Forensic Physics: Glass Types and Composition Basics

UGC-NET Forensic Science Unit VII drill on glass classification, composition, and physical properties as trace evidence. Covers the four principal glass types encountered in forensic casework: soda-lime glass (windows, bottles, refractive index 1.51-1.52), borosilicate glass such as Pyrex (laboratory ware and headlamps, RI approximately 1.47), tempered or toughened glass (vehicle side windows, dice fracture pattern), and laminated safety glass (windshields, polyvinyl butyral interlayer). Additional types include lead crystal glass (tableware, high PbO content) and multilayer bulletproof or armoured glass. The Pilkington float process, by which most flat glass is manufactured by floating molten glass on a tin bath, is examined as a process fundamental. Refractive index ranges across glass types provide the principal discriminating property used by forensic examiners, alongside density and elemental composition.

The Indian forensic context addresses glass examination as practised at the Central Forensic Science Laboratory (CFSL), Chandigarh and Kolkata, under ASTM E1967 (refractive index of glass by oil immersion and phase contrast microscopy) and ASTM E2926 (LIBS elemental analysis of glass). Saferstein's Criminalistics (12th edition) and Sharma B.R.'s Forensic Science in Criminal Investigation and Trials are the primary textbook references. Glass fragments from windshields, side windows, headlamp lenses, bottles, and spectacle lenses represent the most common forensic submissions, and identification of glass type aids in hit-and-run reconstruction, building entry analysis, and contact trace interpretation under Locard's exchange principle.

Topics covered:

• Soda-lime glass: SiO2, Na2O, CaO composition and RI 1.51-1.52
• Borosilicate glass (Pyrex): B2O3 content, RI approximately 1.47
• Tempered glass: thermal treatment and dice fracture pattern
• Laminated glass: PVB interlayer and windshield application
• Bulletproof and armoured glass: multilayer polycarbonate construction
• Float process (Pilkington): molten glass on tin bath
• Lead crystal glass: PbO content and high RI
• Forensic glass sources: windows, headlamps, bottles, spectacles

Calibrated for first-pass UGC-NET Forensic Science Paper II preparation and NFSU MSc Forensic Chemistry entrance revision. Allow 30 minutes.