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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. Basements

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 accidental fire.

b. Attics

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.

c. Closets

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.

B. Fuels

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 of these;
  • 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 days later).

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 signs.

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 manner.
  • 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.

C. Ignition

Primary ignition devices include--

1. Flame

Key facts:

  • 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.)

2. Fuse

Key facts:

  • 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 behind .

3. Cigarettes

Key facts:

  • 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

Key facts:

  • includes mixtures of aluminum powder and potassium perchlorate or potassium nitrate;
  • black powder and flashpowder from small "firecrackers" can ignite exposed fuels or large powder charges.

5. Improvised Chemical Delays

Key facts:

  • Dry chlorine tablets like those used for swimming pool purification can spontaneously ignite if mixed with glycol-based liquids such as brake fluid.

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 of heat.

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 ignite.

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 multiple points.

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, Inc.


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.

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