Glass Analysis

Mrs. Ashley

Glass Analysis

• How broken?• Link a suspect to a crime scene• Fingerprints• Blood• Match pieces by density, refractive index,

thickness, fracture patterns, color

Fractures

Concentric

Radial

bsapp.com

bsapp.com

Glass Composition

• There are thousands of different uses for glass• Formers-major portion of what makes up the

glass• Fluxes- change the temperature at which the

formers melt• Stabilizers- strengthen the glass and make it

resistant to water

Glass Ingredients

– SAND = silicon dioxide (Major Component)– SODA = Sodium carbonate to lower the melting

point– LIME = Calcium oxide. This is a METAL oxide and is

added to prevent the “SODA-LIME GLASS” from dissolving in water.

6 Major Types of Commercial Glass

• Soda-lime glass 90% of all glass, container glass and flat glass, resistance to high temperatures and sudden changes in temperature not good

• Lead glass will not withstand high temperatures, art glass, electrical insulating properties

• Borosilicate glass – high resistance to temperature change; Pipelines, light bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and bake ware

Types of Commercial Glass

• Aluminosilicate glass used as resistors for electronic circuitry

• Ninety six percent silica glass- resistant to heat shock up to 900 degrees

• Fused silica glass very difficult to make and can withstand temperatures up to 1200 degrees for short periods

Glass Uses

• Windows are made from FLOAT glass. Float glass gets its name because the SODA-LIME Glass is cooled on top of a bath of molten tin

• Tempered glass is used in the side and rear windows of automobiles.

• Laminated glass is two sheets of ordinary glass bonded together with a plastic film.

• Windshields of automobiles are made with laminated glass.

Properties of Glass• Mechanically Strong – Glass has great inherent strength and is

weakened only by surface imperfections, which give everyday glass its fragile reputation. Special tempering can minimize surface flaws.

• Hard surface – Glass resists scratches and abrasions. (Because the composition can vary, so can the hardness. On average the hardness of glass is about 5.5 on the Moh’s scale)

• Elastic – Glass “gives” under stress – up to a breaking point – but rebounds exactly to its original shape

• Chemical corrosion-resistant – Glass is affected by few chemicals. It resists most industrial and food acids.

Properties of Glass

• Thermal shock-resistant – Glass withstands intense heat or cold as well as sudden temperature changes.

• Heat-absorbent – Glass retains heat, rather than conducts it. Glass absorbs heat better than metal.

• Optical Properties – Glass reflects, bends, transmits and absorbs light with great accuracy.

• Electrical Insulating – Glass strongly resists electric current. It stores electricity very efficiently.

Additives and Color

Oxide• Iron• Manganese• Cobalt• Gold• Antimony• Copper• Lead/antimony

Color• green, brown• Amethyst• deep blue• Red• White• light blue• yellow

Direction of

Penetration

A projectile hole is inevitably wider at the

exit side

bsapp.com

Direction of

Penetration

Concentric

Radial

bsapp.com

Comparison of Radial and Concentric Fractures

Radial Fracture Concentric FractureWhen Formed? First (primary) Second (secondary)On which side of the glass

Opposite the side of force or impact

Same side as force or impact

Description Lines originate from point of impact and extend to edge of glass

Series of circles one inside the other sharing the same center

Crime Scene

• Backscatter- when window breaks, most of the fragments will be carried forward, but some will be projected backwards, called backscatter

• Heat fracture-In a fire, glass may break due to heat fracture that looks like wavy fracture lines towards the heat source

Stress MarksThe perpendicular edge of stress marks always face the surface on

which the crack originated

bsapp.com

Successive PenetrationsA fracture always terminates at an

existing line of fracture bsapp.com

bsapp.com

Forensic Examination of Glass• Review definitions and map out a plan of analysis. The FBI

definition of glass is "...an inorganic product of fusion that has cooled to a rigid condition without crystallizing.” For the analytical methods the FBI recommends, see below.

• Collect, handle, and identify the evidence.• Make your initial examination. If possible, you'll want to

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

Forensic Examination of Glass• Now measure the density of the sample. Density tells you something about the

composition and thermal history of the glass.• Measure the refractive index of the evidence. 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 methods so the FBI recommends measuring density and refractive index first (both methods are destructive). – You can use scanning electron microscopy, – X-ray fluorescence spectrometry, inductively coupled plasma-optimal emission

spectrophotometry, – inductively coupled plasma-mass spectrometry, – laser ablation-inductively coupled plasma-mass spectrometry, – atomic absorption spectrophotometry.

What is the only way to individualize glass fragments found at a crime scene to a single source?

• The ONLY way to individualize glass fragments found at a crime scene to a single source is to be able to put the pieces of crime scene glass fragments together like a jig saw puzzle with suspect glass fragments.

Tempered Glass

What physical properties are used most often to characterize glass particles? What is the main drawback of using these

properties to characterize glass?

Do the glass particles from suspect and crime scene fit together? – Most of the time the glass evidence is too

fragmentary or too minute to permit a comparison of this type.

- Composition of the glassThe general composition of most glass is the same.

• Density• Refractive Index

Refractive IndexRefractive index is represented by the letter n.The refractive index is a known quantity for different

types of glass.The FBI has a data base of over 200 refractive

indexes for different glass types and compositions.The higher the refractive index, the denser the glass.The higher the refractive index, the more the light

will bend toward the normal (produce a smaller angle)

Immersion Method of Refractive Index

The immersion method submerges the glass fragments in a liquid with a known refractive index.

The temperature of the liquid is then increased or decreased until the glass fragments disappear.

Increasing or decreasing the temperature of the liquid produces predictable changes in the refractive index.

When the glass fragments disappear- THE INDEX of REFRACTION of the glass is the SAME as the index of refraction of the liquid.

Becke Line

• A Becke line is a bright halo observed near the border of a particle immersed in a liquid of a different refractive index.

• When the Becke line is located INSIDE the glass fragment, the fragment has a higher index of refraction than the immersion liquid.

• When the Becke line is located OUTSIDE the glass fragment, the fragment has a lower index of refraction than the immersion liquid.