The Arson Set

"The Arson Set," pp 405-411 from
Kirk's Fire Investigation, 4/E by John D. DeHaan, ©1997.
Reproduced by permission of Prentice-Hall, Inc., Upper Saddle River, NJ.
Permission from Prentice-Hall is required for all other uses.

The professional and, to a lesser extent, the amateur arsonist wants to produce a maximum of destruction no matter how small the first ignition. To this end, most rational arsonists will be sure there are the fundamental requirements for a large, spreading fire: ready availability of fuel, proper ventilation, and reliable ignition. While much attention has been given to the ignition devices used, the most important considerations to the success of the fire are sufficient fuel and access of air. Only when delayed ignition is essential is there any need for unusual methods of igniting the fire. Many more arsonists choose the time and circumstances of the fire to minimize their chances of being seen than rely on any unusual (and often undependable) igniting device. Perhaps the preoccupation of some investigators with such devices results primarily from their novelty, which adds much interest to their discovery. In one study by the author, only 12.3% of the arson cases examined involved the use of any type of ignition device. By far the most common "set" was the direct pouring of a flammable liquid with ignition by match, which was found in 61.2% of the cases.²⁷ This trend continued in cases examined in the author's laboratory during the 1980s. The extent and nature of preparations, such as building or assembling a device, preparing the scene (providing kindling of a particular type or arranged in a certain fashion), providing accelerants, ensuring fire spread, and the like, are all indications of the organization characteristics of the fire setter. Such characteristics (organized versus disorganized) can be used to focus on a possible motive and even develop a personality profile of the arsonist for use in the investigation.²⁸

Arranging the Fire--Location

Once the target is selected, the arsonist must make several choices that are critical to the success of the fire. A point of origin must be selected so that flames will rise into fresh fuel. Although flames will spread fastest across the underside of a ceiling or overhang, it is usually impractical to arrange a point of ignition high up on a wall. The floor adjacent to a flammable wall is the likeliest point for initiating the fire. Walls covered with gypsum sheetrock do not offer much fuel, and once the paint and surface paper burn off, the fire will die back rapidly. Unless holes have been broken or cut through the sheetrock to expose the flammable wall studs or ceiling joists before the fire, such covering will seriously inhibit the spread of the fire. Wooden paneling products vary considerably in their fire resistance but eventually they will all add greatly to the heat release rate of a room. Furniture may be used if present in the correct type and sufficient quantity. Older furniture stuffed with horsehair and covered with wool fabrics is difficult to ignite and does not burn readily once ignited. Nearly all modern furniture contains large quantities of polyurethane foam padding that can be readily ignited with a match and that burns with great speed and intensity. Lighter upholstery fabrics of cotton and synthetics can be flammable, adding further to the ease of ignition and the fuel load. In a commercial or industrial building, stocks of finished goods, packing materials, or raw materials may be moved about to provide a large fuel load in a readily combustible form.

Within the structure, the point selected for the origin may be limited by what areas of the building are readily accessible and the "ideal" spot may be protected by a locked door, gate, or elevator. The arsonist may then have to select a less perfect location. Basements are very suitable since the walls and ceilings are often not covered, the entire structure above the fire providing maximum fuel. A basement location is remote, providing the best concealment, and the utilities and trash often found in basements provide many possibilities for an "accidental" fire. Any basement fire should be investigated with special care, because of all possible locations, this one is generally most suited to the arsonist's purposes.

Attics, although they provide convenient fuel in exposed structural members, trash accumulations, and points of possible "accidental" ignition from exposed wiring are generally not selected as a point of origin. The availability of fuel is limited to the roof and adjacent structure. Insurance fraud fires are sometimes started in the attic, however, for that very reason. The ceiling and roof are destroyed, the building is rendered unusable, and business is interrupted. When the insurance for these claims is collected, "repairs" are begun and the building, while being rebuilt, suffers another more complete fire; the insurance on an "upgraded" building is then collected.

Closets provide both concealment and some fuel in the form of clothing stored there. Other than nuisance, spite, or juvenile-set fires, arson fires in closets are rare. A careful arsonist realizes that the poor flammability of some clothing, the limited air, and the difficulty of encouraging fire spread to other rooms make a closet a poor choice for a point of origin. Clothes closets and beds, when confirmed as points of origin, strongly indicate a revenge/spite motive, rather than fraud. Although, when smoke damage is the desired end (for insurance fraud-to gain remodeling expenses, stock replacement, etc.) rather than a lot of fire, a fire set in a closet may be the best place to ignite it since ventilation and spread can be controlled.

In an attempt to simulate accidental ignition, some arsonists will kindle the fire in the immediate vicinity of a fireplace, heater, dryer, or similar heat-producing appliance. It is hoped that a cursory inspection of the scene will place the point of origin in the vicinity of such a "normal" cause, and the fire will be written off as an accidental malfunction of the appliance. All such fires should be examined carefully, and the investigator must not prejudge such fires and fall into the arsonist's trap. With very few exceptions, accidental fires associated with such appliances will start above floor level and travel upward. A fire set with a flammable liquid will usually burn down to the floor or even below it. If the diagnostic signs of fire behavior indicate a very low burn, the entire area should be examined carefully, as described in Chapter 7.

Fuels

Arson investigators find themselves investigating a fire set by a truly professional "torch" only on rare occasions. This is probably because of two guidelines used by such professionals: (1) the simpler the better, and (2) use fuels available at the scene. Both guidelines minimize the materials that must be brought to the scene by the arsonist, and the fewer the number of times the individual is seen in the area (especially carrying containers or odd packages) the less the chances of being spotted by a witness. Simpler sets minimize the chances of elaborate devices failing to ignite, while they maximize the chances that the entire set will be destroyed by the fire or even normal suppression activities.

The use of accumulated trash as an ignition point obeys both guidelines. Trash piles provide an inherent risk of ignition by accidental causes and many of them are written off as accidental by investigators without careful examination. In the experience of the author, a small quantity of flammable liquid on a pile of loose paper and plastic cartons will ensure complete ignition and yet will be completely undetectable in the postfire debris. Trash accumulations are frequent in basements, attics, storage rooms, and around back doors (inside and out). A trash-filled basement is ideal for producing a large destructive fire. These can be difficult fires to establish as arson-caused because there is nearly always some possible means of accidental ignition, such as defective electrical wiring, furnace, water heater, or appliance. These all must be eliminated as causes in order to prove the crime of arson. Outside trash piles are often used by the arsonist who does not have access to the inside of the structure. Since such fire setters usually do not intentionally provide a path for the fire to spread to the structure, the fires do not always spread to the interior of the nearby building.

Flammable Liquids

Many arsonists, especially amateur ones, do not rely on solid fuels available at the scene and will heavily supplement them with flammable liquid accelerants to ensure complete ignition. Although such flammables have the ability to produce a great deal of fire in any given location, there is no automatic assurance that the flames so rapidly produced will find other fuel in their path so that they can build into a conflagration. The larger the quantity of flammable liquid used the larger the initial fire may be, but if a volatile fuel is involved, the more likely the vapor air mixture will be above the upper explosive limit (meaning no ignition). A very large flammable liquid fire may also deplete the oxygen in the room or structure and result in self-extinguishment before other fuels in the room can be ignited. It is difficult (and often ineffective) to transport and disperse more fuel than can be readily carried by hand. Except in rare instances, less than a few gallons will be used. In instances where large drums of volatile hydrocarbons are readily available (such as in an industrial situation) or where the fuel tanks or vehicles can be used as sources, massive quantities of flammable liquids can be employed. Such unusually high fuel loads can be used to ignite a secondary source of fire such as a large accumulation of automobile tires which burn ferociously and are all but impossible to extinguish.

The most frequently used amount of an accelerant is that amount a person can carry conveniently, without attracting undue attention. A person can easily carry an amount sufficient for kindling a very large fire, so this is not necessarily a serious limitation. The amateur nearly always uses much more of a volatile accelerant than is necessary. By using it judiciously on top of loosely piled flammables, a person can start a very satisfactory fire in a building using only a very small quantity of accelerant. If one were trying to burn a large building in an urban area where detection and suppression would quickly follow ignition, it may be desirable to employ a large reservoir of liquid fuel over a period of time. A metal drum of a suitable fuel such as gasoline, paint thinner, or other petroleum distillate may be placed at the desired point of origin with enough fuel under and around it to start the fire and to heat the drum to the point of rupture. A well-filled drum is not difficult to burst by heating, and the ruptured drum will then spray fuel into the fire for a long time with virtually no possibility of extinguishing it by conventional water or fog sprays. There is always the possibility of an explosion from such a device when flammable vapors are suddenly mixed into a large fire, but maximum damage is usually desired by the arsonist. Such a drum, placed at the origin of the fire kindled with additional fuel may not be suspect as an instrument of arson because there are many legitimate reasons for the storage of liquid flammables on most industrial or commercial premises.

The amateur arsonist does not always select the placement of the liquid accelerant that would provide the most effective ignition. Instead, the most common situation is the pouring of an accelerant over carpeting, furniture, and open floor, a pile of paper or trash, or a combination of these. The pouring of a flammable liquid onto an absorbent surface, such as soil, newspapers, or carpet, allows some fuel a chance to escape combustion. A stack of newspapers, for example, soaked in gasoline allows the fire to burn only on the outside of the stack, with the absorbent papers feeding additional fuel to the fire in the same manner as the wick of a kerosene lamp. Fuel near the center of the stack is not exposed to air or even to extreme temperatures and remains for detection days later, especially if the debris has been water-soaked to minimize evaporation. A pool of paint thinner in a carpet may burn a distinct ring with the carpet pile in the center of the ring barely scorched and a substantial portion of the fuel still present where it was not exposed to the flame-air interface. Fuel soaked into the soil likewise is poorly consumed and then only at the surface. Loscalzo, DeForest, and Chao poured gasoline on plywood, carpet, and soil, ignited it, and tested detectability of the residue at various time intervals. Depending on the length of time the gasoline was originally allowed to burn, gasoline residues were identifiable by gas chromatography as long as 24 hours after burning on carpet and as long as 162 hours on soil.²⁹ More recent tests showed that small quantities of kerosene (unburned) were detectable by standard forensic methods on denim jeans only for 24 hours and for only 72 hours on shoes, stored indoors at 22°C (68°F). Unburned gasoline was detectable for less than 24 hours on shoes and denim jeans under the same conditions, but was still detectable on carpet after 7 days of storage outside at 5­15°C (40­50°F).³⁰

The pouring of a flammable liquid directly on the floor is probably the most commonly detected form of arson. It is readily detected because a considerable portion of the fuel remains unburned and what does burn can leave characteristic burn patterns or diagnostic signs. Poor ignition is due in part to the amateur arsonist's misconceptions about properties of such 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. (With a direct pour, the liquid fuel tends to protect the floor covering by evaporation rather than damaging it.) 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. Remember that if the room subsequently goes to flashover, the radiant heat produced may obliterate many of the more subtle indicators on the floor. On ground-level floors, sometime enough flammable liquid is poured out that it seeps completely through the floor and into the soil beneath. In such cases the wooden subflooring, joists, and soil should be checked for the presence of volatile fuels. Another common mistake for the amateur is to dump the fuel in the center of the room without ensuring that the flames will reach furniture, walls, or other fuels to keep the fire going.

Multiple ignition points are common in arson fires. Not satisfied to gamble on a single point of ignition, the arsonist thinks that "more is better," with the intent of producing a larger fire that spreads more quickly and widely than that from one point. These multiple points are usually connected by a trailer of flammable liquid, smokeless gunpowder, rags, twisted ropes of newspaper, waxed paper, or wood packing material (excelsior). In a few cases, a trailer of flammable liquid has been poured from electrical outlet to outlet along a baseboard, apparently in hopes of simulating a massive electrical fire. Although multiple sets can provide a more complete destruction of the target, they present many problems that the arsonist would just as soon avoid. They require additional materials and fuels; there is also the chance that one of the elements will fail, leaving intact sets to be discovered. It requires more time to distribute fuel in several sites than one and, while the distribution is proceeding, vapors from the earlier sets may be forming an explosive mixture that may deflagrate, involving the arsonist. The shoes and clothing often are splattered with the fuel and may ignite. Especially if the fuel has a high volatility, the hazards from these sources are likely to outweigh any advantage that is gained from multiple points of ignition. The time required also prolongs the chances of the earlier sets being detected and the perpetrator being spotted while still at work. A low-volatility fuel such as kerosene may be suitable for such operations, but ones like gasoline or lacquer thinner can be extremely dangerous.

It should be noted that multiple points of ignition are not necessarily proof of arson. Heat from a general fire can melt and ignite the polystyrene light diffusers used in ceiling lighting fixtures or the asphalt from a collapsing roof. These flaming "liquids" can produce multiple, apparently unconnected, points of origin throughout a large area. Such multiple points of origin have been mistaken in the past for arson sets.

Of all the remote-ignition devices known, the Molotov cocktail is the most common. It consists of a breakable glass container of a flammable liquid which depends on a lighted wick or a chemical incendiary device to provide ignition. Its effects are maximized when it can be thrown against a hard surface (wood is often not hard enough) to break the bottle and mechanically atomize the gasoline into an explosive mist. The ignition then produces a fireball that can spread at very high speeds (up to 10 meters per second). Although a Molotov is usually ignited by its own flaming wick, hypergolic mixtures have been used to avoid the use of an open flame. A packet containing sugar and potassium chlorate is taped or tied to the outside of the bottle, while sulfuric acid is added to the gasoline inside. Upon breaking, the acid reacts with the sugar and chlorate to initiate a very hot flame which, in turn, ignites the gasoline. Although the production of a thickened "napalm" has been recommended in terrorist literature, the minimally enhanced burning effects are generally not worth the extra time and effort required to mix the gasoline with soap flakes or paraffin wax.

Although the use of such a device is convenient from the standpoint of the arsonist, its easy detection at a fire scene makes it less than ideal. A broken bottle, often a broken window, and perhaps eyewitnesses accompany its use. Tracing it to the arsonist may be difficult but often it is not. The broken bottle will usually be at the bottom of the fire where it is exposed to the minimum amount of heat, so fire damage to the fragments is minimal. The portions of the bottle bearing the most information about its origin-the neck with its labeling and the base with its cast-in production data (date and place of production and product code)-are the pieces most resistant to mechanical and fire damage and usually survive intact. The investigator should endeavor to collect all or nearly all of the pieces to allow reconstruction of the device for identification or even a search for fingerprints. (As noted previously, the oily components of latent prints are somewhat soluble in gasoline and may be washed off by the accelerant as it seeps from a broken container, but new techniques now promise recovery of prints even in the presence of gasoline.)

Plastic bottles or containers filled with fuel have been encountered in a number of fires. These may be placed over room heaters to soften and melt, but are more commonly surrounded by loose paper and trash that is set afire to provide a means of softening them and igniting the spilled contents. If properly supported by combustibles, these containers can burn sufficiently to make identification impossible.

One variation on this is the use of a toy balloon or plastic bag containing gasoline that is suspended by a string and set swinging over a candle. Once the oscillations die out, the balloon stops over the candle, which then melts a hole in the bag; the fuel released quickly spreads the fire to the surrounding trash. Although effective if small quantities of accelerant are used, the amateur will often use a large trash bag with lots of fuel that is more likely to extinguish the candle as it rushes out than be ignited by it.

Type of Accelerant

As we have seen previously, the type of flammable liquid used as an accelerant is determined in great part by the experience, motivation, and intent of the arsonist. In a premeditated structure fire set by someone for money (note that this does not imply the person is a professional torch), some thought will be given to selecting an appropriate fuel. Gasoline is by far the most commonly detected accelerant because it is an efficient and easily ignited flammable liquid and because it can be purchased and transported even in large quantities without arousing the suspicions of the seller or authorities. Paint thinners and kerosenes are the next most common fuels.³¹ They are effective fuels and can be purchased in moderate quantities without suspicion because there are legitimate uses even in the private home. Turpentines and lacquer thinners are almost never encountered as premeditated fuels. They are very costly and not often used in bulk by private individuals. The chemistries of new paints make their use more of an exception than the rule it once was. Most of the paints sold today for household use are the water-based latex type which do not use ignitable liquids as thinning or cleaning agents.

Although most arsonists seem to give little thought to the accelerant's volatility, it is a property that has great bearing on the fuel's suitability to the task. Extremely volatile fuels such as gasoline, lacquer thinner, or alcohols may evaporate excessively at high temperatures, interfering with the intended ignition and spread of the fire. The vapors generated 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. This makes it necessary to set the accelerant very quickly and to ignite it from a safe distance, thereby reducing the arsonist's control of the set. Kerosenes, paint thinners, and proprietary lantern fuels are easier to use because of their lower volatility. The varieties of lantern and campstove fuels can vary considerably in their volatilities and flash points. Because they may contain heavier hydrocarbons with commensurately higher boiling points, however, they are more resistant to evaporation and will remain in the debris longer as residues than will, say, lacquer thinners.

At the opposite end of the petroleum distillate scale is fuel oil. It is very difficult to burn except when it is dispersed into a mist and well mixed with air. When applied to paper or trash, it can burn vigorously as a thin film and thereby add to the fuel load, but it will not be readily ignited as a pool.

Alcohols, both methyl (wood) and ethyl (grain), are as volatile as gasoline and carry the same hazards. Because of their flammability and their reduced chances of detection because of their miscibility with water, they would appear to be good choices as flammable accelerants. They are not, however, efficient fuels, and it takes a larger quantity of burning alcohol to generate sufficient heat to bring other nearby combustibles to their ignition points. They will still burn when mixed with some water, but with a lower risk as well as effectiveness. Potable liquors of 90 to 100 proof (45% to 50% alcohol) can be used as accelerants, though not very effective ones.

The foregoing discussions deal with premeditated fire sets involving accelerants. Depending on the motivation and opportunity of the arsonist, such fuels may not be acquired beforehand. When it comes to dealing with fires set using materials already at the scene, ingenuity knows no bounds. 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 all have been used as effective arson accelerants. It is under these circumstances that an inventory of the scene and a thorough interview with owners, tenants, and maintenance people may be vital to the success of the inquiry. Although it is commonly known that the duplicating fluid used in the old-style "spirit" duplicators or mimeograph machines is highly flammable methyl alcohol, it is not commonly known that the copier toner used in some modern office "dry" copiers contains a petroleum distillate or isoparaffinic blend, similar to paint thinners. Many offices have cases of this fluid stored where it is easily accessible to burglars intent on erasing signs of their presence. The same holds true for commercial insecticides that are often dissolved in a light paint-thinner-like petroleum distillate. When such unusual fuels are suspected by the investigator, these suspicions should be relayed to the laboratory examiner along with control samples of all possible fluids.

Kirk's Fire Investigation, 4/E by John D. DeHaan, ©1997. Reproduced by permission of Prentice-Hall, Inc., Upper Saddle River, NJ. Permission from Prentice-Hall is required for all other uses.