Where did the traditional list of fire burn indicators come from?
The traditional list of burn indicators used in fire investigation traces to a 1977 survey by the Law Enforcement Assistance Administration and to a field handbook the National Bureau of Standards published three years later, in 1980. Fire files still arrive, now and then, with a photograph of cracked window glass or a chipped slab and a line saying the fire burned hot and fast, and the reasoning behind that line is older than most of the people using it.
The Law Enforcement Assistance Administration’s 1977 publication, “Arson and Arson Investigation: Survey and Assessment,” described the burn indicators then in use and warned that “[a]lthough burn indicators are widely used to establish the causes of fires, they have received little or no scientific testing.” The Law Enforcement Assistance Administration survey recommended planned experiments to establish the reliability of those burn indicators, and a field handbook built on the results.
The National Bureau of Standards handbook arrived three years later; the planned experiments did not. As Lentini recounts in Forensic Science Review (2019, volume 31, number 1, pages 37 to 44), Chapter 1 of the National Bureau of Standards handbook printed the burn indicators anyway, among them “Alligatoring of wood. Slow fires produce relatively flat alligatoring. Fast fires produced humpback, shiny alligatoring” and “Spalling of concrete. An indication of intense high-heat fire.”
Does crazed glass show that a fire burned hot and fast?
Crazed glass does not show that a fire burned hot and fast: testing published in the Journal of Forensic Sciences found that glass crazes only when its temperature is rapidly decreased, and the usual way to rapidly cool hot glass in a structure fire is a hose stream. The belief being tested was that closely spaced random cracking in window glass recorded a rapid buildup of heat.
Lentini tested that belief in the Journal of Forensic Sciences (1992, volume 37, number 5, pages 1358 to 1362), heating glass slowly and rapidly to 800 degrees Celsius, in a propane flame, and to low fire temperatures. The paper reports that “the test results show that glass will not craze, except when its temperature is rapidly decreased.”
The conclusion of Lentini’s 1992 paper is blunt: “A finding of crazed glass in a fire scene has no special meaning regarding the temperatures to which the glass was exposed.” Because a hose stream is the usual way hot glass is rapidly cooled in a structure fire, crazed glass is a record of suppression.
Does spalled concrete indicate an accelerated or incendiary fire?
Not by itself: explosive spalling of concrete has its own research literature, reaching back to its first written account in 1854, and the mechanisms in that literature turn on the concrete rather than on the fuel. The 2017 American Association for the Advancement of Science assessment of fire investigation lists spalled concrete among the artifacts “once thought to indicate incendiarism but now known to be of little value in classifying the cause of fires”.
Kirnbauer, Bund and Robisson reviewed the history of explosive spalling research for the 2nd International Conference of Civil Engineering in 2023 and set out the theories still considered plausible. Pore-pressure theories include the moisture-clog theory of Shorter and Harmathy, in which water driven ahead of the heat front condenses in cooler pores and pressurizes the pore structure. Thermal-stress theories hold that stress near the heated surface exceeds the compressive strength of the concrete.
The conclusion Kirnbauer, Bund and Robisson reach is that spalling results from a combination of thermo-mechanical and thermo-hydro changes that trigger cracking. Spalling is a concrete problem, not a fuel problem: a spalled slab tells a fire investigator about the slab.
Do melted metal and shiny char blisters show that a fire involved an accelerant or abnormally high temperatures?
Not by themselves: melted metals and large shiny char blisters are both among the artifacts that the 2017 American Association for the Advancement of Science assessment of fire investigation lists as “once thought to indicate incendiarism but now known to be of little value in classifying the cause of fires”, and a field study of fifty structures destroyed in the October 1991 fire in the hills east of Oakland, California, found apparently melted steel items and melted copper to have no probative value as to liquid accelerants, abnormally heavy fuel loads or abnormally high temperatures.
The October 1991 fire in the hills east of Oakland, California, supplied a large sample. Lentini, Smith and Henderson examined the remains of fifty structures destroyed in that fire and published the results as “Baseline Characteristics of Residential Structures Which Have Burned to Completion: The Oakland Experience” in Fire Technology (1992, volume 28, pages 195 to 214).
Lentini, Smith and Henderson looked for the traditional indicators of abnormal fire behavior in each structure, including apparently melted steel items, melted copper, and melted as well as crazed glass. The published finding is that these “were found to have no probative value as to the presence of liquid accelerants, abnormally heavy fuel loads, or abnormally high temperatures.” The indicators turned up across structures burned to completion in a single fire, which is what an indicator of abnormal fire behavior cannot afford to do.
Can irregular burn patterns on the floor of a fully involved room show that a fire was accelerated?
Irregular floor burn patterns in a fully involved room do not by themselves show that a fire was accelerated: in full-scale room burns, experiments using the same method of ignition produced significantly different patterns, an inconsistency attributed to ventilation effects, and post-flashover ventilation-generated patterns have been misread as evidence of an accelerated fire.
Anthony Putorti’s Full Scale Room Burn Pattern Study was published by the National Institute of Justice as NIJ Report 601-97 in December 1997, the experiments having been run at the National Institute of Standards and Technology under National Institute of Justice sponsorship. Four instrumented full-scale burns were conducted in compartments furnished as residential bedrooms, two ignited at an upholstered chair and two with a gasoline spill. The discussion in Putorti’s study reports that significant differences in the condition and appearance of the burn rooms and furnishings were present between experiments using the same method of ignition, in severity of burning and in the locations and types of patterns, and attributes that inconsistency to ventilation effects.
The reason is ordinary fire dynamics. The 2017 American Association for the Advancement of Science (AAAS) assessment of fire investigation describes flashover as the transition in which exposed combustible surfaces ignite almost simultaneously under radiant heating from a hot gas layer that has reached 500 to 600 degrees Celsius. Past flashover the enclosure is typically ventilation-limited, the most intense burning occurs where fuel vapors meet air entering through the ventilation openings, and post-flashover fires generate new ventilation-generated patterns while obscuring earlier ones. The AAAS assessment records that those ventilation-generated patterns have been misread as evidence of an accelerated fire.
What is fire investigation’s current position on traditional burn indicators?
Fire investigation’s current position is set out in the 2017 American Association for the Advancement of Science (AAAS) assessment of fire investigation by Almirall, Arkes, Lentini, Mowrer and Pawliszyn, which lists specific traditional burn indicators as artifacts “once thought to indicate incendiarism but now known to be of little value in classifying the cause of fires”. The 2017 AAAS assessment states the position plainly: the evidence left by fully involved accidental fires is often indistinguishable from that left by fully involved incendiary fires.
The artifacts on the 2017 AAAS assessment’s list are downward burning, charring of floors and baseboards, charring on the undersides of surfaces, large shiny char blisters, irregular fire patterns, melted metals, crazed glass, and spalled concrete.
NFPA 921, Guide for Fire and Explosion Investigations, now in its 2024 edition, is the document the field works to. An origin and cause opinion resting on an item from the AAAS list, with nothing corroborating it, rests on a retired inference.
Should crazed glass, spalled concrete and melted metal still be documented in a fire investigation report?
Yes: retired as a shortcut to cause does not mean worthless, and spall locations, melted metal, glass condition and char depth are real data about the post-fire state of a structure that belong in the fire investigation documentation.
What changed is the direction of the reasoning. Spall locations, melted metal, glass condition and char depth now feed a hypothesis about heat exposure, duration and suppression that has to be tested against fire dynamics, ventilation, fuel load and the electrical evidence, rather than being read off as a verdict.
What can a fire scene examination establish, and what can it not settle?
A competent fire scene examination can establish where the fire burned longest, what the ventilation openings were and when they failed, what the fuel package was, what the electrical system records in its arc damage, and whether laboratory analysis confirms an ignitable liquid residue in a sample, but it cannot convert a chipped slab or a crazed pane into a conclusion about how the fire started.
The burn indicator that survives testing is the one that behaves differently in accidental and incendiary fires. Crazed glass, spalled concrete, melted metals, large shiny char blisters and the other artifacts listed in the 2017 American Association for the Advancement of Science assessment of fire investigation are the ones that do not.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.