Forensic Glass Analysis: Properties, Identification, and Techniques

Physical Properties

Examples include weight, volume, density, color, boiling point, melting point, refractive index, etc.

Chemical Properties

Examples include burning, formation of gas, rusting, decomposition, color change.

Wavelength and Frequency Relationship

Wavelength and frequency have an inverse relationship meaning that as one increases, the other decreases and vice versa.

Spectrophotometer

An instrument that measures the quantity of radiation (light) that a particular material absorbs as a function of wavelength and frequency.

Intensive Property

A property that does not depend on the amount of material (ex: density, refractive index).

Density Formula

Density = mass/volume (D=M/V).

Density of Glass Fragment

Density = mass/volume = 0.195 g/mL.

Refraction

The bending of light caused by the change of velocity from one medium to another.

Refractive Index

The measure of how much the light is bent; it is the ratio of the speed of light in a vacuum (air) to the speed of light in the medium under examination.

Factors Affecting Density & Refractive Index

Temperature.

GRIM 3

A computer/video system used with a hot stage microscope to automatically adjust the temperature of the immersion oil & calculate RI of glass at match point.

Crystalline Solid Refraction

It splits the beam into two, causing double refraction.

Birefringence

The numerical difference between the two refractive indices of the double refraction.

Glass Solid Type

Glass is considered an amorphous, noncrystalline solid.

Birefringence in Glass

No, glass does not exhibit birefringence.

Soda-Lime Glass Composition

Contains Sand (SiO2), Soda (NaO2), and Lime (CaO).

Examples of Soda-Lime Glass

Windows and Bottles.

Borosilicate Glass Composition

Contains Sand (SiO2) and Boron oxide.

Purpose of Boron Oxide

To increase thermal (heat) resistance.

Examples of Borosilicate Glass

Test tubes, laboratory glassware, headlights, kitchenware.

Tempered Glass Production

Made by rapidly heating and cooling it.

Laminated glass

Made by sandwiching a plastic layer between two pieces of glass.

Flotation Method

Used to compare the density of glass fragments by suspending them in a liquid.

Density

One of the two most important physical properties used to identify and compare glass fragments.

Immersion Method

Used to find the refractive index of glass by immersing it in a liquid with a known refractive index.

Becke Line

When the evidence sample is immersed in the liquid and the Becke Line around the sample disappears, it indicates that the refractive indices are the same.

Radial cracks

Form on the opposite side of the force's origin.

Concentric cracks

Form on the same side as the force's origin.

3R Rule

To determine the direction of impact; Radial cracks form a Right angle on the Reverse side of the force.

Gunshot penetration analysis

The first shot's crack will be uninterrupted by the following shots.

Collecting glass fragments

Collect all the glass found if there is a possibility that glass fragments can be pieced together for an individual fit.

Evidence collection for glass

Evidence collectors must submit all glass evidence found in the possession of the suspect along with a representative sample of broken glass remaining at the crime scene.

Best containers for glass collection

Rigid, sealable containers like pill bottles.

Packaging for clothing or shoes

Individually wrapped in paper.

Example of laminated glass

Windshields.

Example of glass that shatters

Shower doors, side car windows, rear car windows.

Density comparison in flotation

If the evidence sample floats on top of the liquid medium, the glass evidence is less dense than the liquid.

Refractive index comparison

If the Becke Line is inside the border of the sample, the glass has a higher RI than the liquid.

First cracks to form

Radial cracks.

Second cracks to form

Concentric cracks.

Initial Examination

If possible, determine the color, fluorescence, surface features, curvature, and thickness of the glass.

Examine Fractures

Are they radial or concentric? Crater or hackle? A fracture match is an absolute means of identification.

Measure the density

Density tells you something about the composition and thermal history of the glass.

Measure the refractive index

Refractive index is the most commonly measured property in forensic glass analysis. It can give you the same information as density, but you only need a very small piece of glass.

Testing both density and refractive index

Tells you more than just checking one or the other.

Determine major, minor, and trace elements in the glass

These methods are destructive and are used AFTER density & refractive index are determined.

Scanning Electron Microscopy (SEM)

A method used to determine elements in glass.

X-ray Fluorescence Spectrometry (XRFS)

A method used to determine elements in glass.

Inductively Coupled Plasma-Optimal Emission Spectrophotometry (ICP-OES)

A method used to determine elements in glass.

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)

A method used to determine elements in glass.

Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS)

A method used to determine elements in glass.

Atomic Absorption Spectrophotometry (AA)

A method used to determine elements in glass.

Physical properties

Characteristic of a substance that can be observed or measured without changing the identity of the substance.

Examples of physical properties

Weight, volume, density, color, boiling point, melting point, & refractive index.

Chemical properties

Describes the substance's behavior when it reacts or combines with another substance in a chemical reaction.

Examples of chemical properties

Burning, formation of a gas, rusting, decomposition, color change.

Density & Refractive Index

Used to identify different types of glass.

Headlight glass density

Density = 2.47-2.63 g/mL.

Window Glass density

Density = 2.46-2.49 g/mL.

Color Change in drug tests

Color change in a drug color test can be used to identify the drug that was seized.

Heroin + Marquis Reagent

Turns purple.

Density definition

Density is defined as the mass per unit volume (D = m/V).

Density calculation example

If an object had a mass of 1.3 g and a volume of 1.0 mL, the density would be calculated by dividing mass by volume (1.3 g/1.0 mL) so the density of the object would be 1.3 g/mL.

Density calculation for given mass and volume

Calculate the density of an object that has a mass of 1.54 g and a volume of 1.37 mL: D = 1.54 g/ 1.37 mL = 1.12 g/mL.

Density - Water Displacement Method

Use water displacement method for finding the volume of an irregularly shaped object!

Density - Water Displacement Example

An irregularly-shaped sample of glass is put on a balance and found to have a mass of 7.8 g. The initial volume reading is 25.5 mL, and after the glass is added, the water level has risen to 41.7 mL. Find the density of the glass sample in g/mL.

Change in Volume (ΔV)

ΔV = V2 - V1 = 41.7 mL - 25.5 mL = 16.2 mL

Density Formula

D = m/V

Density Calculation

D = 7.8 g/16.2 mL = 0.48 g/mL

Density - Flotation Method

The flotation method is a rather precise and rapid method for comparing glass densities.

Flotation Method Process

A standard/reference glass sample of a known density is immersed in a liquid. The density of the liquid is carefully adjusted by the addition of small amounts of an appropriate liquid until the glass sample remains suspended in the liquid medium.

Flotation Method Conclusion

At this point, the standard/reference glass sample and the liquid medium have the same density.

Evidence Sample Comparison

Now, the glass evidence sample can be added to the same liquid for comparison. If the evidence sample is suspended alongside the standard sample, their densities are SIMILAR. If the evidence sinks or floats in the liquid, the evidence sample is NOT made from the standard sample.

Density Table of Glass Types

(g/mL)

Theory of Light

Knowledge of nature & behavior of light is fundamental to understanding physical properties important to the examination of forensic evidence.

Characterization of Substances

Forensic scientists have learned to characterize different chemical substances by the type and quantity of light they absorb, often using a spectrophotometer.

Models of Light Behavior

Two models describe the behavior of light: Light traveling in space is described as a continuous wave. However, once light is absorbed by a substance it behaves as a stream of photons or energy particles.

Frequency Definition

Number of crests (or troughs) passing any one given point in a unit of time is frequency.

Wavelength Definition

Distance between two consecutive crests (high points) or troughs (low points) is wavelength.

Wavelength and Frequency Relationship

Wavelength & frequency have an inverse relationship, meaning as one increases, the other decreases.

Refraction of Light

Refraction is the bending of light as it changes speed when it moves from one medium to another.

Effect of Medium on Light

The speed of light changes due to the density of the medium which causes the light to bend or refract.

Refraction Direction

When light travels from a fast medium into a slow medium, it bends towards the normal line. When light travels from a slow medium into a fast medium, it bends away from the normal line.

Refraction Index (RI)

Refractive index (RI) is a measure of how light bends as it passes from one substance to another. It is the ratio of the speed of light in a vacuum (air) to the speed of light in the medium under examination. RI is always greater than 1.

Refractive Index Example

At 25°C, the refractive index (RI) of water is 1.333. This means that light travels 1.333 times faster in a vacuum (air) than it does in water.

Density and Refractive Index

Density and refractive index are intensive properties. This means they stay the SAME regardless of sample size.

Temperature Effect on Properties

These properties stay the same at the same temperature. When temperature changes, density and refractive index will also change.

Refraction Definition

Refraction is the bending of the light wave.

Cause of Refraction

Refraction is caused by the change of density from air to a different medium.

Refraction

The bending of the light wave.

RI - Submersion Test

Used to determine the refractive index (RI) of glass.

RI - Submersion Test Method

Involves placing the glass fragment into a series of different liquids of known refractive indexes.

Glass and Liquid RI Match

If the glass and a liquid have the same refractive index, the glass fragment will seem to disappear when placed in the liquid.

GRIM 3

Computer/video system used with a hot stage microscope to automatically adjust the temperature of the immersion oil & calculate RI of glass at match point.

Match Point

Glass particle is immersed in a liquid medium whose refractive index is adjusted by varying its temperature until the RI is equal to the RI of the glass particle.

Becke Line

A halo-like ring that appears around the edge of glass when placed in a liquid medium with a refractive index that differs from the refractive index of the glass.

Becke Line Apparent

The Becke Line is apparent, so the RI of the glass and the medium are NOT the same.

Becke Line Not Apparent

The Becke Line is NOT apparent, so the RI of the glass and the medium are the same.

Becke Line Position with Higher RI

Becke line on outside of glass when RImed is higher.

Becke Line Position with Lower RI

Becke line on inside of glass with RImed is lower.