Fire investigation is part science, part detective work. From burn patterns to chemical analysis, here's how forensic fire investigators piece together the story of a fire.. The Scene of the Fire Every fire tells a story. The job of the fire investigator is to read the physical evidence — burn patterns, char depth, witness testimony, CCTV footage, and chemical analysis — to determine where and why a fire started. The conclusions can determine criminal prosecutions, insurance claims worth millions, and future building safety regulations. The Investigation Process Phase 1: Scene preservation Secure the fire scene before investigation begins Prevent disturbance of evidence (particularly important after firefighting operations) Document the scene condition with photography and video Identify and protect potential evidence locations Phase 2: External examination Survey external fire damage patterns Identify fire spread direction from external evidence Document building construction and modifications Interview firefighters about conditions on arrival Phase 3: Internal examination Systematic examination from least damaged to most damaged areas Following the 'fire trail' to the area of origin Identifying the point of origin through convergence of fire patterns Excavation of the origin area to identify potential ignition sources Phase 4: Evidence collection Collection of samples for laboratory analysis Packaging in nylon bags or glass jars (not plastic — it can contaminate accelerant evidence) Chain of custody documentation Comparison samples from unaffected areas Reading the Fire Patterns Fire investigators use physical evidence to trace a fire's path: V patterns: Fires typically create V shaped burn patterns on walls The base of the V indicates the fire's origin area Multiple V patterns may indicate multiple points of origin (potential arson indicator) Char depth: Deeper charring indicates longer burning duration or higher intensity Char depth analysis helps establish the fire's timeline Alligator charring (large, shiny blisters) was once thought to indicate accelerant use — modern research has disproven this Smoke patterns: Clean burn (light coloured) vs. dirty burn (dark, sooty) indicates ventilation conditions Smoke staining on glass indicates whether windows were open or closed during the fire Ceiling smoke patterns show airflow direction Melted materials: Different materials melt at known temperatures (copper: 1,085°C, aluminium: 660°C, lead: 327°C) Melted materials help establish temperature profiles The location of melted vs. unmelted materials maps the fire's intensity pattern Laboratory Analysis Gas chromatography mass spectrometry (GC MS): The gold standard for detecting accelerants (petrol, diesel, paraffin) Can detect accelerant residues in quantities as small as 0.1 microlitre Must be interpreted carefully — many household products contain similar compounds Electrical examination: Arc mapping: Identifying electrical arcing points to determine if electrical failure caused or was caused by the fire Wire analysis: Primary arcing (cause) vs. secondary arcing (consequence) can be distinguished by metallurgical examination Equipment examination: Recovered electrical equipment examined for manufacturing defects or user modification The Expert Witness Role Fire investigators frequently serve as expert witnesses in: Criminal trials (arson, manslaughter, corporate manslaughter) Civil litigation (insurance disputes, product liability, professional negligence) Inquests (coroner's investigations into fire deaths) Public inquiries (Grenfell Tower Inquiry relied heavily on fire investigation evidence) The investigator must maintain objectivity and present findings impartially, regardless of who instructed them. Magnus Opifex provides fire investigation and expert witness services for insurers, solicitors, and building owners. Contact us.