glass as evidence

Forensic Science: Fundamentals & Investigations, Chapter 14 1

Chapter 14 Glass Evidence By the end of this chapter you will be able to:

explain how glass is formed list some of the characteristics of glass provide examples of different types of glass calculate the density of glass use the refractive index to identify different

types of glass describe how glass fractures analyze glass fracture patterns

All Rights Reserved South-Western / Cengage Learning © 2009


Introduction and History of Glass Introduction and History of Glass

Analysis of glass found at a crime scene can yield trace evidence.

The earliest human-made glass objects (beads) date back to about 2500 B.C. Egypt.

Specialized glass production was an art, a science, and a state secret in the republic of Venice over a span of hundreds of years.

By the fourteenth century, the knowledge of glass production spread throughout Europe.

The industrial revolution brought the mass production of many kinds of glass.


What Is Glass? What Is Glass?

Once it cools, glass can be polished, ground, or cut for useful or decorative purposes. Glass blowers can form glass into many different shapes.

Crystalline solids have a regular atomic structure (illus-trated above, left). But glass is an amorphous solid and so has an irregular atomic structure (illustrated above, right),

Because of this, glass breaks in a variety of fracture patterns.


Types of Glass Types of Glass

Glass is an amorphous solid usually made from silica, calcium oxide, and sodium oxide.

Because glass is made of a variety of compounds, there are many types of glass.

Adding different metal oxides to glass mixtures, for example, yields different colors.

And because different types of glass have different densities, refraction indexes and fracture patterns, it is possible to compare one type of glass with another.


Density

The formula for calculating density is: D = m / V

The mass (m) of a fragment of glass can be found using a balance beam device.

Place the fragment of glass into a beaker filled with water and measure the volume (V) of overflow.

Divide the mass (in grams) by the volume (in milliliters) to find the density (D) of the glass fragment.


Refractive Index

When a beam of light moves from one medium into another, its speed changes. That change causes the beam to change direction, bend.

In the illustration above, the green line to the right of the red line shows the direction if the beam had not changed direction. But the black line shows it bent toward the red line.


Refractive Index

Refraction is the change in the direction of light as it changes speed when moving from one medium into another.

The direction of the light forms two angles with the normal .

If the light passes into a denser medium (the gray area), its direction will bend toward the normal.


Application of Refractive Index Application of Refractive Index to Forensicsto Forensics

If light passes from water into air, will it bend toward or away from the normal?

If light passes from air into glass, will it bend toward or away from the normal?

Light passed through two fragments of glass—one found at the crime scene and the other connected to a suspect—may have the same refraction and, so, link one with the other.


Application of Refractive Index Application of Refractive Index to Forensicsto Forensics

If the glass fragments found at the crime scene, or connected with the suspect, or both are too small to check for this consistency by passing light through them, other methods such as the submersion method can be used to provide estimates.

Use the illustration above to describe the submersion method for obtaining refraction estimates.


Fracture Patterns in Broken Glass

Glass can stretch slightly when hit. And because it’s an amorphous solid, it will not

break into regular pieces with straight line fractures.

That means fracture patterns can provide clues about the direction, rate, and sequence of the impacts.


Why Radial and Concentric Fractures Form

Impacted glass is compressed on the side it is hit. It will stretch on the opposite side of the glass, and

the tension there will radiate breaks in the glass outward from the point of impact.

Then fractures form in the shape of concentric circles on the same side of the impact.


Bullet Fractures

As a bullet passes through glass, it pushes a cone shaped piece of glass out of the glass ahead of it.

This makes the exit side of the hole larger than the entrance side of the hole.

Radiating fracture lines from a subsequent shot will stop at the edge of the fracture lines already present in the glass.


Path of a Bullet Passing through Window Glass

The entry hole will be round if the bullet was fired perpendicular to the pane.

If fired from an angle, glass pieces will be forced out to the opposite side from the shot.

The angles at which bullets enter window glass can help locate the position of the shooter.

Bits of the glass can fly backward (backscatter), possibly creating trace evidence.


Handling of Crime Scene Glass Samples

1. Identify and photograph any glass samples before moving them.

2. Collect the largest fragments that can be reasonably collected.

3. Identify the outside and inside surface of any glass. 4. If multiple panes are involved, make a diagram. 5. Note trace evidence such as skin, hair, blood, or

fibers. 6. Package all materials collected to maintain the chain

of custody.


. . . . . . . . . . . . . . . . . Summary. . . . . . . . . . . . . . . . . Summary

Different kinds of glass can be analyzed for their density, refractiveness, and fracture patterns.

The different compounds that make up a kind of glass affect its density.

The refractiveness of glass can be measured in a number of ways.

Fracture patterns can provide information about such things as the direction, the rate, and the sequence of the impacts.

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glass as evidence