Glass as Evidence

Unit OverviewUnit Overview

• Most often the composition of glass is Most often the composition of glass is uniqueunique and therefore and therefore identifiableidentifiable..

• In science In science what type of propertieswhat type of properties can we can we test?test?

PhysicalPhysical vs. vs. ChemicalChemical PropertiesProperties

• The forensic scientist can determine The forensic scientist can determine distinguishingdistinguishing physical physical and and chemical chemical characteristics.characteristics.

• Glass can be identified its Glass can be identified its chemicalchemical compostioncompostion, , densitydensity, and also by its , and also by its refractive indexrefractive index..

Physical vs. Chemical PropertiesPhysical vs. Chemical Properties

• The The bending bending of light waves because of a of light waves because of a change in velocity is called change in velocity is called refraction.refraction.

Forensic Science: Fundamentals & Investigations, Chapter 14 5

Application of Refractive Application of Refractive Index to Forensics Index to Forensics

Submersion Submersion methodmethod—used—used when glass when glass fragmentsfragments found found at the crime at the crime scene are scene are small small

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Application of Refractive Application of Refractive Index to Forensics Index to Forensics

• Place the glass Place the glass fragment into different fragment into different liquids of known liquids of known refractive indexesrefractive indexes

• The glass fragment will The glass fragment will seem to disappear seem to disappear when placed in a liquid when placed in a liquid of the same refractive of the same refractive indexindex

Video

• http://forensics-glassevidence.weebly.com/video.html

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Application of Refractive Index to Application of Refractive Index to

ForensicsForensics

• Becke LineBecke Line a halo-like effecta halo-like effect appearing appearing at the edges of a glass fragment when at the edges of a glass fragment when the the reflective indexreflective index of the glass and liquid of the glass and liquid are are differentdifferent– If the line is If the line is insideinside the glass perimeter, the the glass perimeter, the

glass index is glass index is higherhigher than the index of the than the index of the liquidliquid

– If the line is If the line is outsideoutside the glass perimeter, the the glass perimeter, the glass index is glass index is lowerlower

Glass has higher refractive index-note white line inside

• Glass has lower refractive index-note white line outside

Becke Lines

What is Glass?What is Glass?• Glass is an Glass is an amorphousamorphous solidsolid usually usually

made from silica, calcium oxide, and made from silica, calcium oxide, and sodium oxide.sodium oxide.

• Silica’s chemical name is Silica’s chemical name is silicon silicon dioxidedioxide

Would this structure affect HOW glass Would this structure affect HOW glass breaksbreaks??

11

Types of GlassTypes of Glass

• Glass used in vehicle wind shields have Glass used in vehicle wind shields have been created to not break apart when hit.been created to not break apart when hit.

What type of glass is this?

How is glass used by Forensic How is glass used by Forensic Scientist?Scientist?

Glass as evidence

• By By analyzinganalyzing the the fracture patternsfracture patterns, forensic , forensic investigators can determine investigators can determine what what object hit object hit the glass and the glass and from which directionfrom which direction the the object came, object came, sequencesequence of events and of events and compositioncomposition of specialized glass of specialized glass

CLASS or INDIVIDUAL CLASS or INDIVIDUAL EVIDENCEEVIDENCE ? ?

Glass is ClassUnless….

Glass evidence is considered class Glass evidence is considered class evidence, because it is mass-producedevidence, because it is mass-producedHowever However if pieces of broken glass fit… if pieces of broken glass fit…

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Fracture Patterns inFracture Patterns inBroken Glass Broken Glass

• Fracture patterns provide clues about Fracture patterns provide clues about the the directiondirection, , raterate, and , and sequencsequence of the e of the impacts impacts

What pressure is found on ENTRY and EXIT side of glass?

Compression and Tension

ROT

• Radial-----Opposite side----Tension

CCS

• Compression---Concentric---Same side

•

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Path of a Bullet Passing through Path of a Bullet Passing through Window Glass Window Glass

• The The anglesangles at which bullets enter window glass at which bullets enter window glass help help locate the positionlocate the position of the shooter of the shooter

• Bits of the glass can fly backward (Bits of the glass can fly backward (backscatterbackscatter), ), creating trace evidencecreating trace evidence

perpendicular to the glass

shot from the left

shot from the right

Entry and Exit holes

Blunt Object FracturesBlunt Object Fractures

Heat FracturesHeat Fractures

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Handling of Crime Scene Handling of Crime Scene Glass Samples Glass Samples

1.1. Identify and photographIdentify and photograph any glass samples before any glass samples before moving them. moving them.

2.2. CollectCollect the largest fragments that can be reasonably the largest fragments that can be reasonably collected. collected.

3.3. IdentifyIdentify the outside and inside the outside and inside surfacesurface of any glass. of any glass.

4.4. If multiple panes are involved, If multiple panes are involved, make a diagrammake a diagram. .

5.5. Note trace evidenceNote trace evidence such as skin, hair, blood, or fibers. such as skin, hair, blood, or fibers.

6.6. PackagePackage all materials collected to maintain the all materials collected to maintain the chainchain of of custodycustody. .