Glass What is glass? Forces that Fracture Glass

How Is Glass Used: Glass fragments can be used as evidence to help place a suspect at the scene of a crime. Because 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

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.

Different Densities for Different Glass Forensic scientists use physical properties of glass to associate one type of glass fragment with another. One of these properties is density Because different types of glass contain different combination of metal oxides, they have different densities Density refers to a material’s mass per unit volume D=mass/volume

Density Density of a substance remains constant, no matter what the size of the substance A simple 3 step method to determine density: 1. weigh the sample to find its mass 2. determine the volume of the sample 3. Divide the mass of the sample by its volume

Example Initial volume of water in beaker = 300 mL Add a rock New volume= 500 mL Volume of rock = 200 mL

Glass can provide valuable evidence about a crime Comparisons possible with broken or fractured glass include: physical match, probability of common origin, direction of impact, and sequence of impact Physical match and probability of common origin of two or more pieces of glass can be determined by comparing the physical characteristics of glass samples. The penetration of glass by a high speed projectile, such as a bullet, can leave evidence as to the direction of impact. If there is more than one hole in glass from flying projectiles, the sequence of their impact can be determined

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

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