Definition
Glass is an amorphous solid formed by the rapid cooling of a molten substance (commonly silica with various additives), which prevents the formation of a regular crystal lattice. It is hard, brittle, and transparent or translucent, with wide applications in industry, construction, and forensics.
Properties:
Amorphousness
Brittleness
Hardness
Transparency
Density
Chemical Resistance
Electrical Insulation
Thermal Expansion
Thermal Shock Resistance
Refractive Index
Compressive Strength
Tensile Strength
Non-crystallinity
Optical Clarity
Durability
Smoothness
Fragility
Rigidity
Low Thermal Conductivity
Impermeability
Types of glass:
- Soda-Lime Glass
Definition: The most common type of glass, economical and versatile, widely used for everyday applications.
Composition: Silica (SiO₂), Soda (Na₂O), Lime (CaO)
Uses: Window panes, bottles, jars, drinking glasses, tempered car windows, architectural glass, light bulbs, glass containers, packaging, household items
- Borosilicate Glass
Definition: A heat-resistant and chemically durable glass used in environments with rapid temperature changes and chemical exposure.
Composition: Silica, Boron Oxide (B₂O₃), Soda, Lime
Uses: Laboratory glassware (beakers, test tubes), cookware (Pyrex), lighting bulbs, telescope lenses, chemical containers, pharmaceutical glass, heat-resistant windows, scientific instruments
- Lead Glass (Crystal Glass)
Definition: Dense glass with high refractive index, prized for its brilliance and ability to block radiation.
Composition: Silica, Lead Oxide (PbO)
Uses: Decorative glassware, chandeliers, fine glassware, optical lenses, radiation shielding, glass art, optical prisms, telescope components
- Aluminosilicate Glass
Definition: A strong, scratch-resistant glass designed for use in high-impact and demanding environments.
Composition: Silica, Aluminum Oxide (Al₂O₃), other oxides
Uses: Smartphone/tablet screens (Gorilla Glass), aerospace windows, durable protective glass for industrial use, bulletproof glass components, high-strength glass covers, optical components
- Fused Silica Glass
Definition: Ultra-pure silica glass known for exceptional thermal stability and optical clarity in high-precision applications.
Composition: Nearly pure silica (SiO₂)
Uses: Furnace windows, laser optics, semiconductor manufacturing, optical fibers, high-temperature industrial applications, UV transmitting optics, telescope lenses, precision laboratory equipment
- Flint Glass
Definition: Lead-containing glass with a high refractive index, used mainly for decorative and optical applications.
Composition: Silica with high lead oxide content
Uses: Decorative glass items, camera lenses, optical instruments, fine optical lenses, prisms, luxury glassware, optical filters
- Alkali-Free Glass
Definition: Chemically inert glass with very low alkali content, ideal for sensitive pharmaceutical and electronic applications.
Composition: Low alkali content, high purity silica and alumina
Uses: Pharmaceutical packaging (vials, ampoules), medical instruments, semiconductor components, electronics, chemical containers, optical fibers for telecommunications
- Specialty Glass (Chalcogenide Glass)
Definition: Glass containing chalcogen elements (sulfur, selenium, tellurium) that transmit infrared light.
Composition: Contains sulfur, selenium, or tellurium instead of oxygen
Uses: Infrared optics, fiber optics, infrared sensors, night vision devices, military and defense applications, thermal imaging, communication equipment
- Phosphate Glass
Definition: Glass where phosphorus pentoxide partially replaces silica, often bioactive or used in specialized optical fibers.
Composition: Phosphorus pentoxide replaces some silica
Uses: Optical fibers, bioactive glasses for bone repair and implants, laser materials, glasses with controlled solubility for medical applications, optical devices
- Techtile Glass
Definition: Modified silicate glasses engineered for advanced technological applications requiring flexibility or special properties.
Composition: Modified silicates tailored for specific tech uses
Uses: Flexible electronics, advanced display panels, sensors, wearable devices, microelectronics, smart devices, bendable glass applications
- Selenide Glass
Definition: Selenium-based glasses known for infrared transparency and used in thermal imaging and sensor technologies.
Composition: Selenium-based glasses
Uses: Infrared optics, thermal imaging cameras, sensors, night vision equipment, infrared lasers, spectroscopy devices
- Heavy Metal Oxide Glass
Definition: Dense glass with high heavy metal oxide content, used in radiation shielding and advanced optics.
Composition: Heavy metal oxides like bismuth, lead, or tellurium
Uses: Optical amplifiers, radiation shielding (medical and nuclear), lasers, fiber optic components, non-linear optical devices, gamma-ray shielding
Collection and Packaging of Glass Evidence
Collection Steps:
Identify the source of broken glass, windows, bottles, headlights, etc.
Wear gloves and use protective tools like tweezers or forceps.
Use oblique or alternate light to find small or transparent fragments.
Photograph and document all glass before collecting it.
Carefully collect both large and small fragments.
Check clothing, shoes, tools for adhered glass particles.
Take a control sample from the unbroken part of the suspected glass source.
Place collected fragments in temporary containers and label them properly.
Packaging Steps:
Wrap each glass piece separately using tissue, cotton, or bubble wrap.
Use appropriate containers:
Small pieces → paper bindle → envelope or vial
Large pieces → wrapped → rigid box
Clothing → air dry (if wet) → paper bag
Shoes/tools → do not remove glass → pack entire item
Label clearly with case number, date, time, collector’s name, and description.
Seal properly with tamper-evident tape.
Maintain chain of custody with a log of every person handling the evidence.
Types of fractures
Radial Fractures
- Cracks that radiate outward from the point of impact (like wheel spokes).
- Start on the side opposite to where the force was applied.
Forensic use: Helps identify the direction of force.
Concentric Fractures
- Circular cracks that form around the point of impact.
- Appear on the same side as the force.
Forensic use: Often form after radial fractures; used to understand the sequence of impacts.
Conchoidal Fractures
- Smooth, curved break patterns, like ripples or shell-like surfaces.
- Common in brittle materials like glass.
Forensic use: Help in matching glass pieces and analyzing fracture surfaces.
Mechanism – Fracture of Glass
- Application of Force or Stress
When an external force acts on glass, such as a hit from a hammer, a bullet, or sudden pressure, it introduces stress in the material. Stress is basically an internal force per unit area and can be:
Tensile stress: pulling or stretching force
Compressive stress: pushing or squeezing force
Shear stress: sliding force
Even though glass is strong under compression, it is very weak under tension, so tensile stress is critical in breaking.
- Stress Concentration at Flaws
Glass is never perfectly uniform; it contains microscopic imperfections like tiny cracks, scratches, or air bubbles. These flaws cause stress concentration, meaning the stress is locally much higher at these points than elsewhere in the glass.
Think of these flaws as weak spots where cracks are more likely to start because the material can’t handle as much stress there.
- Crack Initiation
Once the localized tensile stress at a flaw exceeds the fracture strength of glass (which is lower at the flaw), a crack starts to form. This is called crack initiation.
At this stage, the crack is microscopic, but it marks the beginning of failure.
- Crack Propagation
After initiation, the crack spreads quickly through the glass. This happens because:
Glass stores elastic energy when stressed.
As the crack grows, this energy is released, driving the crack forward rapidly.
The crack tends to propagate in certain characteristic patterns:
Radial fractures: Cracks radiate outward from the point of impact, usually starting on the side opposite the force application. These look like spokes on a wheel.
Concentric fractures: Circular cracks form around the point of impact on the same side as the force, developing after radial fractures.
The rapid spreading of cracks causes the glass to break suddenly.
- Energy Release and Fragmentation
When the crack extends, stored elastic energy converts into mechanical energy that breaks the bonds in the glass network.
The speed of crack propagation in glass is extremely fast, close to the speed of sound within the material. This rapid breakage leads to fragmentation.
Depending on the glass type:
Annealed (regular) glass breaks into large, sharp, irregular shards because it doesn’t have internal stress treatment.
Tempered glass is pre-stressed by heat treatment and breaks into small, relatively harmless cube-shaped fragments (dicing) because the internal stresses cause it to shatter safely.
Laminated glass has a plastic layer between glass sheets, so cracks may appear, but the layer holds fragments in place.
Forensic Examination
3R Rule in Glass Fracture
“Radial cracks form at Right angles to the Rear of the glass relative to the force.”
This rule gives us the “3 Rs”:
- Radial cracks
- Form at Right angles
- To the Rear side of impact
How to Apply the 3R Rule:
When a projectile (like a stone or bullet) hits a glass surface, radial cracks form.
These cracks start from the opposite side of the glass (the rear) and travel outward.
At the point where a radial crack meets the surface, it makes a right angle on the rear side of the glass.
Visual Examination of Glass Fracture
Purpose:
To observe and interpret the fracture patterns, edges, and characteristics on glass to understand the cause, direction, and sequence of breakage.
Key Aspects:
- Fracture Patterns
Radial Fractures:
Appear as cracks radiating outward from the point of impact like spokes on a wheel.
These cracks form on the opposite side of the applied force.
Concentric Fractures:
Circular cracks that form around the point of impact.
Usually develop on the same side as the force applied, often due to continued pressure after the initial fracture.
Secondary Fractures:
Additional cracks forming after initial fracture due to further stress or impacts.
- 3R Rule (Right Angle Rule)
Radial cracks create “rib marks” that make Right angles on the Reverse side from where the force was applied.
This helps determine the side of impact.
- Fracture Edge Characteristics
Examine the edges for signs of smoothness, sharpness, or chipping.
The fracture initiation point usually has a small crater or conchoidal mark indicating the impact site.
- Wallner Lines (Stress Marks)
Microscopic lines on fracture surfaces that show the direction of crack propagation.
These lines curve and help identify the force direction.
- Other Observations
Thickness and color variation: Can indicate type or origin.
Presence of inclusions or contaminants: May provide clues about the glass manufacturing or environmental exposure.
Tools Used:
Magnifying glass or low-power microscope to observe crack details.
Photography with scales for documentation.
Comparative analysis by physically matching fractured edges.
Physical Examinations of Glass
Purpose:
To analyze the physical properties of glass samples, helping to identify and compare them or infer the conditions under which they fractured.
Key Physical Examination Methods:
- Fracture Pattern Analysis
Radial and Concentric Cracks:
Observe and measure crack patterns to determine the point of impact and direction of force.
Sequence of Fractures:
When multiple impacts occur, examine which fractures terminate at others to establish the order of impacts.
- Thickness Measurement
Use calipers or micrometers to measure the thickness of glass fragments.
Variations in thickness can help distinguish between different sources of glass.
- Density Measurement
Calculate density by:
Mass
Volume
Density
Volume
Mass
Measure the mass with a precision balance and the volume (often by water displacement or geometric calculation).
Helps differentiate glass types (e.g., tempered vs. annealed).
- Hardness Testing
Using scratch tests (Mohs hardness scale) to determine surface hardness.
Can help differentiate glass types or detect surface treatments.
- Surface Wear and Weathering
Examine surfaces for scratches, abrasions, or weathering that might indicate the glass’s environment or handling before fracture.
Tools Used in Physical Examination:
Calipers and Micrometers: For thickness and dimension measurement.
Precision Balance: For accurate mass measurement.
Magnifying Glass or Microscope: To examine surface details and fracture edges.
Hardness Kits: For scratch testing.
Importance in Forensics:
Physical measurements help match glass fragments to a particular window or source.
Fracture pattern and sequence provide clues to how and when the glass broke.
Density and hardness aid in identifying glass types and treatments
Chemical Examination of Glass
Purpose:
To determine the chemical composition of glass samples, which helps in identifying the glass type, origin, and potentially matching fragments to a source.
Key Chemical Examination Techniques:
- Qualitative and Quantitative Chemical Analysis
Elemental Analysis:
Identify the presence of major and trace elements such as silicon, sodium, calcium, lead, boron, etc.
Elemental composition varies by glass type and manufacturer.
- Common Chemical Techniques
Inductively Coupled Plasma Mass Spectrometry (ICP-MS):
Highly sensitive technique that vaporizes the glass sample and measures elemental composition at trace levels.
Destructive method because the sample is consumed.
Atomic Absorption Spectroscopy (AAS):
Measures concentration of specific elements by absorption of light; requires sample dissolution.
X-Ray Fluorescence (XRF) Spectroscopy:
Non-destructive method that excites atoms in the sample with X-rays and detects the emitted secondary X-rays characteristic of elements present.
Useful for elemental analysis without damaging the sample.
Fourier Transform Infrared Spectroscopy (FTIR):
Identifies chemical bonds and molecular structure by measuring absorption of infrared light.
Usually non-destructive.
- Chemical Tests for Glass Types
Lead Content Test:
Determines presence of lead oxide, indicating lead glass or crystal.
Boron Content Test:
Identifies borosilicate glass (used in labware).
Importance in Forensics:
Helps differentiate glass types with similar physical characteristics but different chemical compositions.
Provides evidential value by matching fragments based on elemental fingerprints.
Can trace glass back to specific manufacturers or batches based on chemical markers.
Optical Examination of Glass
Purpose:
To analyze the optical properties of glass, which can help identify glass type, detect stress patterns, and compare samples for matching.
Key Optical Examination Techniques:
- Refractive Index (RI) Measurement
What It Is:
Measures how much light bends (refracts) when passing through the glass.
Why It Matters:
Different types of glass have characteristic RI values; comparing RI helps match fragments to the same source.
Methods:
Becke Line Method:
A bright halo (Becke line) moves when the glass and immersion liquid’s RI differ; observing this helps estimate RI.
Refractive Index Comparators:
Instruments that provide precise RI measurements without damaging the sample.
Considerations:
Tempered glass or laminated glass may have varying RI values; careful calibration is necessary.
- Polarized Light Microscopy
What It Is:
Uses polarized light to examine birefringence and stress patterns in glass.
Why It Matters:
Reveals internal stresses or strain patterns caused by manufacturing or impact.
Applications:
Identify tempered vs. annealed glass.
Visualize stress marks related to fracture propagation.
- UV-Visible Spectroscopy
What It Is:
Measures absorption of ultraviolet and visible light by the glass.
Why It Matters:
Different glass types or colors absorb light differently; helps classify colored or treated glass.
- Fourier Transform Infrared Spectroscopy (FTIR)
What It Is:
Identifies molecular bonds by measuring infrared light absorption.
Why It Matters:
Provides chemical fingerprint without damaging the sample. Often complements other optical methods.
Importance in Forensics:
Enables matching of glass fragments based on optical characteristics.
Detects manufacturing defects or treatments (e.g., tempering).
Helps distinguish glass types with similar physical but different optical properties.
Tools Used:
- Refractive Index Microscope or Comparator
- Polarized Light Microscope
- UV-Vis Spectrophotometer
- FTIR Spectrometer
.