Glass Analysis 2011 SV

8/10/2019 Glass Analysis 2011 SV

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Forensic Examination of Glass

Goals in examining glass evidence: Determine the ____________________________

Determine _______________________________

Use _____________________________________

________________ the glass to a source

Compare physical and chemical characteristics:

_______________________: surface wear,

striations from manufacturing, thickness,

surface film or dirt, hardness, density

__________________: color and refractive index

_________________: additives or trace elements


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What is Glass?

Fusion of ________ (SiO2), __________(Na2CO3) &_________ (CaO) that produces a transparent solid

when cooled.

An extended, 3D network of atoms which lacks the

repeated, orderly arrangement typical of crystallinematerials.

_______________ properties: hard, elastic, brittle,

non-conductor of electricity, softening point,

density, refractive index, etc.


3/22

What Types of Glass Are There?

____________: made by a float glass process;molten glass is floated on a pool of tin whilecooling. Commonly found in doors and windows.

_____________________: used in windshields,

two sheets of glass with plastic between them.

__________________________: used in car sidewindows and designed to break into tiny pieces;potassium (K) replaces sodium (Na) on the

surface.


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____________________: most common type, madefrom silica (SiO2), soda (Na2CO3) and lime (CaO).

__________________________: 5% borax (Na2B4O7)is added to resist breaking when heated or cooled.

____________________: metal oxides or colloidaliron (Fe) & sulfur (S) are added to change its color.

_________: Pb increases refractive index & density

What Types of Glass Are There?


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Each force causes a deformation that may leave avisible mark or fracture the glass. This can be used

to determine the direction and amount of force.

Initially, glass acts initially as an _______________surface and bends away when a force is applied.

How Does Glass Break?


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__________ cracks form first and are propagatedin short segments on the side opposite the force.

____________ cracks come later from continued

pressure on the same side as the force applied.



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Edges of broken pieces of glass will show__________ (stress) marks.

In a radial crack, the rib marks are

perpendicular to unloaded side andparallel to loaded side. The arrow shows

the side that received the impact.

3R rule: R____ cracks give rib marks that make

R______ angles on the

R______ side from where the force was

applied


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Exceptions to the Three R Rule

Tempered glass_________ without forming ridges

Very small windows held tightly in frame

cant bend or bulge appreciably

Windows broken by ___________________

no point of impact

curved, ________ edges at break points


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Types of Fractures by Projectiles

Bullets are a projectile force (load) that can passthrough glass.

__________________ is the entrance side;

unloaded side is the exit side.

_______________________ projectiles: rib marksmay indicate where breaking force was applied

As the bullets _______ increases, the central hole

becomes smaller, cracking patterns become

simpler, and the exit hole becomes _________than the entrance hole.


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Fractures by Projectiles

Stress lines on the glass edge of_________cracks

form a rightangle on the reverseside from the force.

Stress lines on the glass edge of___________ cracks

form a rightangle on the same side from the force.

________(unloaded)

side is________than

entry (load) side.

Which side was the bullet fired from?


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Which Bullet Hole Was First?

In the figures above, which impact occurred first?

The sequence of impacts can be determined sincecrack propagation is stopped by earlier cracks.


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Putting it Back Together Again?

Examiners can fit together two or more pieces ofglass that were broken from the _________ object.

Because glass is __________________, no two

glass objects will break the same way.


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Glass Transfer Evidence

When broken, glass flies ___________from all parts of the object where cracks

appear not just from point of impact.

This creates a shower of ________ glassparticles and a transfer of evidence.

__________________________depends

finding and measuring properties thatwill associate one glass fragment with

another while eliminating other sources.


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Optical Properties of Glass

Make side-by-side comparisons using similar-sized fragments.

Place samples on a ________ surface using

____________ light.

Use both ______________ and _____________

light to determine the glasss color.

Visual color analysis is very ______________.

_________ and ____________ can be almost

impossible to extract.


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Nonoptical Physical Properties of

Glass

__________________________________ Rollers leave parallel ream marks on sheet glass

Markings may indicate the glasss orientationwhen pieces are ______________

Surface scratches, etchings, and other markingsmay also be used to individualize evidence

______________________

Hardness=5-6 on _____ scale; use a scratch test.

Determinations of curvature can distinguish flatglass from container, decorative, or ophthalmic.


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Why Measure Density?

Can be used as a __________ ___________ withlarge numbers of fragments.

Useful in identifying ____________ sources present

in the known and/or questioned samples.

It is _______________ and an ___________ property

(not dependant on sample mass).

Need to measure very precisely in parts per

_______________________________.


17/22

Glass Density

Density can be measured by: directly determining ______ and

_______ (usually by displacement)

comparison by ____________

comparison using a density gradientcolumn

Density gradient column method:

Fragments of different densities

settle at different ___________ in thecolumn

Technique is not accurate forfragments that are ____________


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Density by the Flotation Method

A glass particle is immersed in a liquid. The densityof the liquid is adjusted by the addition of ___________________________________ until the glass chipremains _______________.

At this point, the glass will have the ______ densityas the liquid medium and can be compared to otherrelevant pieces of glass which will remain__________________________.


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Why Measure Refractive Index?

Refractive index: ratio of the velocity of light in a__________ to the velocity of light in any other

_______________.

For example, at 25oC the refractive index of water is1.333. This means that light travels 1.333 timesfaster in a vacuum than it does in water.

Like density, refractive index is an ______________

property but it can be measured very precisely(0.0002) and does not ____________ the sample.

Refractive index of glass varies with small changes

in composition or by how it is manufactured.


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Chemical Analysis of Glass

___________________

Under UV radiation, many glasses exhibitfluorescence (glow)

Caused by heavy metals (including tin) fromfloat process or organic coatings

_______________________________________________________________________

Can determine many elements simultaneously

Surfaces of samples (>50 mg) can be analyzed

___________________________________________

You must first know which elements are present

Can analyze ppm levels of elements present


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The Wave of the Future: _____________

Laser Ablation Inductively Coupled Plasma MassSpectrometry (LA-ICP-MS)

Laser burns off a microscopic sample

Elements are ionized by plasma

Detects 46 trace elements and their isotopes

simultaneously in glass at < 1 ppb

Glass Analysis 2011 SV

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