Dr. Sangeetha BalakrishnanPG Department of Chemistry

Women’s Christian CollegeChennai – 600 006

Glass

24 January, 2017

Forensic Science PCH/NM/01

What is Glass?• Glass is a hard, amorphous material made by

melting sand (silicon dioxide), lime (Calcium oxide) and sodium oxide at very high temperatures.

• Sodium oxide is added to reduce the melting point of sand.

• Calcium oxide is added to prevent glass from being soluble in water.SiO2 + CaO + Na2O (melt, high temp) Glass

Glass is called an amorphous solid because its atoms are arranged in a random fashion. Because of its irregular atomic structure, when glass is broken, it produces a variety of fracture patterns.

Properties of glass that are used in forensic investigations

• Density

Density = mass/volume Unit: g/mL

• Refractive Index (µ): It is a measure of how light bends as it passes from one medium to another.

µ = speed of light in vacuum / speed of light in medium(medium = glass)

Microscopic Determination of Refractive Index of Glass

• Place the glass on a slide surrounded by a liquid of known refractive index.

• Place the slide on the microscopic stage and focus the lens.

i) If the glass seems to disappear, then the glass and the liquid have the same refractive index.

ii) If the glass seems visible, then the glass and the surrounding medium have different refractive indices.

• Look for Becke Lines.

• Becke Lines: When the refractive index of the glass is

different from the refractive index of the medium, then a halo-like ring appears on the edge of the glass. This halo-like effect is called a Becke Line.

The Becke Line is located in the medium that has a higher refractive index.

Info obtained from Becke Line

• If the Becke Line is located inside the perimeter of the glass, then the refractive index of the glass is higher than the refractive index of the surrounding medium.

• If the Becke Line is located on the outside the perimeter of the glass, then the refractive index of the surrounding medium is higher than the refractive index of glass.

Refractive indices of common liquids

Fracture Patterns in Glass

• Atoms in glass are arranged in a random order; it is an _________ solid. It is for this reason that glass breaks into fragments, and not into regularly sized pieces.

Fracture Patterns

Radial Fractures

Concentric Fractures

Radial Fractures Concentric Fractures

• Start at the point of impact. • Radiate outwards from there. • Formed on the side opposite to the point of impact. • Also called primary fractures.

• Take the form of concentric circles around the point of impact.• They appear on the same side as the point of impact. • Also called secondary fractures.

What can you say about this image?

Is it possible to determine the direction of impact based on the type of fracture on the glass?

Why do fractures appear at all?• When an object (bullet/rock) hits the glass, it stretches.• On the side of impact, the glass is compressed

(squeezed together).• Due to the impact, the opposite side of glass stretches,

and is under tension. • Glass is weaker under tension than under compression. • It therefore breaks first on the weaker side, ie the side

opposite to the side of impact. This produces radial fractures first.

• After these, the concentric fractures appear.

Appearance of radial and concentric fractures when glass is hit

A Recap!

Fracture Patterns on Glass

Bullet fractures in glass

1) Exit holes and entry holes• As a bullet passes through glass, it pushes

some glass ahead of it. • It causes a cone-shaped piece of glass to exit

along with the bullet. • This cone of glass makes the exit hole larger

than the entrance hole of the bullet.

Source: Saurabh Bhargava, SlideShare

Source: Saurabh Bhargava, SlideShare

2) Order of shots fired• The first shot produces the first set of fracture

lines. • These fracture lines set the boundaries for

further fracturing by the subsequent shots.• Radiating fracture lines from a second shot

stop at the edge of fracture lines already present in the glass.

Bullet fractures in glass… cont’d

3) Angle of firing• Information on the angle of firing can help locate the

position of the shooter.• If the bullet was fired perpendicular to the windowpane,

the entry hole of the bullet will be round.• If the shooter was firing at an angle coming from left,

glass pieces will be forced out to the right. The bullet’s exit hole will form an irregular oval as it exits

to the right.• If the shooter was firing at an angle coming from right,

glass pieces will be forced out to the left. The bullet’s exit hole will form an irregular oval as it exits

to the left.

Bullet fractures in glass… cont’d

Handling of Crime-Scene Glass Samples• Identify and photograph any glass samples

before moving them.• Collect the largest fragments that can be

reasonably collected.• If multiples window panes are involves, indicate

their relative position in a diagram.• Look for trace evidence embedded in glass. Eg:

hair, fibre, blood etc.• Properly package all materials collected.• Glass pieces should not be transported in glass

vials. Why?