Types and Properties of Glass

Glass

• Glass is a hard, amorphous material made by melting sand, lime, and sodium oxide at high temperatures.
• Its atoms are randomly arranged.
• Main component is silicon dioxide (silica).

Characteristics of Glass

• Hard, amorphous solid
• Usually transparent
• Primarily composed of silica with oxides
• Brittle
• Exhibits conchoidal fracture

Types of Glass

• Soda-lime Glass: Most common (approx. 90%). Used in windows, containers, and light bulbs. Inexpensive to produce and can be reheated and recycled easily.
• Soda-lead Glass: Used in fine tableware and art objects. Higher density and refractive index than soda-lime, making it more brilliant.
• Borosilicate Glass: Heat-resistant, like Pyrex. Used for cooking and laboratory equipment. High resistance to temperature changes and chemicals.
• Silica/Fused Silica Glass: Used in chemical equipment and space shuttle windows. Strongest and most thermally stable.
• Tempered Glass: Used in car windows and phone screens. Stronger and safer than annealed glass; shatters into small, less sharp pieces when broken.
• Laminated Glass: Consists of two sheets of tempered glass bonded with PVB. Used in car windshields for safety and security.
• Colored/Tinted Glass: Tinted with metallic salts during manufacturing.
• Wire Mesh Glass: Glass with embedded metal wire. Helps hold broken pieces together.

Glass as Forensic Evidence

• Broken glass is important evidence.
• Different types have unique properties.
• Fragments can travel up to 9 feet.
• Can be found on clothing, shoes, or hair.
• Fracture patterns provide clues about the object and the angle of impact.
• Experts analyze the breakage to understand projectiles causing the break.

Handling of Crime Scene Glass

• Identify and photograph any glass samples.
• Collect largest fragments possible.
• Look for trace evidence (hair, fibers, blood).
• Package fragments carefully in boxes padded with cotton.
• Submit with a control sample of the type of glass from the crime scene.

Forensic Examination of Glass

• Compare glass samples' physical and chemical properties.
• Analyze optical properties (color, refractive index).
• Evaluate non-optical characteristics (surface wear, thickness, and density).
• Examine chemical properties (additives, trace elements).
• Goals are: -Identify glass types present. -Determine how the glass was fractured. -Individualize glass fragments to a source.

Physical Characteristics of Glass

• Color
• Thickness
• Fluorescence
• Patterns/Designs
• Markings (striations, dimples)
• Density
• Refractive index

Physical Matching of Glass

• Match larger fragments based on physical similarities.
• Counting fragments and how they fit together to determine source.

Refractive Index Study

• A measure of how glass bends light.
• Immersion method: Immerse a glass fragment in liquid with similar refractive index to see if invisible or a halo ("Becke line") appears
• Becke line: used to observe if the fragment's refractive index matches the liquid.
• Observation:
• No Becke Line: Fragment and Liquid have same refractive index.
• Becke Line inside glass: Fragment's refractive index is higher than the liquid.
• Becke Line outside glass: Fragment's refractive index is lower than the liquid.
• Hot stage: Increasing the temperature can get to the match point when the Becke line disappears

Density Study

• Measures of density (mass/volume) using displacement.
• Different types of liquids with varying densities
• Determine the glass type from its density.
• Direct method: Measuring of mass and volume using displacement method.

Fracture Study & Sequencing

• Fracture patterns are unique and help determine the origin, direction of force, and sequence of impacts.
• Radial fractures: originate on the opposite side of the force.
• Concentric fractures: originate on the same side as the force.
• Cone fractures: Caused by high-velocity projectiles, resulting in a crater-shaped hole surrounded by fractures.

Soil

• Soil is any disintegrated material on or near the earth (natural or manufactured).
• Soil has unique properties useful for forensic examination.
• Types of Soil
• Sediment
• Sandy Soil
• Silt Soil
• Clay Soil
• Loamy Soil
• Peaty Soil

Soil as Evidence

• Soil from a crime scene or related items can be compared to standard/reference samples for a comparison.
• Collected for comparison within a 100-yard radius of the crime scene, from similar environments.

Collection of Soil Samples

• Handling/Collection:
• Use appropriate tools (spatula, trowel) for sample collection and placement in containers.
• Remove the lids from containers to allow for gradual drying if necessary
• Containers (rigid, airtight, waterproof) for sample storage.

Preservation of Soil Samples

• Store samples immediately after collection, using refrigerated or dark, cool areas.
• Avoid freezing.

Microscopy Observations

• Examine soil particles visually under a stereomicroscope. -Examine under several magnification levels for more detail
• Examine particle color, shape, and size.

Chemical Reagent Observations

• Observe the reaction of the soil under chemical reagents for possible indication of substances.
• Confirm identification of observed substances using confirmatory reagents.

Particle Size Distribution of Soil

• Separate soil using a set of sieves to divide the soil samples by size.
• Measure the weight of each soil size for percentage analysis.

Ignition Test (Soil)

• Heat the sample to determine water content and weight loss.
• Compare the weight loss of a suspect soil sample with a known soil sample from a given area (comparison soil sample)

pH Measurement (Soil)

• pH is useful in determining soil acidity/alkalinity.
• Prepare a known soil sample in a liquid of controlled volume.
• Measure and compare the pH values of the soil samples to determine compatibility.

This quiz explores the composition, characteristics, and various types of glass, including soda-lime and borosilicate glass. Learn about their uses, properties, and how they differ from one another. Test your knowledge on this important material in both everyday items and specialized applications.