RAKESH KUMAR MISHRAID. NO.15MSFS035M.Sc.(FORENSIC SCIENCE) 3rd semester

GLASS…..•Glass is a collidle mixture of silica with oxide of two or more metals. •Fusion of sand (SiO2), soda (Na2CO3) & lime (CaO) that produces a transparent solid when cooled.

•Physical properties: hard, elastic, brittle, non-conductor of electricity, density, refractive index, etc.•Chemical: resistant to all but fluorine and very strong bases.

PROPERTIES OF GLASS..

Types of Glass….

Flat glass: made by a “float glass process”; molten glass is floated on a pool of tin while cooling. Commonly found in doors and windows.Laminated glass: used in windshields, two sheets of glass with plastic between them.Tempered safety glass: used in car side windows and designed to break into tiny pieces; potassium (K) replaces sodium (Na) on the surface.Light sensitive glass.. These glass contain of collide particle of silver halide.

SODA LIME GLASS.. This is the most common type of glass being used for manufacturing window , bottle , container, light bulb.BOROSILICATE GLASS… Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituentsLab glassware, thermometers, cookware.

WIRED GLASS… Singal sheet of glass with the layer of mixed wire completely inside the glass.

Bulletproof Glass:- Bulletproof glass is a combination of two

or more types of glass, one hard and one soft.

The softer layer makes the glass more

elastic so it can flex instead of shatter.

The index of refraction for both of the glasses used in the bulletproof layers must be almost the same to keep the glass transparent and allow a clear view through the glass.

Bulletproof glass varies in thickness from three-quarter inch to three inches.

Goals in examining glass evidence

Determine the types of glass at the scene

Determine how the glass was fractured

Use physical characteristics to classify it

Individualize the glass to a source

Methods of examination

1 Physical examinationPhysical comparisonPhysical measurementsFluorescence under UV radiationPhysical matching Density or Specific gravityRefractive Index2 CHEMICAL examinationScanning Electron Microscopy Energy Dispersive X-ray

AnalysisAtomic Absorption Spectroscopy

Physical compariso

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Appearance: Transparent / milky /

ground / colored.Flat mirror / curved / patterned / polished

Type of glass: Ordinary / Pyrex /

borosilicate / laminated / tempered / toughened /

wired glass.Window pan / glassware / automobile glass / head light glass / optical glass bottle glass etc.

Physical measurements

Edge thickness: A micrometre is used to measure accurately the edge thickness of

the glass fragments. the broken edged to find out at which point, the crime exhibits

matches with any portion of the broken glass

Curvature: A spherometer is measure the radius of curvature of the glass fragments having curved surface. The radius of curvature of the fragment is calculated using the formulae… R = (l2/6h)+(h/2)Where, I = the mean distance between the legs of the spherometer. h = height of the curved surface

Fluorescence under UV radiation

Some types of glass fluorescence under ultraviolet radiation with different colours which may be brown, violet, purple, blue or green

This examination has to be conducted in a dark room and the glass pieces to be exposed to UV radiation should be of similar size and thickness and they are to be thoroughly washed with acetone or any other suitable solvent to remove

any grease or dirt.

When there is a clear difference in the fluorescence of the two glasses, it indicates different sources of their origin.

Physical matching…

This is most conclusive proof of source correspondence, since no two fractures will ever be identical over any appreciable length. A complimentary lateral fit along the broken

edges over a length of quarter inch (1/4) or more establishes that the two glass garments were continuous before breakage.

By naked eye or under a microscope, we should search carefully the edges of the samples, which will exactly fit into each other, taking into consideration, the factors contributing to the matching, such as general appearance, colour, edge thickness, shape of breakage, all

the irregularities and striations near the broken surfaces.

Lateral pressure has to be applied, in order to see that the small irregularities intermesh and hold the pieces together, so that we can feel the sense of exact fit along the broken

edges. This is especially important for matching of smaller fragments.

MEASURMENT OF DENSITY

DENSITY ???

Density as the mass per unit volume. (D = M/V) Density is an intensive

property of matter, meaning it remains the same regardless of sample size.

It is considered a characteristic property of a substance and can be used as an aid in identification.

Why Measure Density?

Can be used as a screening technique with large numbers of fragments.

Useful in identifying multiple sources present in the known and/or questioned samples.

It is nondestructive and an intensive property (not dependent on sample mass).

Need to measure very precisely in parts per hundred or thousand or better.

Flotation Method…..The flotation method is used for finding the density of glass.

An unknown glass fragment is immersed in a liquid of known

density.

The density of the liquid is carefully adjusted until the glass chip floats (remains suspended in the liquid).

When the glass fragment floats, it has

the same density as the liquid.

COMPARISON USING A DENSITY GRADIENT COLUMN…

Glass tube is filled with liquids of different densities ( bromoform &

bromobenzene ) 9:0 ,8:1 , 7:2 , 6:3 , 5:4 , 4:5 , 3:6 , 2:7 , 1:8 , 0:9

Glass is added and particles will sink to the portion of the tube that has a

density of equal value and remain there suspended

The density distribution pattern of glass particles can be obtained and

compared to other specimens with the same method

Measurement of Refractive index...Refractive index ?????

•Refractive index: ratio of the velocity of light in a vacuum to the velocity of light in any other medium.

Why Measure Refractive Index?

Refractive index is an intensive property but it can be measured very precisely (±0.0002) and does not destroy the sample.

Refractive index of glass varies with small changes in composition or by how it is manufactured.

Refractive index by immersion method:

Immersing a glass particle in a liquid medium (silicone oil) whose refractive index can varied with temperature until it

is equal to that of the glass particle.

At this point, known as the match point, the Becke line disappears and minimum contrast between liquid and

particle is observed: RI oil = RI glass.

The Becke line is a bright halo near the boarder of a particle that is immersed in a liquid of a different refractive

index.

Refractive index by hot stage method:the glass is emerged in high boiling

liquid usually silicon oil & it was heated at a uniform rate until the

matched point appear.

Melter hot stage is commonly used for changing & maintaining the exact temperature i.e. 0.1˚C.

The emersion liquid used must have property to change refractive index with

change in temperature & quantity.

CHEMICAL ANALYSIS OF GLASS

• Can determine many elements simultaneously

• Surfaces of samples (>50 mg) can be analyzed

Scanning Electron

Microscopy Energy

Dispersive X-ray Analysis

• You must first know which elements are present

• Can analyze ppm levels of elements present

Atomic Absorption

Spectroscopy

How Do Glass Windows Break? Each force causes a deformation that may leave a visible mark or fracture the glass. These markings can be used to determineDirection of the force, amount of force applied, sequence of impactsGlass acts as an elastic surface and bends away when an initial force is applied. When the force increases beyond its tensile strength, it CRACKS

Fracture pattern of

glassRadial cracks—form FIRST and

span out in short (spider-like)

segments on the OPPOSITE side of

the force

Concentric cracks—

come SECOND on the SAME side as the force applied

http://videos.howstuffworks.com/discovery/33704-time-warp-regular-glass-break-video.htm

3R rule:Radial cracks give rib marks that make

Right angles on the

Reverse side from where the force was applied

Exceptions to the Three R Rule..Tempered glass

“dices” without forming ridges

Very small windows held tightly in frame

can’t bend or bulge appreciably

Windows broken by heat or explosion

no “point of impact”curved, smooth edges at break points

No point of impact

Curved or wavy fractures

Smooth edges at break points

Heat fractures….

Bullet fractures…Bullets are a projectile force that can

pass through glass

EXIT side of a bullet fracture is WIDER than then entry side

As bullet velocity increases, central hole becomes smaller & cracking

patterns become simpler