Crime and Clues

Interpretation, Collection and Preservation of Glass Fragments

A man is found dead in an abandoned house. Upon examination the medical examiner finds that the man was killed by a single gunshot to the head. Three local teens where observed near the house two days earlier. They are picked up and questioned by police. When pressured, the teens admit to having stolen one of the boys' father's guns. They had taken turns shooting at the windows of an abandoned house.

The teens admit that John shot the gun first, then Jay and last was Fred. At the scene the investigators find three bullet holes in a window. They analyze the angle of each bullet hole in relation to the victim and find that the bullet which passed through the far right side of the window pane is the one which fatally wounded the man.

It is up to the investigators to determine the order of the shots. This will tell them which boy fired the fatal shot and allow the investigators to work out what the charges will be against the shooter. Examination of the fractures in the glass will tell them this.

When a projectile such as a bullet or stone hits a glass surface the impact causes changes, in the form of fractures, to occur within the glass. When a projectile smashes into the surface of a glass pane the glass bends slightly before it breaks. Some of the energy required to propel the bullet forward is absorbed by the glass .When the glass reaches a certain point of distortion it breaks and the projectile passes through the glass. In accordance with the laws of physics the energy from the projectile which was absorbed by the glass will dissipate along the path of least resistance. Cracks occur in the glass as energy dissipates through the glass. Shock waves of energy emanate from the point of impact causing specific types of damage to the glass. This is what makes it possible to determine the order of fractures. As well, the various types of fractures in glass can tell an investigator the angle of impact and the side of a glass pane on which the impact occurred.

When the projectile hits a glass surface the glass bends and energy dissipating from the point of impact causes 'radial' fractures to occur on the side of the glass opposite the point of impact.

Waves of energy from the projectile reverberate from the point of impact causing the glass to fracture in a cone shape. The result is small entrance hole and a larger exit hole in the glass. Surrounding the hole made by a projectile there will be concentric rings know as 'concentric fractures'.

Radial fractures will always travel along the path of least resistance. They will always end if they encounter an existing fracture line.

Radial fractures are also useful in determining the direction a pane of glass was struck from. Stress marks on the edges of the radial fractures conform in a specific way which will show the direction of impact. Stress marks are curved. One end of the curve runs parallel to the glass surface. The other end of the curve runs perpendicular to the glass. The side of the glass which has stress marks which are parallel to the surface is the side which sustained the impact (Saferstein, 2004).

Very often the glass in question is shattered and must be pieced back together. An investigator can start with the frame which is labeled as to which side is the inside and which is the outside. Loose pieces of glass can be matched up with pieces still adhering to the frame.

Once the pieces are all in place the point of impact can be determined. The radial fractures can be examined to determine from which side the glass was broken.

Determining the side from which glass was broken is important in burglaries and other crimes involving home invasion. Arson investigators have particular interest in knowing whether a pane of glass was broken from the inside or the outside. Windows broken from the inside at the top of a structure can indicate an attempt to ventilate the building. Ventilation causes the fire to spread upward following the source of oxygen. A fire started at the bottom of a structure can be made to race through a structure more rapidly than normal by ventilating the upper levels.

The concentric fractures form on the same side of the glass surface as the point of impact. When glass breaks most of the glass falls inward. However, the tiny shards produced in the formation the concentric fractures spray backwards towards the person wielding the weapon or tool (Lane, 1992) (Innes, 2002). These tiny bits of glass can get caught in the hair and clothing of a person near the point of impact. Even after washing, tiny fragments of glass can remain caught in the fibers of clothing and footwear. When a gun is fired at a glass surface, tiny particles of glass can spray back up to 18 feet (Innes, 2002)

Tempered glass and laminated glass do not behave in the ways described above. Tempered glass breaks into small squares when it is broken. Laminated glass, used in windshields and side windows in vehicles, has a layer of plastic imbedded between tow sheets of glass (Innes, 2002).

Glass fragments as Evidence

Physical evidence can broken be down into two general categories. There are types of physical evidence which can be considered have "class" characteristics. Evidence with class characteristics can be said to have similar characteristics within a group but can not be linked to a particular individual. An example of evidence which has class characteristics is as blood types (Saferstein, 2004). All blood types exist in any given population. The blood type itself only indicates the percentage of people in a population from which the sample could have come from based on the ratio of occurrence in that population. It does not tell an investigator what particular individual the sample came from.

Still evidence with class characteristics can be useful. Several different kinds of evidence such as hair, blood type and glass fragments together may significantly raise the probability of linking a suspect or victim to a crime scene (Saferstein, 2004)( Crocker, 1999).

The second type of evidence is that which exhibits 'individual' characteristics. This type of evidence has numerous points of comparison. Each point of comparison from an 'unknown source' which matches a 'known source' sample raises the probability of the two samples having the same origin (Saferstein, 2004). In a case of a hit and run where it is possible to recover the headlight or taillight and the broken bits from the scene an investigator may be able to fit all the pieces together. This situation is where individualization of the evidence is possible.

Glass comes in many types: headlight glass, window glass, ophthalmic glass, windshield glass, light bulb glass and many more. When fragments of glass are found during an investigation they may be analyzed by testing the density and refractive index of the sample.

The density is determined by weighing the sample and then measuring the volume. A formula of: Density = mass/volume is used to find the density.

The refractive index is the number assigned to a sample of glass which reflects the bending of light as it passes through a sample. An automated system known as the Grim 2 is used to calculate the refractive index of a glass sample.

From these values the analyst can usually determine the type of glass the sample came from. Determining the type of glass gives investigators evidence which is considered to have 'class' characteristics.

Only in cases where the suspect fragments exactly match fragments from the crime scene is it possible to consider this type of evidence as showing 'individual' characteristics which point to a specific source. The likelihood that all of the fragments from both locations will be found and fit together is extremely remote (Saferstein, 2004).

Spectrography is also used to analyze the composition of glass. This technique is useful in determining the profile of a specific sample but is destructive in that is requires heating of the sample to the melting point (Saferstein, 2004)

Two men leave the local bar at 2 a.m.. A few blocks away the driver of their vehicle looses control of the vehicle and slams into an oncoming car. The occupant of the other car is killed. Both men survive the crash. By the time the police arrive both men have exited the vehicle and are found sitting on the curb next to the wreckage. Both men are injured and neither remembers who was driving. The police notice a pair of shattered eye glasses in the first man's hand. The vehicle is processed and the glass fragments are collected from the vehicle. Each set of samples are labeled as to the location in the vehicle it was found.

Analysis of the fragments show that the fragments collected from the driver's side floor included windshield glass and ophthalmic glass. The ophthalmic glass was of the same prescription strength as the first man's glasses. The police now know who was driving the vehicle when it crashed.

In a case recounted by Gengre (2002) police analyzed the headlight glass found at the scene of a hit and run. They publicized the make and model of the vehicle based on analysis of the headlight glass found at the scene. Shortly after the public broadcast a man turned himself in as the driver stating he assumed they had a witness because they had a description of his car.

It is extremely important to collect all glass found at a scene. Labeling is equally important. Each sample must be labeled accurately with the case number, time, date, and name of the person who collected the sample.

How to Collect and Package Evidence

Reference samples of any glass remaining at the crime scene should be collected for purposes of comparison with glass found on a suspect. Reference samples can be collected from the glass remaining at a scene. Approximately 1 square inch of glass from the point of breakage is required (Saferstein. 2004). Reference samples must be placed in containers which will prevent crushing during transport to the laboratory. These samples should be packed in such a way as to prevent movement inside the container which could damage the edges of the fragment. Damage to the edges would prevent the laboratory personnel from matching the sample with other fragments from a suspect.

In cases where broken headlights, tail lights, light bulbs or bottles are found the entire object can be removed and sent to the laboratory. Again, these items should be packaged in such a way as to prevent further breakage.

When dealing fragments or objects which might contain biological evidence, such as blood or saliva, the investigator should insure that the sample is clearly labeled as containing biological evidence. All biological matter must be dry before it is packaged. Fingerprints can also be recovered from glass and samples should be handled using gloves.

All fragments of glass from a scene should be collected if the aim is to determine the angle or point of impact.

The F.B.I recommends that small fragments be packaged in plastic vials such as a film canister. The vials should be packed so that the fragments do not bounce around inside the container. The fragments must not be allowed to rub together.

When fragments are removed from a frame each fragment should be labeled indicating the inside and outside surface. As well the position in the frame from where the sample was taken should be indicated. For instance the label might read 'top right corner'.

Small fragments can be damaged by tweezers during collection. Tape may be an alternate method when the pieces are very small.

Finally, all clothing and footwear should be placed in a paper bag, sealed and labeled. Precautions should be taken to ensure the bag is not torn causing the loss of some of the evidence.