Glass

How Is Glass Used: Glass fragments can be used as evidence to help place a suspect at the scene of a crime. different kinds of glass have different physical characteristics, types of glass can be distinguished from one another. For example, a chip of glass from a broken window may fall into a perp’s trouser cuff or shoes. A forensic scientist can identify the chips as part of the broken window. Similarly, parts of a broken headlight found at the scene of a hit and run can be used to identify the suspected vehicle.

Composition of Glass Hard Brittle Made of silicon oxides (sand), lime, soda, and oxides of metal The metal oxides found in most window glass are sodium, calcium, magnesium, and aluminum. Auto headlights and other heat-resistant types of glass such as Pyrex, contain boron oxides

Types of Glass Glass is a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxides. Amorphous solids have their atoms arranged randomly, unlike crystals. Tempered glass is stronger than normal glass due to rapid heating and cooling. Laminated glass found in car windshields has a layer of plastic between two pieces of ordinary window glass.

Safety Glass Broken glass can be sharp and dangerous This is why car manufacturers use tempered and safety glass in vehicles. Tempered glass is made strong by a rapid heating and cooling process that introduces stress to the glass surface When tempered glass breaks, it fragments into small squares that do not have sharp edges Therefore, tempered glass is not so dangerous Windshields are made of laminated or safety glass. This type of glass is strong and break resistant because it is made by sandwiching a layer of plastic between two ordinary pieces of window glass.

Glass Fragments For the forensic scientist, the problem of glass comparison is one that depends on the need to find and measure those properties that will associate one glass fragment with another while minimizing or eliminating other sources. To compare glass fragments, a forensic scientist evaluates two important physical properties: density and refractive index.

Flotation Method The flotation method is a rather precise and rapid method for comparing glass densities. In the flotation method, a glass particle is immersed in a liquid. The density of the liquid is carefully adjusted by the addition of small amounts of an appropriate liquid until the glass chip remains suspended in the liquid medium. At this point, the glass will have the same density as the liquid medium and can be compared to other relevant pieces of glass which will remain suspended, sink, or float.

Refractive Light Method Crystalline solids have definite geometric forms because of the orderly arrangement of their atoms. These solids refract a beam of light in two different light-ray components. This results in double refraction. Birefringence is the numerical difference between these two refractive indices. –Not all solids are crystalline in nature. For example, glass has a random arrangement of atoms to form an amorphous or noncrystalline solid.

Refractive Light Method

Immersion Method The flotation and the immersion methods are best used to determine a glass fragment’s density and refractive index, respectively. The immersion method involves immersing a glass particle in a liquid medium whose refractive index is varied until it is equal to that of the glass particle. At this point, known as the match point, the Becke line disappears and minimum contrast between liquid and particle is observed. The Becke line is a bright halo that appears near the border of an object immersed in a liquid of a different refractive index.

Analyzing Cracks The penetration of window glass by a projectile, whether it is a bullet or a stone, produces cracks which radiate outward (radial fractures) and encircle the hole (concentric fractures). By analyzing the radial and concentric fracture patterns in glass, the forensic scientist can determine the direction of impact.

Glass Fracture Patterns Two parts: –Radial –Concentric Can tell direction of force from these

Straight and Circular Lines When glass is penetrated by a projectile, it fractures in two ways Radially –Extends from the point of impact Concentrically –Circular line of broken glass around the point of impact When a high speed projectile hits glass, it bends the glass as far as possible, then breaks it

How does Glass break? Forensic Fracture Analysis

Analyzing Cracks A high-velocity projectile such as a bullet often leaves a hole that is wider at the exit side, and hence its examination is important in determining the direction of impact. The direction of impact can also be accomplished by applying the 3R Rule: Radial cracks form a Right angle on the Reverse side of the force. The sequence of impacts when there have been successive penetrations of glass is frequently possible to determine because a fracture always terminates at an existing line of fracture.

Determining direction of force Radial edge of glass –3 R’s Radial cracks have Right angles on the Reverse side of applied force Concentric edge of glass Direction of force

Bullet holes When a high-speed projectile, such as a bullet, penetrates glass, it leaves an exit hole that is larger than its entrance hole. This helps the investigator determine the direction of impact. The hole produced is often crater-shaped, and surrounded by concentric and radial fractures. A piece of glass may be penetrated by more than one projectile It is possible to determine the order in which the penetrations occurred by examining the fracture lines. A new fracture line will always stop when it reaches an existing fracture line Therefore, fracture lines from the first penetration will not end at any other fracture lines.

The left fracture came First, because the right Lines terminate at the Lines of the left one

Collection of Glass If even the remotest possibility exists that glass fragments may be pieced together, every effort must be made to collect all the glass found. When an individual fit is thought improbable, the evidence collector must submit all glass evidence found in the possession of the suspect along with a representative sample of broken glass remaining at the crime scene.

Collection of Glass The glass fragments should be packaged in solid containers to avoid further breakage. If the suspect’s shoes and/or clothing are to be examined for the presence of glass fragments, they should be individually wrapped in paper and transmitted to the laboratory.