PHYSICAL PROPERTIESGLASS

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?