Forensic Glass Analysis: Physical, Chemical Properties & Light Behavior

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.

Chemical properties

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

Example of physical property in forensic science

Density & Refractive Index are 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.

Example of chemical property in forensic science

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

Heroin + Marquis Reagent

Turns purple.

Density

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 a specific object

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 Method 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.

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

Density Result

D = 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.

Spectrophotometer

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

Light Behavior Models

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

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

Wavelength

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.

Refraction Cause

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.

Refractive 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 Fill in the Blank

Refraction is the ________ of the light wave. Refraction is caused by the change of ________ from air to a different medium.

Refraction Definition

Refraction is the bending of the light wave.

Refraction

The bending of the light wave.

Cause of Refraction

Caused by the change of speed from air to a different medium.

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.

Match Point

When the RI of the glass particle is equal to the RI of the liquid medium.

Becke Line

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

Becke Line Apparent

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

Becke Line Not Apparent

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

Becke Line Position

Becke line on outside of glass when RImed is higher.

RIglass (1.34) < RImed (1.525)

Indicates the glass has a lower refractive index than the medium.

RIglass (1.6) > RImed (1.525)

Indicates the glass has a higher refractive index than the medium.

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.

Understanding Check Question

Is the RI index of the medium higher or lower than the RI index of the glass?

Understanding Check Answer

Higher.

Glass

A hard, brittle, amorphous material.

Amorphous solid

A solid in which the atoms are arranged in a random fashion.

Birefringence

The numerical difference between two refractive indices in crystalline solids.

Non-crystalline structure

A structure that does not experience birefringence.

Silica

The primary ingredient in glass, also known as silicon dioxide (SiO2).

Calcium oxide

Also known as lime (CaO), added to prevent glass from becoming soluble in water.

Sodium oxide

Also known as soda (Na2O), added to reduce the melting point of silica.

Formers

Substances that make up the bulk of the glass, such as silicon dioxide (SiO2), boron trioxide (B2O3), and phosphorus pentoxide (P2O5).

Fluxes

Substances that change the temperature at which the formers melt, such as sodium carbonate (Na2CO3) and potassium carbonate (K2CO3).

Stabilizers

Substances that strengthen the glass and make it resistant to water, such as calcium carbonate (CaCO3).

Oxides in glass

The raw materials for making glass, which can be expressed in terms of percentage of each oxide used.

Window or bottle glass composition

Approximately 73.6% Silica (SiO2), 16.0% Soda (Na2O), 5.2% Lime (CaO), 0.6% Potash (K2O), 3.6% Magnesia (MgO), and 1.0% Alumina (Al2O3).

Soda-lime glass

The most basic, common, inexpensive glass used for manufacturing windows and bottles.

Borosilicate glass

Glass that can withstand a wide range of temperatures without breaking, used in kitchen and laboratory glassware.

Tempered glass

Stronger than ordinary glass, strengthened by rapid heating and cooling, breaks into small squares.

Laminated glass

Constructed by bonding two ordinary sheets of glass together with a plastic film, used for windshields.

Soda (NaCO3)

A component used in glass making.

Lime (CaO)

A component used in glass making.

Borosilicates

A type of glass that uses boron oxide and can withstand high heats.

Glass Elasticity

Glass has a certain degree of elasticity and breaks when its elastic limit is exceeded.

Radial Fractures

Fractures that first form on the side of the glass opposite to where the impact originated.

Concentric Fractures

Fractures that always form on the same side of the glass as the impact originated.

Sequence of Bullet Holes

Radial fractures from a second bullet hole terminate into the fractures from the first bullet hole.

Determining Bullet Direction

Compare the size of the entrance hole to the size of the exit hole; exit holes are always larger.

3R Rule

Radial cracks form a right angle on the reverse side of the force; concentric cracks are the opposite.

Collecting Glass as Evidence

Avoid loss or contamination of evidence samples and photograph all glass samples before moving them.

Glass Fragment Collection

Collect the largest fragments and identify the outside and inside surfaces of any glass.

Relative Position of Window Panes

Indicate the relative position of multiple window panes in a diagram.

Trace Evidence

Note any other trace evidence found on or embedded in the glass, such as skin, hair, blood, or fibers.

Chain of Custody

Package all collected materials properly to maintain the chain of custody.

Physical Property Separation

Separate the glass by physical property, such as size, color, and texture.

Cataloging Samples

Catalog the samples and keep them separated to avoid contamination between two different sources.

Examining Clothing for Glass Fragments

Examine any clothing related to the crime scene for glass fragments and other trace evidence.