Forensic Science 1- Glass Examination-University of Southern Mississippi

Physical Property

Describes a substance without reference to another substance (e.g., weight, volume, color, boiling point, melting point).

Chemical Property

Describes how a substance behaves or reacts with other substances.

Importance of Properties in Forensics

Forensic scientists use distinguishing properties to give matter a unique identity for comparison and identification.

Temperature

Measure of heat intensity; in science, measured on the Celsius scale (0°C = freezing, 100°C = boiling).

Weight vs. Mass

Weight: Force of gravity on a body.

Mass: Amount of matter an object contains, independent of gravity.

Two Models of Light

Light as a continuous wave.

Light as discrete particles (photons).

Dispersion

Separation of white light into colors through a prism, forming a continuous spectrum.

Visible Spectrum Range

Red to violet—only a small part of the electromagnetic spectrum.

Wavelength

Distance between two successive wave crests or troughs.

Frequency

Number of wave crests passing a given point per second.

Relationship Between Frequency and Wavelength

Inversely proportional — as wavelength increases, frequency decreases.

Electromagnetic Spectrum

The full range of electromagnetic radiation from cosmic rays to radio waves; visible light is a small portion.

Basic Composition of Glass

Primarily silicon oxides (sand) mixed with metal oxides.

Additives in Glass Manufacturing

Sodium carbonate (soda): Lowers melting point.

Calcium carbonate (lime): Prevents glass from dissolving in water.

Soda-Lime Glass

Most common type; used in windows and bottles.

Float Glass

Formed by cooling molten glass on a bath of molten tin; used for flat windows.

Tempered Glass

Strengthened through rapid heating and cooling; breaks into small, safe fragments.

Laminated Glass

Two layers of glass with plastic (PVB) in between; used in windshields.

Purpose of Glass Comparison

To associate fragments by analyzing physical properties while minimizing other possible sources.

Key Properties Used in Glass Comparison

Density and refractive index.

Definition of Density

Mass per unit volume (D = M/V); an intensive property that doesn't change with sample size.

Importance of Density in Forensics

Characteristic property useful for identifying materials or matching fragments.

Refraction

Bending of light as it passes from one medium to another due to a change in velocity.

Refractive Index (RI)

Ratio of the velocity of light in a vacuum to its velocity in the material.

RI of Water at 25°C

1.333 (light travels 1.333× faster in a vacuum than in water).

Importance of RI

Like density, RI is an intensive property used to characterize materials.

Flotation Method Purpose

Determines the density of a glass fragment.

Process of Flotation Method

Immerse glass in a liquid mixture (bromoform + bromobenzene).

Adjust liquid density until the glass remains suspended.

Matching densities → possible common origin.

When Density Differs

Glass samples are from different sources.

When Density Matches

Proceed to test refractive index.

Purpose

Determines refractive index by immersing glass in a liquid whose RI is varied until it matches the glass.

Becke Line

Bright halo around glass in a liquid of different RI; disappears at the match point when RIs are equal.

Match Point

Temperature or condition where the glass and liquid have identical refractive indices.

FBI Glass Database

Contains density and refractive index values from glass samples; used to estimate frequency and evidentiary value.

Radial Fractures

Radiate outward from the point of impact (form first, opposite side of force).

Concentric Fractures

Circular cracks around the point of impact (form second, same side as force).

3R Rule

Radial cracks form Right angles on the Reverse side of the impact.

Direction of Force

Determined by examining the perpendicular edge of radial cracks—it faces the side opposite the impact.

Sequence of Impacts

Later fractures terminate at lines of earlier ones; the first crack continues, the second stops.

Collection Guidelines

Collect all possible fragments for reconstruction; package in solid containers to prevent breakage.

Handling Clothing or Shoes

Wrap each item separately in paper to preserve trace glass.

Individualization of Glass

Rare; best evidence from physically matching fragments. If not possible, compare density and refractive index.