Forensic Properties of Glass and Soil Evidence Module Overview

Evidence found at crime scenes is found in all shapes and sizes. Whether it is a fingerprint, a blood spatter, glass fragment or pair of muddy boots, a Forensic Investigator must understand the value of the evidence and how to collect and preserve it for analysis in the lab. Two substances most commonly examined by forensic scientists for their physical properties in a crime lab are glass and soil. Both are so common that the forensic scientist must determine properties that distinguish characteristics in an effort to find a unique identity that matches the evidence to the victim, the crime scene, or the perpetrator.

Essential Questions

1. What are the various types of evidence and how do they differ?
2. What types of crimes involve glass and soil?
3. In what ways do glass and soil evidence aid in solving a crime?
4. How are glass and soil evidence collected and preserved?

Key Terms

Evidence - Matter produced before a court of law in an attempt to prove or disprove a point in issue, such as the statements of witnesses, documents, material objects, etc.
Physical Evidence - Also referred to as real evidence, consists of tangible articles such as hairs, fibers, latent fingerprints and biological material.
Testimonial Evidence - Statements or the spoken word from the victim(s) or witness(es).
Circumstantial Evidence - Evidence based on suggestion rather than personal knowledge or observation.
Indirect Evidence - Evidence provides only a basis for inference about disputed fact.
Individual Evidence - Material that can be related to a single source or person, this individualization always involves a comparison.
Class Evidence - Material that can be associated only with a group of items that share properties or characteristics.
Weight - The force on an object due to gravity.
Density - Mass per unit volume, or g/ml. Density= Mass/Volume.
Refractive Index - A measure of the extent to which a substance slows down light waves passing through it. The index of refraction of a substance is equal to the ratio of the velocity of light in a vacuum to its speed in that substance. Its value determines the extent to which light is refracted when entering or leaving the substance.
Crystalline - Resembling crystal, as in transparency or distinctness of structure or outline.
Amorphous - Lacking organization, formless, or random distribution.
Borosilicate Glass - Any of a range of heat- and chemical-resistant glasses, such as Pyrex, prepared by fusing together boron(III) oxide, silicon dioxide, and, usually, a metal oxide.
Tempered Glass - Type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering creates imbalanced internal stresses which cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards. The granular chunks are less likely to cause injury.
Laminated Glass - Type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer, typically of polyvinyl butyral (PVB), between its two or more layers of glass.
Flotation Method - A quick method for comparing glass densities. Glass particles are immersed in a liquid the density of the liquid is adjusted until one or more glass particles remain suspended in the liquid. When the glass is suspended, it has the same density as the liquid and the density can then be inferred from the liquid. The comparison pieces of glass will either suspend, float or sink depending on their density relative to the liquid.
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. The Becke Line will disappear when the refractive index is matched.
Radial Fractures - Cracks in the glass, which radiate outward from the point of impact.
Concentric Fractures - fractures forming in an approximately circular pattern around the point of impact
Mineral - Any of a class of naturally occurring solid inorganic substances with a characteristic crystalline form and a homogeneous chemical composition
Density Gradient - A column containing a liquid mixture that undergoes a gradual decrease in density from the bottom level to the top level.
Becke Line - A bright halo near the border of a glass particle that is immersed in a liquid of a different refractive index.
Soil - The top layer of the earth's surface, consisting of rock and mineral particles mixed with organic matter.
Sand - A loose granular substance, typically pale yellowish brown, resulting from the erosion of siliceous and other rocks.
Clay - A stiff, sticky fine-grained earth, typically yellow, red, or bluish-gray in color and often forming an impermeable layer in the soil.
Silt - Fine sand, clay, or other material carried by running water and deposited as a sediment, esp. in a channel or harbor.
Soda Lime Glass - The most common type of glass it is used in plate and window glass, glass containers, and electric light bulbs.
Mass - A measure of how much matter is in an object.

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