicon dioxide (a component in sand), calcium carbonate (a component in stone), and sodium carbonate (a component in soda) produces a new chemical product called glass. During the heating process, there are additional other chemicals added to change the qualities of the glass, and these various impurities become incorporated into the final glass product. It is up to the criminalist to determine the chemical composition of these additives and impurities. Knowing these facts can aid in either matching an unknown sample to a known source or weeding it out.

Scientifically studying the physical, optical, and chemical makeup of glass can point to a particular manufacturer to aid in a criminal investigation. A piece of glass from a truck windshield or turn signal lamp may indicate the make, model, and sometimes the year of a vehicle involved in a hit-skip incident, used in a crime, or involved in a motor vehicle accident.

Measuring glass density: In order to measure glass density, a cylindrical-shaped container filled with liquids of known various densities is used.

The most common liquids used in the cylinder are bromoform and bromobenzene. Both liquids are mixed together, and then the glass sample is added. Then another one or the other of the liquids is added until the glass sample is suspended in the mixture; i.e. the sample is neither floating at the top nor sinking at the bottom of the container. Once the glass sample is suspended, it can be concluded that the glass and liquid mixture have the same density. Since both the liquids have known densities, the crime scene technician can calculate the density of the mixture and of the glass with little problems. Scientifically speaking, when a known piece of glass is suspended (control), and an unknown piece is dropped into the container (experimental), if the unknown is suspended too, the criminal scientist can conclude that the two pieces of glass have the same densities. On the other hand, if the unknown piece of glass either sinks or floats, the two pieces of glass do not share consistent density and therefore do not come from the same source.

Analyzing light transmission, reflection, and refraction: Different glass products can transmit, reflect, and refract light in different ways. Light transmission refers to light passing through glass much like sunlight through a windshield. Light reflection refers to light bouncing off glass. Light refraction refers to light passing through glass but its pathway is bent. An example of refracted light is when you stick a straight stick into water and note that the stick is “bent.” It is the medium, water (like glass), that changes slightly the direction of light giving you the appearance that the stick is bent. Different kinds of glass vary in one or more of these optical properties. Experimentally, a crime scene laboratory technician can show that two pieces of glass are similar, if they have the same optical properties.

For instance, you can check the refractive index by submerging a piece of glass in liquid with a refractive index that changes when the temperature of that liquid changes. Silicon oil is a good liquid to use. As heat is added and raises the temperature of the oil, the temperature at which the glass piece looks like it is going to disappear indicates that the refractive indices of both the glass and the oil are the same. Since the refractive index of silicon oil is known throughout any temperature, measuring the temperature of the liquid at its heated state shows the refractive index of the unknown piece of glass.

Analyzing chemical properties: A forensic laboratory technician can check for any additional chemicals or impurities in glass with an inclination toward matching the chemical composition of the known and unknown samples. Any differences found by the criminal scientist allow him to rule out the known source as the source of the unknown piece of glass.

Knowing the density, optical properties, and chemical composition of glass found at the scene of a crime can bring crime scene investigators one step closer to solving a crime.

The next time you watch your favorite CSI or Forensic Files TV show, you will have a better understanding of how glass is studied in a crime lab in order to bring a perpetrator to justice.

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