Shattered: Forensic Glass Shattered: Forensic Glass AnalysisAnalysis

What is Glass?What is Glass? ““An inorganic product of fusion which An inorganic product of fusion which

has cooled to a rigid condition without has cooled to a rigid condition without crystallizing” crystallizing”

Uniform amorphous solidUniform amorphous solid No specific m.p.No specific m.p. Softens over a temperature Softens over a temperature rangerange

'Ordinary' sheet glass -made by passing the molten glass through rollers; process gives an almost flat finish but with distortions. -glass is used in glazing greenhouses and garden sheds where the visual distortions do not matter.

Float glass (plate) Float glass gets its name from the method of production used to manufacture it. -molten glass is 'floated' onto a bed of molten tin-produces a glass which is flat and distortion free-process imparts a fluorescence to the glass [UV light]

Rolled Glass Float Glass

Float Glass Process

Toughened (Safety glass) -glass is produced by applying a special treatment to ordinary float glass after it has been cut to size and finished. -treatment involves heating the glass so that it begins to soften (about 620 C) and then rapidly cooling it. -produces a glass which, if broken, breaks into small pieces without sharp edges.

Laminated glass -glass is made up of a sandwich of two or more sheets of glass (or plastic), bonded together by a flexible, normally transparent material. -if cracked or broken, the flexible material is designed to hold the glass fragments in place.

What’s in Glass? What’s in Glass?

Network Components-Formers:Network Components-Formers:SiOSiO22, B, B22OO33, P, P22OO55, GeO, GeO22, V, V22OO55, As, As22OO33, Sb, Sb22OO55

Fluxes–Softeners [lowers melting point]:Fluxes–Softeners [lowers melting point]:

NaNa22O, KO, K22O, LiO, AlO, LiO, Al22OO33, B, B22OO33, Cs, Cs22OO Stabilizers–Chemical/Corrosion Stabilizers–Chemical/Corrosion

Resistance:Resistance:

CaO, MgO, AlCaO, MgO, Al22OO33, PbO, SrO, BaO, ZnO, , PbO, SrO, BaO, ZnO, ZrOZrO

Major Types and UsesMajor Types and Uses

Soda lime silicate glassSoda lime silicate glass

SiOSiO22 + Na + Na22O / KO / K22O + CaO / AlO + CaO / Al22OO33 / MgO / MgO Flat glass, container glass, electric light Flat glass, container glass, electric light

bulbsbulbs Borosilicate glassBorosilicate glass

> 5% B> 5% B22OO33 (replaces Na (replaces Na22O)O)

Lab glassware, thermometers, cookware, Lab glassware, thermometers, cookware, sealed-beam headlightssealed-beam headlights

Aluminosilicate glassAluminosilicate glass Higher percentage of aluminumHigher percentage of aluminum higher temperatures than borosilicatehigher temperatures than borosilicate Aluminoborosilicate – 50:50 Al/BAluminoborosilicate – 50:50 Al/B Labware, cookware, and glass fibersLabware, cookware, and glass fibers

Lead alkali silicate (leaded glass)Lead alkali silicate (leaded glass) Up to 80% PbOUp to 80% PbO high refractive indexhigh refractive index ““Crystal” tableware, Crystal” tableware,

Different glasses-domestic windows, car headlamps, light bulb glass-are produced differently from different chemistries.

Forensic Individualization of Forensic Individualization of GlassGlass

Flat Glass–Soda lime silicateFlat Glass–Soda lime silicate

--Rolling or FloatingRolling or Floating Tempered: Rapid CoolingTempered: Rapid Cooling

Adds StrengthAdds Strength Dices when brokenDices when broken Automotive windows & security windowsAutomotive windows & security windows Float Glass floresces when excited at 254 Float Glass floresces when excited at 254

nm.nm.

Coated: Surface modificationCoated: Surface modification MirrorsMirrors

Laminated: Sandwiched around Laminated: Sandwiched around plasticplastic

Automotive windshieldsAutomotive windshields Headlights: often borosilicateHeadlights: often borosilicate Light bulbs: soda lime glassLight bulbs: soda lime glass Heat absorbing/ UV filteringHeat absorbing/ UV filtering

tintingtinting Eyeglasses: prescription Eyeglasses: prescription

lenses/photosensitivelenses/photosensitive

Container GlassContainer Glass Lower magnesium, higher sodiumLower magnesium, higher sodium Clear vs. greenish (window)Clear vs. greenish (window)

Glass FibersGlass Fibers Fiberglass insulationFiberglass insulation Alumino-borosilicateAlumino-borosilicate Binder (red or yellow) to hold fibers in Binder (red or yellow) to hold fibers in

bundlesbundles

Forensic Analysis of GlassForensic Analysis of Glass

Fundamental Question: Is it Glass?Fundamental Question: Is it Glass? Isotropic vs. AnisotropicIsotropic vs. Anisotropic

Glass: IsotropicGlass: Isotropic dark when rotated under crossed polarizersdark when rotated under crossed polarizers

Crystalline solids and plastics: Crystalline solids and plastics: AnisotropicAnisotropic

Change retardation when rotated under Change retardation when rotated under crossed polarizerscrossed polarizers

HardnessHardness SolubilitySolubility

Examination of Glass ChipsExamination of Glass Chips

1. Physical – thickness, uniformity, color, curvature, surface conditions, soiling, density

Sink-Float MethodSink-Float Method CHBrCHBr33, C, C22HH22BrBr44, Sodium Polytungstate, Sodium Polytungstate

Density Range:Density Range:

2.465-2.540 g/cm2.465-2.540 g/cm33

2. Optical – Refractive Index

3. Elemental – Composition of Glass

Determine the Refractive IndexDetermine the Refractive Index(in a liquid medium or solid sample)(in a liquid medium or solid sample)

Oblique Illumination

Dispersion staining color

Movement of the Becke Line: most popular; see a halo or brightness near the edge of sample due to increase of distance between sample & microscope objective (Fig 16.11, p327)

When 2 substances have different RI, light passing through them produces a Becke Line

A Becke line is a band or rim of light visible along a grain/crystal boundary in plane-polarized light.

Glass analysis is often performed using our oldfriend the PLM

-the goal of glass analysisby PLM is to individualize an evidence glass fragment to anexemplar from a crime scene[primary or secondary]

How does Glass break?Forensic Fracture Analysis

RadialRadial

Begin at impact point and radiateoutward from point of impact-initial crack is on the sideopposite the applied force

ConcentricConcentric

-Cracks seeming to make a circle around -Cracks seeming to make a circle around point of impactpoint of impact

-Motion of the projectile through the glass -Motion of the projectile through the glass

puts tension on the front surface of the puts tension on the front surface of the glass, causing concentric fractures. glass, causing concentric fractures.

-high-speed projectile [a rock] penetrates -high-speed projectile [a rock] penetrates the glass, it leaves an exit hole larger the glass, it leaves an exit hole larger than it‘s entrance hole. than it‘s entrance hole.

-helps to determine the direction on impact. -helps to determine the direction on impact.

Conchoidal Lines – stress marks shaped like arches that are perpendicular to 1 glass surface and curved nearly parallel to the opposite surface; perpendicular surface faces side where crack originates; primarily found with bullet holes

A determination of the direction of force in breaking a window pane:

-direction of the rib marks[stress marks on broken edges of glass that are perpendicular to one side of glass]

For radial fractures (radiating from the center):- the direction of the force is on the same side as the tangential parts of the rib marks.

Bullets and GlassBullets and Glass

It’s possible to determineIt’s possible to determine

the bullet's direction bythe bullet's direction by

noting the side of the cone-noting the side of the cone-

shaped hole left by theshaped hole left by the

bullet.bullet.

-small opening is on the-small opening is on the

entrance entrance

sideside

Counting Bullet HolesCounting Bullet Holes

Determination of the sequence of bullet holes can be made by noting the radial fractures.

Radial fractures caused by the passage of a bullet stop at pre-existing fracture.