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Lecture 21: Glass : A Multitasking Class of Evidence. Introduction. Glass is an important class of associative evidence.

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Lecture 21 glass a multitasking class of evidence

Lecture 21: Glass : A Multitasking Class of Evidence

Lecture 21 glass a multitasking class of evidence


  • Glass is an important class of associative evidence.

    • It is found in multiple crime types. From the smallest of splinters to an intact window with bullet holes, glass is evidence that tells a story about its relationship to the crime.

  • Example

  • A burglar breaks a window in the basement of a house and climbs through.

    • He figures out the house is alarmed but a quick inspection reveals that the basement windows were not.

  • Perhaps the owners decided that the basement didn’t need alarming, rationalizing the windows were too small for someone to crawl through. Perhaps it was a financial decision. Regardless, the window is broken and there has been a burglary.

  • Broken glass provides specific pieces of investigative information, such as the

    • Direction of the break and the

    • Location of the entry point for the burglary.

Lecture 21 glass a multitasking class of evidence

Why is Glass Important?

  • Leads to other classes of evidence: footwear impressions, fingerprints, DNA, etc.

  • Glass Offers Information:

    • For example, maybe the robbery had been staged by the people living at the house.

    • If that was the case, inspecting the glass might reveal the direction of the break – outside in or inside out.

      • If the break had been inside-to-out, it narrows the possibilities to two - assuming the window had not been broken prior to the burglary.

  • People in the house broke the window to stage burglary,

  • OR was exit point for burglar.

Lecture 21 glass a multitasking class of evidence

Always Consider Glass From It’s Forensic Value

  • Value as forensic evidence

    • A means to answer investigative questions

    • As a way to add to the puzzle of the investigation

      • Pinpointing entrance/egress points

      • Identify characteristics of the crime

        • Direction of gunshot

        • Shooter positions

        • Direction of window breaking

  • Investigator Concerns

  • Finding glass evidence,

  • Making important on-scene determinations & interpretations

  • Collecting and preserving.

Lecture 21 glass a multitasking class of evidence

  • Defined:

  • “Any of various amorphous materials formed from a melt by cooling to rigidity without crystallization: as a : a usually transparent or translucent material consisting typically of a mixture of silicates b : a material (as obsidian) produced by fast cooling of magma.”

  • Any amorphous material that melts and forms a rigid structure without crystallizing after cooling is a “glass.” This means a glass is not necessarily made of silicon. Wikipedia gives plastics and resins as examples[2].

  • Why understand in the composition of glass?

  • Different types of glass have different purposes and thus different chemical compositions … valuable because knowing the general composition of glass can give scene investigators information about how to search for it.

  • Certain types of glass that appear uncolored and transparent to the unaided eye have elemental compositions that can make them appear colored or fluoresce under UV light or the ALS.

Glass types
Glass Types

  • Different types of glass depending on the manufacturing process

    • Flat glass:

      • Sheet Glass

        • Made by rolling through rollers

        • Used for house windows

      • Plate Glass

        • Made by rollers

        • Thicker than sheet glass & highly polished

      • Float glass discriminated from double-ground & polished plate

        • Manufactured by “floating” molten glass onto surface of bath of melted tin

        • Tin diffuses into the glass & fluoresces under UV

          • Blue to yellowish glow & not on both surfaces

        • Very smooth & polished

    • Leaded Glass

      • Blue-white under UV

      • Rings like a bell after pinging with forefinger

    • Old Glass

      • Antique glass (yellow/Vaseline glass - Uranium oxide)

        • Fluoresces bright yellow/green under UV

      • Often has undissolved silica & other imperfections

Glass in a case
Glass in a Case

  • Characteristics

    • Many uses

    • Possesses different qualities

    • Breaks easily

  • Glass at the Scene

    • Ideal transfer evidence

      • Ejects very small particles in different directions

        • Retained by fabrics, footwear

Shards of glass from

Crime Scene

Glass as evidence multitasking associative evidence
Glass as EvidenceMultitasking & Associative Evidence

An example of multitasking, associative evidence … it occurs in many crime types AND provides associative forensic information that can often

significantly help the on-scene investigation

  • Homicide/burglary/rape

    • Associative evidence

      • Origin estimation

      • Broken glass tied to assailant/Burglar

  • Hit & Run

    • Associative evidence

      • Glass transfer

        • Window glass

    • Headlight glass

    • Direction of break

  • Shooting Incident Scenes

    • Direction of shot

    • Sequence of shots







Hit & Run




of Shot


of Break

How glass breaks understanding is critical to determining direction of force
How Glass BreaksUnderstanding is Critical to Determining Direction of Force

Lecture 21 glass a multitasking class of evidence

Direction of Force

  • How Glass Breaks: Understanding how glass breaks gives insight into how to determine the direction of the force that broke the glass, say a window.

  • First it bends and then begins to fracture, beginning with the formation of radial fractures, R1. As the stress on the glass continues, concentric fractures begin, C1. Fracturing continues alternating with radial (R2) and concentric (C2), etc, until the breaking process ends.







Lecture 21 glass a multitasking class of evidence

Glass Failure (Breaking)


glass failure

fully tempered

glass failure

Annealed glass failure


Lecture 21 glass a multitasking class of evidence


Direction of Force

  • When the first radial fracture begins, stress occurs along the edges closest to the point of impact,

    • Stress lines form called conchoidal fractures.

    • These form right angles opposite the side from where the force is applied,

Lecture 21 glass a multitasking class of evidence

Determining Direction of Force

Conchoidal Stress Fractures on Edge of Radial Fracture




  • Crack starts on side of glass that is opposite the applied force.

  • Remove piece of glass.

  • Locate edge corresponding to radial fracture near break

  • Examine edge for Conchoidal Fractures

  • Find (right angle) at extreme edge of fracture

  • Force came from opposite side of the right angle


of Force

Right Angle Formed

By Chochoidal Fracture

And edge of Radial Fracture

Edge of Radial Fracture

Lecture 21 glass a multitasking class of evidence

Conchoidal Striations

Forensic Examination of Glass and Paint. Caddy, B. Ed.;

Taylor & Francis: London, 1999.

Lecture 21 glass a multitasking class of evidence

Radial Edge Technique Can Fail

  • Not valid for all types of glass.

  • Tempered glass is not easily reconstructed. Actually it is a daunting task. Reconstructing hundreds or thousands of similar looking chunks of sharp-edged glass is like mission impossible … though possible … takes time, perseverance.

  • Laminated glass (automobile windshields) have special problems because of the plastic lamination holding the sandwiched glass together … radial edges are not available to examine because they are held together by the plastic laminate.

  • Thermal glass… fracture is curved … smooth edge …

    • No conchoidal fractures and thus no determinable point of origin

Lecture 21 glass a multitasking class of evidence

Bullet Through Window

  • Premise:

    • Window fractures but remains intact as the bullet passes through it carrying shards of glass with it.

  • Bullet removes cone-like chunk of window …

    • Cone forms on the opposite side of the bullet path (exit side of bullet path.

  • Shape of the cone

  • Helps elucidate direction of bullet.

  • A perfectly shaped (symmetrical) indicates a straight-through bullet

  • Skewed (asymmetric) cone shows bullet path at angle to the window

Lecture 21 glass a multitasking class of evidence

Debris Trail & Direction of Travel

  • The direction of force & approximation of the impact angle … determined by locating the trail of small shards/splinters a bullet carries with it when it exits the window.

  • Investigator must locate shards/splinters by highlighting the glass: ALS wavelength, a UV light or flashlight --- determined by chemical composition of the glass. The trail can be elusive … but it should be there.

  • The impact angle … track shard trail.

    • Bullet creates shard trail by carrying splinters of glass after passing through window. This shard trail follows bullet’s path at angle it hit the window.

Lecture 21 glass a multitasking class of evidence

Determining Approximate

Angle of Impact of Bullet through Window


Perspective: Looking Down

onto Glass Shard Trail

  • Locate glass shard trail using ALS, flashlight and/or UV light source.

  • Measure distance the smallest shards/splinters travel.

  • Using zero edge protractor, determine approximate angle bullet exited from window.

  • Confirm approximate angle by measuring length and width of bullet hole and calculating angle using sine function

Direction of

Shard Trail

Zero Edge



Top Edge

of Window


A=Angle of Impact


of Bullet

Lecture 21 glass a multitasking class of evidence

Sequencing Bullet Shots in Plate Glass

  • Locate the radial fractures and tracing them to their endpoint. If the radial fracture from, bullet 1 ends at a radial fracture from another bullet, bullet 2, then bullet 1 entered the glass second. The radial fractures (green) fired by the second bullet – on the left – were stopped from continuing by the radial fractures coming from the bullet hole on the right (blue radial fractures). Unless there is reason to believe these radial fractures are artifacts, such as elongation of radial fractures because the car had been moved, the radial fractures in green (2nd bullet) impacted second.

Tempered glass automotive glass
Tempered GlassAutomotive Glass

  • Collect all Pieces @the scene

    • Window can remain intact after shooting

      • Very fragile

        • Rarely together

          • Try to collect intact – use tape to hold together

          • Crater @ impact point rarely present

      • Can be put back together if very patient – complex jigsaw puzzle

  • Determining if shards are tempered glass

    • Interference patterns from strain in the glass

      • Observed with polarized light

        • Place over light source w/polarizing filter

        • Observe with second polarizing filter

Lecture 21 glass a multitasking class of evidence

Shot Sequence in Tempered Glass:

00 Buckshot

First Pellet Strike

Only strike that produces

Radial fractures

Radial Fractures from 1st Pellet