PHYSICAL PROPERTIES: GLASS

Physical vs. Chemical Properties Characteristics of matter for UNIQUE identity Physical properties describe a substance without reference to any other substance Weight Volume Color Boiling point Melting point Chemical properties describe the behavior of a substance when it reacts or combines with another substance

Measurement System Used throughout the world METRIC SYSTEM has basic units: Length (Meter) Mass (Gram) Volume (Liter) Common prefixes used in the metric system: Deci (10-1), Centi (10-2), Milli (10-3), Micro (10-6), Nano (10-9), Kilo (103), Mega (106), Giga (109), Tera (1012)

Important Physical Properties Temperature - a measure of heat intensity, (hotness or coldness). Celsius scale: Freezing point of water = 0°C Boiling point of water = 100°C Weight - the force with which gravity attracts a body Mass - the amount of matter an object contains, independent of gravity The mass of an object is determined by comparison to the known mass of standard objects

Important Physical Properties Density - defined as mass per unit volume, or 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

Important Physical Properties Light waves travel in air at a constant velocity until they penetrate another medium, such as glass or water, at which point they are suddenly slowed, causing the rays to bend Refraction - Bending of light waves due to a change in velocity

Important Physical Properties Refractive Index - ratio of the velocity of light in a vacuum to that in the medium under examination RI = Vacuum / examination For example: At 25oC, the refractive index of water is 1.333 This means that light travels 1.333 times faster in a vacuum than it does in water Like density, refractive index is an intensive property and will serve to characterize a substance

Glass Fragments: In Forensics Need to find and measure those properties that will associate one glass fragment with another while minimizing or eliminating other sources Important physical properties for comparison: Density Refractive Index These are both intensive properties!! (Why is this important?)

Glass Fragments Tempered glass is stronger than normal glass due to rapid heating and cooling (Pyrex, car windows) Laminated glass - has a layer of plastic between two pieces of ordinary window glass (windshield)

Density and Refractive Index METHODS: Best methods to determine a glass fragment’s density and refractive index are: Flotation Immersion

Flotation Method The flotation method is a rather precise and rapid method for comparing glass densities In the flotation method, a glass particle is immersed in a liquid The density of the liquid is carefully adjusted by the addition of small amounts of an appropriate liquid until the glass chip remains suspended in the liquid medium At this point, the glass will have the same density as the liquid medium and can be compared to other relevant pieces of glass which will remain suspended, sink, or float

Molecular arrangement – Crystal vs. Glass Glass is a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxides Amorphous solids have their atoms arranged randomly, unlike crystals Crystal Glass

Refractive Light Method Crystalline solids have definite geometric forms because of the orderly arrangement of their atoms These solids refract a beam of light in two different light-ray components resulting in double refraction Birefringence is the numerical difference between these two refractive indices Not all solids are crystalline in nature. For example, glass has a random arrangement of atoms to form an amorphous or noncrystalline solid.

Refraction

Immersion Method Immersion Method - involves immersing a glass particle in a liquid medium whose refractive index is varied 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 The Becke line is a bright halo near the boarder of a particle that is immersed in a liquid of a different refractive index

Analyzing Cracks The penetration of window glass by a projectile, whether it is a bullet or a stone, produces cracks which radiate outward (radial fractures) and encircle the hole (concentric fractures) By analyzing the radial and concentric fracture patterns in glass, the forensic scientist can determine the direction of impact

Annealed vs. Tempered vs. Laminated Glass

Analyzing Cracks A high-velocity projectile (ex: bullet) often leaves a hole that is wider at the exit side - important in determining the direction of impact Direction of Impact 4R Rule: Ridge lines on Radial fractures form Right angles to the Rear. The sequence of impacts when there have been successive penetrations of glass is frequently possible to determine because a fracture always terminates at an existing line of fracture.

Analyzing Cracks A high-velocity projectile (ex: bullet) often leaves a hole that is wider at the exit side - important in determining the direction of impact Direction of Impact 4R Rule: Ridge Lines on Radial fractures are at a Right angle to the Reverse side of the force

Radial or Concentric Edge? Arrow indicates direction of force applied

Radial vs. Concentric Cracks Radial Cracks form first, opposite the side of force Concentric cracks occur afterwards, starting on the same side as the force

Bullet Resistant Glass

Bulletproof Glass

Which bullet came first? A B Sequence of impacts can be determined when there have been successive penetrations of glass because a fracture always terminates at an existing line of fracture

Glass Fractures Sound breaks glass Do the edges of these glass fractures have the characteristic radial or concentric fracture patterns (perpendicular marks opposite or in line with directional force)?

Collection of Glass The glass fragments should be packaged in solid containers to avoid further breakage If the suspect’s shoes and/or clothing are to be examined for the presence of glass fragments, they should be individually wrapped in paper and transmitted to the laboratory

Collection of Glass If even the remotest possibility exists that glass fragments may be pieced together, every effort must be made to collect all the glass found When an individual fit is thought improbable, the evidence collector must submit all glass evidence found in the possession of the suspect along with a representative sample of broken glass remaining at the crime scene