PHYSICAL PROPERTIES GLASS

Physical vs. Chemical Properties The forensic scientist must constantly determine properties that uniquely describe a piece of evidence. Physical properties such as weight, volume, color, boiling point, and melting point describe a substance without reference to any other substance. A Chemical property describes the behavior of a substance when it reacts or combines with another substance.

Important Physical Properties Density is defined as the mass per unit volume (D = M/V) Density is an intensive property of matter, meaning it remains the same regardless of sample size. It is considered a characteristic property of a substance and can be used as an aid in identification.

Determination of Density Remember your density lab from chemistry? A straight mathematical model of calculating density will seldom work with evidence brought into the laboratory. Broken fragments of glass and even plastic are irregularly shaped and their volumes are not accurately measurable.

Flotation Method The flotation method is a precise and rapid method for comparing glass densities. A glass particle is immersed in a liquid of known density. The density of the liquid is adjusted by addition of small amounts of an appropriate liquid until the glass chip remains suspended in liquid.

Refraction Light waves travel at a constant speed until they encounter a new medium such as glass or water. The new medium will slow down the light causing the wave to bend or refract. The amount of this refraction is dependent on the ratio between the speed of light in the two mediums.

Seen Phenomena

Refractive Index Refractive index is a distinguishable physical property of glass. Refractive index is a value always greater than 1. For example the index of refraction for water is 1.3; meaning that light travels 1.3 faster in air than water. It is calculated by taking the ratio between the speed of light in air to the speed of light in the medium.

Index of Refraction

Immersion Method One method for determining the refractive index is immersing it in a liquid with a similar index. With the immersion method the analyst is looking for the Becke Line which is a bright halo surrounding the piece of glass. The index of the liquid is adjusted with temperature until this line disappears.

Becke Lines around Glass Crystals

Glass Structure

What is 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. The added metal oxides provide various properties to glass and also make them unique.

Common Glass Types Tempered glass is stronger than normal glass due to rapid heating and cooling. Tempered glass is used in car windows. Laminated glass found in car windshields has a layer of plastic between two pieces of ordinary window glass. Most homes now are required to use tempered glass for safety.

Broken Tempered Glass

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.

Radial and Concentric Fractures

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 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.

Which bullet hole was first?