Incendiary Fire Basics
excerpted from "Motive, Means, and Opportunity, A Guide
to Fire Investigation."
American Re-Insurance Company, Claims Division, 1996.
Table of Contents
Arsonists are careful about choosing the time and circumstances of fires
they set in the hopes of escaping detection. The most important considerations
to the success of the fire are sufficient fuel and access of air. Sometimes
it's important to delay ignition, and then more unusual methods are employed.
A. Location, Location, Location*
Once the target is selected, the arsonist makes choices critical to the
"success" of the fire. As in real estate, where "location
is everything," so it is in arson. We are referring to the point of
origin within that target. The point of origin for the fire must ensure
flames will continue to reach fresh fuel.
Flames will spread fastest across the underside of a ceiling or overhang.
Thus, the floor adjacent to a flammable wall is the most likely point of
origin. Walls covered with gypsum sheet rock suppress a fire. Unless holes
have been broken through the sheet rock to expose the flammable wall studs
or ceiling joists, once the paint and surface paper burn off, the fire will
die rapidly. Wood paneling varies in its resistance to fire, but generally
add to the fuel load of a room.
There are advantages and disadvantages of new or old furniture at the
site of the targeted fire. If appropriate, the arsonist may use certain
furniture to start and spread the fire. Older furniture stuffed with animal
hair and covered with wool is more difficult to ignite. Once ignited, this
type does not burn too well. More modern furniture often uses quantities
of polyurethane foam padding, which can be ignited easily with a match.
It burns quickly and forcefully. Lighter synthetics and cottons, used for
upholstery, can be flammable and burn well.
In a commercial or industrial building, the arsonist may move readily
available supplies of finished goods, packing materials, or raw materials
around strategically to provide a large load of readily combustible fuel.
1. Points of Origin
A benefit to using this part of a building as the fire's source is that
walls and ceilings are often bare of any fire-inhibiting covering and the
building above provides ready fuel. Basements, ordinarily hidden from view,
can provide a protected environment for the arsonist to work. The litter
or storage in these areas, may be used as an "excuse" for a so-called
Attics, like basements, contain storage materials that can be pointed
to as the source of an "accidental" fire, but the attic has limited
fuel making it one of the less likely sites chosen because the fire may
only destroy the ceiling and roof. One exception may be insurance fraud
fires. The process entails setting the fire to render the building unusable.
When the insurance claim is collected, "repairs" are begun. The
building, while being rebuilt, "suffers" another "more complete"
fire. The owner of the building may then collect a higher insurance payment
on an "upgraded" building.
While closets generally contain clothing or supplies that are useful
as fuel, and they are out of sight, they are not likely places for arson
fires. Some exceptions may be nuisance, spite or juvenile-set fires.
d. Fireplaces, Electrical Outlets, Heaters, Dryers, Furnaces
An area near a fireplace, electrical outlet, heater, dryer or other heat-producing
source or appliance can be used as a place to start a fire in an effort
to mislead investigators. A hasty inspection of a fire scene may lead investigators
to wrongly conclude that the fire was caused by the misuse or malfunction
of the heat-producing source or appliance.
One of the most popular of the electrical devices used by arsonists is
the electric iron. This "plant" is left to appear innocent. An
electric iron may be left on in an apparently natural setting. When it burns
through a shelf, table or ironing board, the resultant ignition appears
to be accidental. Lighting circuits, which are deliberately overloaded are
also used in the furtherance of arson.
These fires should be examined carefully: with few exceptions, accidental
fires caused by malfunctioning or abused appliances start above floor level
and travel upward. A fire deliberately set with a flammable fuel will often
burn down to the floor or even below it. If it appears the fire burned down,
examine the entire area carefully.
Fuels used depend on the mindset of the arsonist. Some want to use fuels
available at the scene for simplicity and ensure they are inconspicuous
to witnesses by not carrying containers or odd packages into a building.
The "thinking" arsonist wants to ignite the fire with a simple
device that is sure to work, and that will be destroyed in the blaze. And
he or she often want complete destruction and will therefore pick a fuel
and ignition device that will thwart normal suppression activities.
1. Trash Trash-filled basements, attics, and storage rooms may be perceived
by authorities as sites for "accidental" fires. Readily available
and treated with a small amount of flammable liquid, trash will generally
ignite easily and provide a cover for the arsonist because these fires may
be explained away as accidents.
2. Ignitable Liquids Amateur arsonists are more likely to use a greater
quantity of an accelerant and tend to place the accelerant in a less effective
position. The professional arsonist achieves a successful blaze with more
judicious use of an accelerant.
Liquid accelerants are often--
- poured over carpeting or furniture;
- poured on an open floor, a pile of paper or trash, or a combination
- placed on absorbent surfaces (e.g. soil, newspapers or carpets);
- poured on piles of newspapers to enable the fire to burn only on the
outside of the stack (fuel near the center of the stack can be detected
Pouring of an ignitable liquid directly on the floor is an easily detected
form of arson--
- a portion of the fuel may remain unburned;
- fuel that does burn leaves characteristic burn patterns or diagnostic
The amateur arsonist is unfamiliar with the properties of ignitable liquids
used as fuels--
- On a smooth, nonabsorbent, "tight" floor, little or no damage
may result if the burning of the fuel has not raised the floor-covering
to its ignition point.
- On a floor with cracks, holes or joints through which the fuel can
penetrate, holes and lines will burn into the floor in a characteristic
- On ground-level floors, sometimes poured ignitable liquid seeps through
the floor and into the soil beneath. (Wooden subflooring, joists and soil
should be checked for the presence of volatile fuels.)
- When fuel is poured in the center of the room, flames won't reach furniture,
walls or other fuels to keep the fire going.
Experience, motivation, and intent usually determines the type of ignitable
liquid used as an accelerant--
- Gasoline is one of the most commonly used accelerants because it's
efficient and easily ignited and because it can usually be bought and transported
without arousing suspicion.
- The next most common fuels are paint thinners and light kerosenes.
Moderate amounts can be purchased without arousing suspicion because there
are legitimate uses for these fuels in private homes.
The fuel's volatility has great bearing on its suitability to the task--
- Extremely volatile fuels such as gasoline, lacquer thinner or alcohols
can evaporate at high temperatures, interfering with the ignition and spread
of the fire. Vapors can travel considerable distances and produce explosive
mixtures in the vicinity of distant "accidental" sources of ignition,
most typically the pilot light on the water heater or furnace. The arsonist
must place the accelerant quickly and set the fire from a safe distance.
This reduces the control he has over the target. Gas isn't a favorite with
the professional arsonist because explosions may occur without setting
a fire, but amateurs may employ it, sometimes to their detriment.
- The volatility and flash points of kerosenes, paint thinners and varieties
of lantern and camp stove fuels vary. Because they contain heavier hydrocarbons
with higher boiling points they are less likely to evaporate and will remain
in the debris as residue longer.
- Fuel oil is very difficult to burn except when mixed with gasoline
or as a secondary fuel.
- Both methyl (wood) alcohol and ethyl (grain) alcohol are more volatile
than gasoline and carry the same hazards. Only a large quantity of burning
alcohol will generate sufficient heat to ignite nearby combustibles. Ninety
to 100 proof (45 to 50 percent alcohol) liquors can be used as accelerants,
but not very effectively.
- Glues, dye solvents, rubber cements and their solvents, mop cleaners,
uncured polyester resin, paints, brush cleaners and the like are common
in homes, schools and businesses and have been used as accelerants. It's
crucial that investigators perform a thorough investigation of the scene,
including interviews with owners, tenants and maintenance people. Copier
toner used in some modern office copiers contains a petroleum distillate
similar to light paint thinners. Some commercial insecticides are often
dissolved in a light paint-thinner-like petroleum distillate. If an investigator
suspects unusual fuels, his suspicions should be relayed to the laboratory
examiner along with control samples of all possible fluids.
Primary ignition devices include--
- easy to create, apply and eliminate as evidence;
- a definite means of starting a fire because it's a fire already and
requires only propagation;
- whatever is left at the scene (like a burnt match) it is assumed will
likely be destroyed. (An exception: a candle which won't often be completely
destroyed, and melted wax will remain in a fire if it accumulates on a
non-combustible surface even though the form of the candle is altered.
Most commonly used is the white drug store variety which will burn at about
a rate of one inch every 46 minutes. The key is that candles give the arsonist
plenty of time to get to another place when the fire starts, furnishing
him or her with an alibi.);
- another source of flame is an altered gas pilot light assembly or broken
gas line set to throw a blowtorch-like flame. (This source may be overlooked
by investigators who may conclude that the fire began because of an accidental
malfunction of these devices.)
- a time-delay device;
- one example: a trailer that connects points of origin and directs flames
to a remote location.
There are several types of fuses--
- pyrotechnic...uses a train of flammable powder packed around a string
core; has an exposed flame at all points along its length; will scorch
and even ignite combustibles upon which it rests; also known as hobby or
rocket fuse; widely available in hobby stores; vary in their burning rates,
from 30 seconds per foot to 10 seconds per foot;
- safety...contains a train of black powder within layers of string and
asphalt wrappings; sometimes a continuous tube of water-resistant plastic;
has an exposed flame only at its ends; asphalt coating will melt and smolder,
but the wrappings will remain in place and damage to surroundings will
be minimal; almost always sold through suppliers of commercial blasting
supplies; burns at 30 to 40 seconds per foot;
- igniter cord...has a core of pyrotechnic powder wrapped in wire; used
only in special blasting operations when high temperatures and quick burning
times are desirable; after burning, a fine multiple spiral of wire is left
- the lighted cigarette in the waste basket is a source of accidental
fires from carelessness;
- more effective in combination with other flame sources;
- commonly found with matches. (The lighted cigarette is placed between
two layers of paper matches in a matchbook. When it burns down to the head
it will ignite the matches and the book will burst into flame.);
- another variation: tying matches around the outside of a cigarette
which burns down to the match heads. (This device is fairly resistant to
burning to completion, especially the match book. Thus, its remains can
be found by careful examination of the point of origin. This is among the
most commonly used delay devices in grass and wildland fires.)
4. Black Powder and Flashpowder
- includes mixtures of aluminum powder and potassium perchlorate or potassium
- black powder and flashpowder from small "firecrackers" can
ignite exposed fuels or large powder charges.
5. Improvised Chemical Delays
- Dry chlorine tablets like those used for swimming pool purification
can spontaneously ignite if mixed with glycol-based liquids such as brake
D. Molotov Cocktails
Molotov Cocktails are among the most common remote-ignition devices and
deserve special attention. They are comprised of a glass container of a
flammable liquid with a lighted wick or chemical incendiary device protruding
from an opening. When thrown against a hard surface the bottle breaks and
sprays the liquid, creating a fireball. Fire can spread very quickly (up
to 100 feet per second).
Hypergolic mixtures can be used in a Molotov Cocktail instead of a flame.
A packet of sugar and potassium chlorate is taped or tied to the outside
of the bottle and sulfuric acid is added to the liquid inside. When the
bottle breaks, the acid reacts with the sugar and chlorate to create a very
hot flame that ignites the liquid.
The use of such a device is convenient, but its easy detection at the
fire scene makes it less than ideal. The broken bottle is usually at the
bottom of the fire, easily detected because it's exposed to the least amount
Plastic bottles or containers filled with fuels may also be used. These
may be surrounded by loose paper and trash that's set on fire and softens
them eventually igniting the liquid inside. Plastic containers can burn
completely and make identification difficult.
While not a Molotov Cocktail, another device to ignite a fire sometimes
used is a balloon or plastic bag containing gasoline is suspended by rope
over a candle. The arsonist pushes the bag to make it swing, giving him
time to get out. Once it stops swinging, the candle melts a hole in the
bag, and the fuel spreads the fire.
E. Multiple Ignition Points
Professional arsonists will often set multiple ignition points connected
by a fire-spreading trailer such as a flammable liquid, smokeless gunpowder,
rags, twisted ropes or newspaper, waxed paper or even fabric softener strips.
A trailer of flammable liquid may be poured from one electrical outlet to
another, in efforts to simulate an electrical fire. Materials most often
used as trailers are toilet paper, black gunpowder, film wrapped in paper
rags, excelsior, kerosene, decorative streamers, cotton bat-ting, kapok,
paper, and combinations of these items. When a fire is extinguished before
destroying the building, these trailers can often be identified. Investigators
should examine the nap of rugs, hardwood floors or their polished surfaces
carefully for sign of these materials.
Multiple ignition points do cause more destruction, but also may present
problems for the arsonist--
- They require additional materials and fuels.
- There's the chance that one of the elements will fail, leaving intact
evidence to be discovered.
- Placing fuel at several sites versus one takes more time and increase
his or her chances of being detected.
- This approach places the arsonist at risk for getting trapped in the
fire because fuel may splash onto his or her shoes or clothing and accidentally
It's important to bear in mind, however, that multiple points of ignition
don't necessarily mean arson has occurred. An accidental fire can melt and
ignite the polystyrene light diffusers used in ceiling lighting fixtures
or the asphalt from a collapsing roof and in this way spread a fire from
This section is based upon, and contains excerpts and quotes
from "Kirk's Fire Investigation," by John De Haan and "Arson
Investigation," by Dr. Henry Lee, Chief of the Connecticut State Forensics
Laboratory. The material is used with the permission of the both authors
and that of the publisher of "Kirk's Fire Investigation," Prentice-Hall,
DeHaan, John D., Kirk's Fire Investigation (Brady Fire
Sciences Series, Third Edition), Prentice-Hall, Inc., 1991, p. 331-338
Lee, Dr. Henry C., Arson Investigation, Connecticut Fair
Plan, undated, p. 11-13
Reprinted with permission.