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CHAPTER 6 Sources of Ignition Courtesy of Jamie Novak, Novak Investigations and St. Paul Fire Dept. KEY TERMS arc, p. 174 rollout, p. 184 short-circ...

CHAPTER 6 Sources of Ignition Courtesy of Jamie Novak, Novak Investigations and St. Paul Fire Dept. KEY TERMS arc, p. 174 rollout, p. 184 short-circuit, p. 179 piloted ignition, p. 185 self-ignition, p. 208 spark, p. 174 resistance, p. 169 service, p. 179 OBJECTIVES After reading this chapter, you should be able to: Describe common ignition sources and their typical heat release rates. Recognize secondary sources of ignition. Identify the role of appliances in starting fires. Discuss the role of hot and burning fragments in kindling fires. Explain the role of smoking as a fire origin. For additional review materials, appendices, and suggested readings, visit www.bradybooks.com and follow the MyBradyKit link to register for book-specific resources. Register for MyFireKit by following directions on the MyFireKit student access card provided with this text. If there is no card, go to www.bradybooks.com and follow the MyFireKit link to Buy Access from there. 167  Introduction to Ignition Sources Ignition can be defined as the initiation of self-sustaining combustion in a fuel. The mech- anisms of ignition were explored in detail in Chapters 4 and 5. As we have seen, the fun- damental properties that influence an object to ignitability are its density, thermal capacity, and thermal conductivity. Taken together, these properties constitute the mater- ial’s thermal inertia. Ignition requires transfer of enough energy quickly enough to over- come the thermal inertia and trigger sufficient combustion to become self-sufficient. If that occurs, the ignition source is said to be competent (for that fuel under those condi- tions). As we have seen from previous chapters, ignition involves bringing at least a part of the fuel to a characteristic temperature by means of conducted, convected, or radiant heat transfer until it can sustain combustion. In gaseous fuels, this process involves rais- ing the temperature of only a small volume to ignition. In solids, it usually involves the surface of the fuel (the exception being self-heating, in which the heat is being generated within the bulk of the fuel). No matter what the nature of the fuel, without some source of ignition, that is, without some source of energy, there will be no fire. Without excep- tion, the source is some type of hot object or mass, chemical reaction, flame, or electric current. Except for the self-heating case, the temperature of the source must exceed the ignition temperature of the fuel, and it must be able to transfer enough heat into a suit- able mass of fuel before there can be ignition. In the case of self-heating, ignition will cause self-sustaining smoldering combustion, in which oxygen from the surrounding air diffuses into the surface of the charring fuel to create enough heat to advance the reac- tion, but smoldering combustion can, of course, be started equally well by an external heat source. Although ignition of smoldering combustion is sometimes of interest to the fire investigator, for the most part the onset of flaming combustion is the event of most concern. Flaming combustion of a solid material occurs when sufficient gases or vapors are generated from the pyrolyzing fuel to support flames. As a general rule, the lower the energy of the ignition source, the closer the source and first fuel have to be for ignition to take place. The ignition source may be small and inconspicuous compared with the destructive fire that follows. There may be intermedi- ate stages from initial ignition to full involvement of the first fuel package and then spread as other fuel packages in the room ignite, and this item-to-item growth must obey the same physical laws. Material that is intentionally heated by electricity, friction, chemical reaction, or a controlled flame may be dropped or blown into fuel that then ignites. These intermediate occurrences must always be taken into consideration in dealing with the source of ignition. The challenge to the fire investigator is to identify the fuel first ignited and then determine how the ignition source managed to transfer enough heat to that fuel for it to kindle into flame. In this chapter we examine a wide variety of heat sources and their thermal proper- ties. This text cannot discuss each source in combination with every possible fuel. The fundamental processes of heat transfer, heat release rate, and fire propagation must be applied to each possible situation. By applying the scientific method to ignition processes, the investigator must evaluate likely sources. Primary Ignition Sources The primary source of ignition of virtually every fire is heat. As we have seen, the heat can originate from a hot surface, a hot particle, a chemical reaction, or a distant radiant heat source, but the small open flame of a common match or lighter is among the most effective primary ignition source for fires. Without question, such sources are responsible for a large percentage of both accidental as well as deliberate fires. Other sources include mechanical sparks (incandescently hot or burning particles) or electric arcs, and the lat- ter can range from very small to lightning bolt size. All these sources generate enough 168 Chapter 6 Sources of Ignition localized heat to ignite many fuels, although not all sources will ignite all fuels. Heat may also be generated by the passage of current through wires (particularly in an overload condition), through a high resistance connection or through an unintended conductive resistance Opposition path. Heat is also generated by numerous chemical reactions of the exothermic type, some to the passage of an electrical current. very slow, some very fast. To clarify the relationship of various ignition sources to various fuels, matches, lighters, torches, and candles will be considered briefly. MATCHES A very common means for initiating combustion, the match is specifically and exclusively designed for this purpose, and it is the most basic source of flame. Whether it directly ini- tiates the destructive fire or merely sets a controlled fire that is later responsible for the larger fire is immaterial to the basic question of ignition but may be very material to the investigation of the larger fire. The match is a stick that is treated to be readily combustible combined with a head that includes both fuel and oxidizer in a form that can be ignited by localized heat pro- duced by friction. Matches are found in two general categories: the strike-anywhere match and the safety match. The strike-anywhere or kitchen match contains an oxidant such as potassium chlorate mixed with an oxidizable material such as sulfur or paraffin, a binder of glue or rosin, and an inert filler like silica. The tip, which is more easily ignitable by friction, contains a high percentage of both phosphorus sesquisulfide (P4S3) and ground glass. The head of the safety match contains an oxidizer and a fuel such as sulfur. It will ignite only when struck against the strip containing red phosphorus, glue, and an abrasive like ground glass. The stick of the match is often treated with a chemical to suppress afterglow. Paper matches are generally impregnated with paraffin to improve both water resistance and the burning characteristics of the head. The heat output of a large kitchen match is on the order of 50 to 80 W (see Table 6-1), and the average flame temperature (once the incendiary tip has burned away) is on the order of 700°C to 900°C (1,300°F to 1,650°F). Because the match flame is laminar and involves wood (or cardboard) and wax, temperatures within the thin combustion zone near the outside of the plume are similar to those of a laminar candle flame and can be as high as 1,200°C to 1,400°C (2,200°F to 2,500°F).1 Such energy and temperature are adequate for ignition of many fuels as long as that energy can be transferred into the fuel. Such a low heat output is not enough to ignite most fuels by radiant heat, so there is usu- ally direct contact between the flame and fuel before ignition. This is true of ordinary combustible solids, but combustible gases or vapors can be drawn into the flame by entrainment flow, so there may appear to be ignition without direct contact. U.S. and Foreign-Manufactured Matches Minor variations in match formulation and manufacturing detail exist among U.S. manu- facturers, although some matches are custom-made with special head or shaft designs for promotional purposes. Large differences in appearance (and sometimes in action) are expected only with matches of foreign origin. For instance, some foreign-made matches have been known to ignite by frictional contact between their heads. Considerations for Fire Investigators It is common to attempt to compare matches from a scene with suspected sources like partial matchbooks on the basis of their length, width, color, and the paper content of the cardboard stems. On some occasions, the torn bases of the paper matches can be physi- cally compared, jigsaw fashion, to the stubs in the matchbook. Most U.S. book matches today, however, are pre-perforated, which makes them tear uniformly and thus reduces the chances for success in such comparisons. It is sometimes possible to trace the origin of partially consumed matches at a fire scene by conducting an elemental analysis on the unburned match heads.2 Chapter 6 Sources of Ignition 169  TABLE 6-1 Heat Release Rates and Burn Times of Common Ignition Sources MAXIMUM TYPICAL FLAME HEIGHT MAXIMUM TYPICAL BURN FLAME HEAT FLUX OUTPUT (kW) TIME (s)a (mm) (kW/m2) Cigarette 1.1 g (not puffed, laid on solid 0.005 1,200 — 42 surface) bone dry Cigarette 1.1 g (not puffed, laid on solid 0.005 1,200 — 35 surface) conditioned to 50% R.H. Methenamine pill, 0.15 g 0.045 90 4 * Candle (21 mm, wax) 0.075 — 42 70b Wood cribs, BS 5852 Part 2 No. 4 crib, 8.5 g 1 190 15c No. 5 crib, 17 g 1.9 200 17c No. 6 crib, 60 g 2.6 190 20c No. 7 crib, 126 g 6.4 350 25c (Crumpled) brown lunch bag, 6 g 1.2 80 (Crumpled) wax paper, 4.5 g (tight) 1.8 25 Newspaper—folded double sheet, 4 100 22 g (bottom ignition) (Crumpled) wax paper, 4.5 g (loose) 5.3 20 Newspaper —crumpled double sheet, 7.4 40 22 g (top ignition) Newspaper double sheet newspaper, 17 20 22 g ( bottom ignition) Polyethylene wastebasket, 285 g, filled 50 200d 550 35e with 12 milk cartons (390 g) Plastic trash bags, filled with cellulosic 120–350 200d trash (1.2–14 kg)f Small upholstered chair 150–250 — — — Upholstered (modern foam) easy chair 350–750 — — — Recliner (PU foam, synthetic upholstery) 500–1000 — — — Gasoline pool on concrete (2 L) 1000 30–60 — — (1/m2 area) Sofa 1000–3000 — — — Sources: V. Babrauskas and J. Krasny, Fire Behavior of Upholstered Furniture, NBS Monograph 173 (Gaithersburg, MD: U.S. Department of Commerce, National Bureau of Standards, 1985). *From S. E. Dillon and A. Hamins, “Ignition Propensity and Heat Flux Profiles of Candle Flames for Fire Investigation” in Proceedings: Fire and Materials 2003 (London: Interscience Communications), 363–76. a Time duration of significant flaming. b Centerline—immediately above flame; 4 kW/m2 outside. c Measured from 25 mm away. d Total burn time in excess of 1800 s. e As measured on simulation burner. f Results vary greatly with packing density. 170 Chapter 6 Sources of Ignition LIGHTERS Basically, there are two general types of lighters: the electric-element type found in vehi- cles, and liquid-fuel lighters. For most fire purposes, the electric lighter may be disre- garded as a source of ignition because it depends on the battery of a vehicle to heat the wire heating element. It is not operative except with its electrical connections to a battery, which greatly restricts its use. The basic liquid-fuel lighter ignites the flammable vapors from a fuel-soaked wick or liquid reservoir by means of a spark produced when a steel strikes a flint. Cigarette lighter “flints” are made of “misch metal,” which includes a rare earth such as cesium, lan- thanum, or neodymium whose sparking potential is very high.3 Thus, a spark or hot par- ticle ignites a fuel vapor under controlled conditions, leading to a small, predictable flame. Such lighters are a convenient substitute for a match and are, no doubt, responsible for many incendiary fires. They are rarely left behind and are therefore unlikely to be recov- ered as evidence. When lighters are recovered, the possible presence of latent fingerprints (especially on a removable fuel reservoir inside the case) and trace evidence should be con- sidered. Some lighters ignite the vapors by catalytic oxidation or piezoelectric effect on a crystal, but since the flame produced is the same as from a “regular” flint lighter, their use as an ignition source remains functionally the same. The heat release from a wick-type lighter is on the same order as that of a large kitchen match (50–80 W), and maximum temperatures are similar to those reported for pool fires of hydrocarbon fuels, ≈1,000°C (1,830°F).4 Traditional lighters that the user refueled with a light liquid petroleum product have largely been supplanted by disposable lighters filled with butane. In the latter, the butane fuel compartment is under pressure, and the fuel is delivered through a meter- ing valve to be ignited (in the manner described earlier) as a small, laminar jet flame. The adiabatic temperatures of such flames have been reported as 1,895°C (3,440°F), so real-world maximum temperatures might be expected on the order of 1,400°C (2,500°F). Research at the University of Maryland on cigarette lighters with port-type and Bunsen-type burner designs (adjusted to produce 75-W flames) reported tempera- tures reaching 2,022 K (1,749°C; 3,180°F) and peak heat fluxes as high as 169 kW/m2 (25 mm from tip axially).5 Tests of a typical disposable butane lighter indicated a fuel release rate of 0.001 to 0.002 g/s and a flame less than 20 mm (0.8 in.) in height. Based on a ∆Hc of 46 kJ/g, this fuel consumption would produce an estimated 40 to 90 W.6 Such lighters can explode if exposed to high temperatures and are subject to leakage when dropped or exposed to reduced atmospheric pressure such as in airplane cabins. The metering valve can some- times be adjusted to release a fairly voluminous stream of flammable gas, and the result- ing flame can be several inches high (with corresponding increase in heat release rate). Because these lighters are nonrefillable and intended to be disposable, they are more likely to be left at the point of ignition of a fire. Innumerable specialty lighters such as barbe- cue or fireplace lighters with an extended delivery tube are also available, as well as nov- elty lighters disguised as guns, pens, and toys. TORCHES Torches of a wide variety of types and fuels find uses in many industrial and residential applications. There are “traditional” blowtorches using manually pressurized “white gas” delivery. There are “drip torches” using kerosene or diesel fuel for setting controlled “back fires” in wildlands. Handheld or long-wand roofing torches use propane, butane or MAPP [methyl acetylene (H3C ¬ C ‚ CH), and propadiene (H2C “ C “ CH2)] as fuels. Acetylene/oxygen and acetylene/air handheld torches are very well known. Certain paint- stripping devices (heat guns) that are either propane-fueled or electrically powered can pro- duce very high gas temperatures and high heat fluxes. Examples are shown in Figure 6-1. Chapter 6 Sources of Ignition 171 FIGURE 6-1 Common hand torches. From left: propane, MAPP (with igniter), blow torch (white gas), butane microtorch. Foreground: acetylene/air hobby torch and oxy- acetylene welding torch. Courtesy of John D. DeHaan. Fire Findings reported that flame temperatures in a propane hand torch ranged from 332°C (629°F ) at the tip to 1,333°C (2,071°F ) at the flame center to 1,200°C–1,350°C (2,200°F 2,468°F ) at the tip of the “inner flame” cone.7 Any of these propane torches can cause accidental (and intentional) fires if they are applied to suitable fuels. Cellulosic materials are most susceptible to ignition by torch application especially if they are finely divided, since brief application of such a flame to a solid wood surface will cause only surface scorching and (possibly) brief glowing combustion that ceases as soon as the flame is withdrawn. Finely divided materials (dried leaves, needles, sawdusts) are very suscepti- ble to ignition, usually in two stages: smoldering followed by flame. More massive mate- rials are susceptible when the torch flame is played into narrow crevices or spaces, where radiant losses from surface ignition are minimized. In these cases, ignition can be caused within the cavity that may not be readily detectable or extinguishable from the outside. Such crevices can also retain sawdust and detritus from termite attack, making ignition even more likely. In hot torch work on roofs, torches are used to cause tar to adhere to roof structures or liquefy the asphaltic adhesive on the flexible membrane as it is laid. Under either condition, smoldering ignition triggered in crevices or in debris is then con- cealed under the hot tar or membrane. Such roof fires following several hours of con- cealed smoldering have been identified. A special variety of small devices is a new generation of microtorches advertised as being intended for welding and brazing operations but that are primarily sold by smoke shops. These use butane as a fuel and premix it in a special torch tip that can produce extremely hot flames. (See Figure 6-1.) These microtorches are small enough to be concealed in the hand and can be ignited and left burning. Due to their ostensible application as welding devices, such microtorches normally lack any child-resistant features. The maxi- mum temperatures measured in the combustion zones of such a microtorch should be the same as for a full-size one (1,200°C–1,350°C; 2,200°F–2,470°F). Such torches are often associated with inhalation of drugs of abuse such as crack cocaine. See Chapter 14 for further discussion of their role in arson. 172 Chapter 6 Sources of Ignition CANDLES Once only a consideration of fire causation during power failures, candles have become extremely popular in many different forms and functions. The National Candle Association estimates that candles are used in 7 of 10 American homes and that candle sales increase by 15 percent each year. The USFA estimates that candles are responsible for more than 9,400 residential fires each year. The CPSC estimated an even higher number—12,800 fires causing 170 deaths and 1,200 injuries.8 The NFPA estimated that in 2002 some 18,000 reported home fires were started by candles, causing 130 deaths, 1,350 injuries, and property losses of $333 million in the United States alone.9 A traditional paraffin or beeswax candle pro- duces about 50 to 80 W of heat with a laminar flame with an average flame temperature of 800°C to 900°C (1,470F–1,650°F), but like a match flame, a candle flame has an outer com- bustion zone that can have temperatures as high as 1,200°C to 1,400°C (2,200°F–2,550°F), as we saw in Chapter 2. Because of the candle’s steady, prolonged flame and high tempera- ture zone, it is possible to melt thin copper or even iron wire in a candle flame if it is held steadily in the very hot combustion zone, but the thermal conductivity of such metals con- ducts the heat away too quickly to permit the melting of larger-diameter wire. The structure and dynamics of candle flames were discussed in detail in Chapter 3. Dillon and Hamins reported that a 21-mm (0.8-in.)-diameter candle had a mass loss rate of 0.09 to 0.11 g/min. With a net ∆Hc of 43 kJ/g, this produced a flame 40 mm (1.6 in.) high, with an HRR of ~65 to 75 kW. They measured the total heat flux along the vertical centerline and showed a maximum of about 70 kW/m2 at the tip of the flame, dropping to ~40 kW/m2 at 50 mm (2 in.) above the tip and ~20 kW/m2 at 170 mm (6.7 in.) above the tip.10 Just 13 mm (0.6 in.) from the centerline, the total heat flux at the level of the flame tip was about 27 kW/m2, showing the significant amount of heat in the buoyant plume. These results confirm direct observations of the propensity for a candle flame to ignite fuels at some distance immediately above the flame but not target fuels at the side of the flame. A well-designed candle melts at just the right rate to maintain a steady supply of wax for the wick to deliver to the flame: too little wax and the candle extinguishes for lack of fuel (as the wick burns up); too much wax and the wick gutters and drowns.11 Ideally the wax burns completely to carbon dioxide (CO2) and water. The combustion of carbon soot occur- ring in the outer high temperature zone is essentially complete; virtually no soot escapes the laminar flame. If the wick is too long and the flame becomes turbulent, soot can escape. If the wax is contaminated with oils or other organic materials, these may not burn com- pletely, and a large quantity of soot can be released. If the wick is not symmetrically placed in the candle, the candle can fail and slump, exposing large lengths of wick. The heat release rate (and therefore the flame length) of a candle is determined by the amount of exposed wick. If charred matchsticks are dropped into a candle, they can act as supplemental wicks, dramatically increasing the size of the flame. The larger flame can impinge on adjacent fuels or heat the container in which the candle is placed (such as the metal cup of a small chaf- ing dish or tea light candle), creating increased risk of accidental fires.12 Candles cause fires when combustibles come into contact with the flame: loose material can move into the flame, the candle can tip over while lighted (especially with taper or column-style candles as opposed to pillar or votive styles), or the candle fails as the wick comes loose or otherwise becomes more exposed. This last situation causes a larger flame. Ignition can also occur when combustibles are included in the candle as decorations. Candles with aromatic oils are being used in large arrays for aromatherapy and then left to burn while the occupant goes to sleep. Large decorative candles with multiple wicks and burning times of several days mean prolonged risks of ignition of draperies, furniture, and clothing.13 Secondary Ignition Sources For purposes of this chapter, secondary ignition sources include sparks and arcs, hot objects and surfaces, friction, radiant heat, and chemical reaction. Chapter 6 Sources of Ignition 173  SPARKS/ARCS spark Superheated, The definition of spark is ambiguous because the term can refer to one of two situations: incandescent particle. an electric arc of brief duration where electric current is discharging through air or arc Flow of current another insulator across an air gap (or a tiny fragment of burning or glowing solid material moving through the air another nonconductor) between two The electric spark is not readily distinguished from the electric arc, except for dura- conductors. tion. Babrauskas has suggested that an arc represents a stable electrical flow, whereas an electric spark represents an initial flow of electric current across a gap between conductors. The arc persists as a discharge for some considerable time interval, while an electric spark is thought of as being virtually instantaneous. It is therefore simpler to regard all such elec- trical phenomena as arcs of various durations and leave the term spark to represent a solid particle or molten droplet heated by some process to incandescence. The longer an arc per- sists, the more time it has to heat its immediate surroundings and to transfer heat to sur- rounding fuel. The energy that can be released from an electric arc can range from millijoules to millions of joules. Because the arc can persist anywhere from microseconds to hundreds of seconds, the total heat released can be within a very broad range—from a tiny, brief arc of static electricity on a household doorknob to a massive lightning strike. The capacity of arcs to ignite fuels, then, depends greatly on their duration, current flow, and the susceptibility (physical and chemical properties) of nearby fuels. A full discussion of electrical sources of ignition including arcs is found in Chapter 10. The latter conception of a spark, that is, a tiny fragment of burning or glowing mate- rial moving through the air, has become the standard usage and will be used here. Sparks, then, can be produced by a sharp blow between two dissimilar materials such as steel and flint, by extreme frictional contact between two moving materials, by extreme heating and melting of conductors in electrical failures, or by the airborne debris of a solid-fuel fire. Such incandescently hot particles are capable of igniting fires in vapors and some solids and will be treated later in this chapter. Mechanical sparks include burning brands or very hot embers, fragments of debris from an existing fire, droplets of metal from extremely hot combustion or electric arcing events, incandescent or burning metal from impact or mechanical friction, or soot/partic- ulates from engines of many types. These will be considered as a special case of “hot objects” in the following section. HOT OBJECTS/HOT SURFACES Most hot objects are heated either by being in or close to a flame, by frictional heating, or by the flow of electric current through them. In electric appliances, ceramic or wire heating elements may be present that are designed to carry current and generate heat as a result. If combustible materials come into contact with these heating elements, they can cause fires by igniting and then spreading the fire to other fuels nearby. Such appliances, then, are not primary sources of ignition but lead to loss of control of a designed heat source. Malfunctions that allow a flow of electricity through conductive paths for which they were not intended or at rates that exceed the design of intended conductors can also lead to the production of hot surfaces. Hot objects play an important role in transmitting the heat of various chemical and physical processes to fuels, thereby spreading a fire. As we saw in Table 3-1, the visible light emitted by the surface of a hot object can give an indication of its temperature. The capacity of this object to ignite a fire depends not only on its initial temperature but also on its mass and thermal capacity, which together deter- mine how much heat this mass contains. The evaluation of a possible ignition of a fuel by hot object or surface is not a simple matter of noting the temperature of the surface or mass or comparing it with the listed AIT of the fuel. Ignition of any fuel will not occur unless enough heat is transferred into 174 Chapter 6 Sources of Ignition a sufficient mass of fuel to establish a persistent flame. The transfer of that heat from a surface depends on the nature and contour of the surface (even its roughness and cleanli- ness), the nature of contact, and whether the fuel can maintain the contact long enough. The AIT of a liquid or gaseous fuel, for example, varies with the test method used, the volume and shape of the test cell, and the material used for the test cell.14 For example, clean, polished metal hot surfaces produce ignitions at lower temperatures than do simi- lar metals that have an oxide layer. Very short contact or residence times do not allow enough heat to be transferred. A volatile liquid fuel dripped onto a flat, hot metal surface is likely to cool the immediate contact area by evaporation; the resulting vapors rise by convection away from the hot surface, thus reducing the residence time. The temperature of the surface, then, would have to be hundreds of degrees hotter than the AIT for that fuel for there to be ignition, and even then it would not be a guaranteed result. Interestingly, in general for liquid fuels, the lower the flash point of the liquid, the higher the temperature of the hot surface necessary to achieve ignition.15 Hot-surface ignition of liquid or gaseous fuels is achieved at lower temperatures if they are confined (in a tube or other enclosure). Other variables include surface type, droplet size, and air- flows near the surface. When hot particles can penetrate or burrow into a solid fuel, res- idence time is no longer a factor, but the total amount of heat that the mass can carry is critical. If the thermal energy of the particle is too low (insufficient mass or density, inad- equate thermal capacity), there may not be ignition even if the object is buried in the fuel and transfers all its heat. Hot objects that land on a rigid surface are even less likely to ignite a solid fuel due to the extra convective and radiative losses to the surrounding air. It is tempting to measure the temperatures of hot surfaces as they occur in open air and then compare those temperatures against ignition temperatures of potential fuels (as has been reported in various recent publications, such as Fire Findings), but such temper- atures are only a guide to the processes taking place (see later section in this chapter.) If the temperature on the surface of a glass lightbulb in the open is measured at, say, 200°C (400°F), that value reflects only the steady-state temperature of that surface achieved by the balance between the heat flux (input) striking the inside of that bulb from the hot fil- ament and the heat losses from convective, radiative, and even conductive processes on the outside of the bulb. If the rate at which the heat is lost is changed by placing the bulb in contact with a surface or by burying it in an insulating material, the temperature of that surface will rise, often dramatically. The new temperature then poses a very different “source” for transfer of heat to a potential fuel. In short, burying even a low-wattage heat source in a deep layer of cellulose insulation poses an ignition risk even when the open- air temperature of that heat source would make it seem “safe.” As a reference point, however, knowledge of open-air temperatures of some hot sur- faces is useful. The surface temperatures of halogen work-light lamps (bulbs) has been reported to be 593°C to 685°C (1,100°F to 1,266°F) (sufficient to ignite flammable vapor/air mixtures as well as solids), and the glass face of a 300-W halogen work light to range from 180°C to 215°C (357°F to 418°F).16 The open-air surface temperatures of heating elements of electric range tops ranged from 370°C (700°F) for small burners to 590°C (1,100°F) for large burners in older appliances, and 500°C to 730°C (925°F to 1,350°F) for equivalent burners in new ranges.17 Most modern electric ranges have a great deal of power in each cooking element, as well as high temperature. Goodson and Hardin reported wattage ratings for various electric range heating elements ranges from 1,200 to 2,500 W.18 As we have seen, with complex fuels such as cellulosics, the increase in temperature may bring about physical and chemical changes in the fuel that make it more susceptible to ignition than the original fuel. The heat transfer rate to the fuel surface is probably more useful than temperature as an indicator of ignitability. A table of minimum critical radiant heat fluxes is found in Appendix K. The chemical and physical nature of the fuel has to be considered, and the effects of radiant heat have to be tested as part of the Chapter 6 Sources of Ignition 175  hypothesis testing of the scientific method. The higher the radiant flux applied to a fuel, the lower the observed ignition temperature will usually be, dramatically decreasing the time to ignition. If the fuel is a thermoplastic, it may melt, sag away from the ignition source, or form a nonporous mass and become less likely to be ignited. Materials that char upon initial heating may become more susceptible with initial heating if the charring produces a porous residue that will support a smoldering combustion. Charring may also produce a pyrolyzed fuel that requires more energy to ignite. FRICTION As a source of ignition, friction is a special case of a “hot object.” Friction between two moving surfaces generates heat (as in the disc brakes of an automobile, which can become extremely hot). As a source of fire, rubbing two sticks together is little used on the mod- ern scene (except traditionally by Boy Scouts). Even they are more likely to supply a bow to spin a wooden point in a wooden depression because of the much greater speed and frictional heat that are generated. Wood is used, not primarily because it is a fuel, but because it is a very poor conductor of heat. It allows heat to be generated by friction faster than it is carried away and dissipated. With enough effort, the local heat may rise to the point of kindling a glow in fine tinder, which can then be aroused into a flame by fanning or blowing. Although this method of igniting fires has historical interest, it is of little or no practical consequence today. However, friction in other contexts may be of great importance in igniting fires, espe- cially in machinery. An overheated wheel bearing or “hot box” on older railroad cars— frictionally heated because of inadequate lubrication—may cause discharge of hot metal fragments or cause adjacent fuel material (e.g., cotton-waste oil wick) to catch on fire. Any bearing that does not have adequate lubrication can become hot through friction, and contact of the hot object with a readily ignited fuel can lead to a fire. This lack of lubrication is without a doubt one of the relatively common sources of fire where machin- ery is in use. Conveyor or power transmission belts can jam or be forced to run against frozen rollers and cause extreme frictional heating. Once ignited, the moving belt can spread fire very rapidly. If frictional heat becomes extreme, one or both of the surfaces may disintegrate, pro- ducing a shower of incandescent particles. Such disintegration may be the end result of massive overheating or very localized surface heating where the temperature of the mass itself may not rise appreciably. The temperatures of hot particles generated by this process are limited by the melting temperatures of the material involved. For instance, a mechan- ical spark generated from a copper-nickel alloy surface may have a maximum tempera- ture of 300°C (570°F) (and not be incandescent at all), while a fragment of tool steel may be at 1,400°C (2,550°F) and be bright sparkling white.19 This situation can occur with any moving machinery. Bearings in conveyor systems, for instance, can clog with abrasive dust or seize from lack of lubrication and create large numbers of incandescent sparks for long periods of time. Bearings in automotive turbochargers can fail in seconds if lubrica- tion is interrupted, due to their very high rotational speeds (up to 25,000 rpm). Not only bearings give rise to dangerous temperatures from excessive friction. High- speed rotors, such as those found in airplane motors, that come into contact with the hous- ing have been known to produce enough heat locally to melt the housing and ignite related hardware. Fragments of overheated brake shoes in malfunctioning train or truck brakes can be thrown from the moving vehicle for miles before being detected. Mufflers, tailpipes, tailgates, chains, and other equipment dragging on the road surface can generate substan- tial showers of incandescent metal particles with enough heat to ignite roadside fuel (dried grass, leaves, or litter). Machining operations (milling machines, lathes) can generate show- ers of hot particles that can ignite waste in or under the tool. Mechanical sparks (hot frag- ments) are discussed in more detail in a later section of this chapter. 176 Chapter 6 Sources of Ignition RADIANT HEAT Although the heat radiated from a fire may at times ignite other fires at a distance, such ignition is not primary but secondary to an already large, established fire. Radiant heat plays a great role in spreading most fires and was examined in Chapter 3. As a primary ignition source, radiant heat alone is much less frequently encountered. Radiant heat from fireplaces, stoves, and heaters has been seen to bring nearby cellulosics to their igni- tion temperatures and thus start fires. When such devices are permanent fixtures, their remains will usually still be detectable after the fire. Other sources may not be so easily identifiable. The investigator must remember that direct physical contact between the heat source and the first fuel is not always necessary when sufficient radiant heat can be brought to bear on the fuel surface. Here the reflective and absorptive qualities of the fuel are critical, as are its density and thermal conductivity. All that is required is for the target fuel to absorb more heat than it can dissipate and for that heat to raise the local (surface) temperature above the fuel’s spontaneous ignition temperature. Rays from direct sunlight (at a typical heat flux of 1 kW/m2) are not intense enough to ignite com- mon fuels, but if they are concentrated or focused by a transparent object that is round in cross section (i.e., spherical or cylindrical) or by a concave reflective surface (such as a shaving mirror or the polished metal bottom of some aerosol cans), they can reach 10 to 20 kW/m2 at the focal point of the light path. If a cellulosic fuel is located at or near that focal point, the fuel can be heated to its ignition temperature and catch fire. Two examples of ignition by sunlight focused by shaving mirrors are shown in Figures 6-2 and 6-3. This mechanism is often blamed for starting wildland fires, but experiments by DeHaan revealed that the conditions have to be just right: the object has to produce a sharply focused area of sunlight, and the dry fuel has to be located precisely at the focal point of that lens or mirror before ignition will take place. Empty glass bottles, broken glass, or flat reflectors do not have a measurable focal length and will not cause ignition by this means. The maximum heat fluxes of some common small flaming ignition sources (at a dis- tance of 25 mm; 1 in.) were shown in Table 6-1. Some tools and appliances, even when operating as designed, can act as nonflaming ignition sources. Radiant heat from floodlights FIGURE 6-2 Sunlight focused on cloth target by shaving mirror can reach heat flux sufficient to ignite fabrics. Courtesy of Jamie Novak, Novak Investigations and St. Paul Fire Dept. Chapter 6 Sources of Ignition 177 FIGURE 6-3A Shaving mirror (foreground) focused sunlight onto FIGURE 6-3B Shaving mirror (on left) focused sunlight from window corner of box above it in parked car. Courtesy of Scott R. Schuett, onto towels, which ignited on rack. Courtesy of Curt Hawk, Fire Cause Lebanon, CT. Analysis. (particularly quartz halogen), projection lamps, heat lamps, and flameless torches can be enough to ignite cellulosic fuels at close range.20 A noncombustible covering (sheet metal, tile, asbestos, or plasterboard, for example) can allow heat to be transferred to a fuel sur- face beneath while impeding the loss of heat by convection, thus making it susceptible to ignition even when concealed. Hot surfaces associated with vehicles such as brake rotors, exhaust manifolds, and catalytic converters have the potential to ignite fuels by both radi- ant and conducted heat and will be discussed in Chapter 9. Radiant heat is also linked to the ignition of wood even when the heat flux is not enough to create ignition temperatures directly. In installations of heaters, furnaces, boil- ers, and even chimneys and vent flues, radiant heat at low levels may induce degradation of the wood to charcoal. Charring of wood can occur when temperatures on the order of 105°C (230°F) or greater are maintained for very long periods of time. This mechanism was discussed in Chapter 5 and will be discussed in greater detail in a later section deal- ing with furnaces. CHEMICAL REACTION A number of chemical mixtures are capable of great heat generation and even formation of flame. They are only of occasional consequence in ordinary investigation of fires. Some accidental fires in warehouse-style home-supply stores have recently been linked to leaks or spills of corrosives (acids or bases) that came into contact with metals, or strong oxi- dizers (swimming pool chlorines) that came into contact with fuels such as automobile brake fluids and underwent an exothermic reaction. They play a role in a limited number of incendiary fires because of the lack of knowledge about them on the part of the arson- ists. Such reactions are more likely to be encountered in industrial or clandestine drug lab fires than elsewhere, and they require the attention of the chemical specialist. Salvage and recycling operations may produce mixtures of chemicals that spontaneously combust on contact, producing dangerous, large fires.21 Many chemical fires are due to ignition or decomposition of hazardous chemicals. The properties of the more common hazardous materials are examined in detail in Chapter 13. There are, of course, biochemical reactions that generate heat—drying oils, decom- posing cellulosic and organic materials, and the like—that can contribute to ignition. They are discussed later in this chapter under the heading “Spontaneous Combustion.” 178 Chapter 6 Sources of Ignition The Role of Services and Appliances as Ignition Sources So many fires are started either by service facilities or appliances, or are attributed to service The conduc- these origins, that consideration of such possible origins is of great importance. When the tors and equipment for delivering electricity origin of a fire is not determined, it is very convenient to decide that it was due to some from the supply system malfunction of an appliance, a short-circuit in electrical wiring, or other similar cause. to the equipment of This decision is especially attractive because, in many fires, there is so much damage to the premises served. wiring, or melting of appliances, that such an opinion can have some credibility. In addi- tion, it is indisputable that malfunctions or failures of services and appliances do cause short-circuit Direct contact between a many fires. Thus, investigators may find themselves on the horns of a dilemma in the current-carrying attempt to separate the real from the claimed origins of fires at such times. In modern conductor and another building construction, electrical wiring is widely distributed as an integral part of nearly conductor. all parts of the structure, and gas lines and appliances are also very common. In all such structures, it is the rare fire indeed that does not engulf some such items to some extent at least, thus possibly focusing attention on them. The investigator must remember that the mere presence of an electrical wire or a gas line does not constitute proof of responsibility for igniting a fire. If an electrical wire is not energized (that is, no voltage applied to it), it cannot provide energy to ignite any fuel. Even if it is energized, there has to be a mechanism, a reason, for it to create heat in an unintended manner. It is the investigator’s responsibility to establish this mechanism and demonstrate how it can create enough heat to ignite the first fuel. Gas lines, even when charged or pressurized with gas, do not constitute an ignition source (except under the most exceptional circumstances when they can conduct misdirected electrical energy) but are merely a source of fuel. GAS APPLIANCES AS IGNITION SOURCES Gas appliances used for nonindustrial purposes are relatively limited to a variety of heat- ing devices: stoves, ranges, room heaters, furnaces, clothes dryers, and water heaters. All these items have open flame burners that create heat. Thus, regardless of the exact type of heating device, the investigations, as well as all the possible hazards that attend the use of the device, are very much the same. The heat output rating of the appliance, whether the flame was correctly adjusted, how the flame could come into contact with com- bustibles, and whether the automatic control mechanisms are functioning, are the features that are of interest to the fire investigator. One of the few ways in which the gas furnace or dryer in normal operation is likely to initiate a fire results from the simultaneous failure of the thermostat and the high-level or “high limit” control. The high-level device is a second thermostat or thermal cutoff (TCO) located in the circulating-air ducting of a furnace, for example. It is usually cou- pled in series with the regular thermostat and is adjusted to break the circuit (and close a valve on the gas delivery line) at a maximum temperature above which the furnace can- not be safely operated. This temperature varies but is usually above 90°C (200°F). A simultaneous failure of both thermostats allows the furnace to continue operating indef- initely with great production of heat and possibly a resulting fire. The failure of pressure regulators (as mentioned in Chapter 4) can cause an appliance to become an ignition source. Although such regulators may or may not be part of the appliance, their failure can cause masses of flame to escape from the burner compartment of the stove, furnace, or water heater affected. These flames can then ignite nearby com- bustibles by radiant heat or by direct flame impingement. Even under normal operation, the burners create considerable heat. Some appliances may not have adequate standoff or may be installed improperly, thereby exposing combustible walls or floors to radiant or conducted heat (as illustrated in Figures 6-4a–c). Inadequate air supplies can result in Chapter 6 Sources of Ignition 179 FIGURE 6-4A Fire in water heater closet as found (unit less than 1 year old). Courtesy of Battalion Chief Kurt Hubele, Richland, (WA) FD. FIGURE 6-4B After cleanup—no floor left in closet. Courtesy of Battalion Chief Kurt Hubele, Richland, (WA) FD. FIGURE 6-4C Exemplar heater in next apartment showed heater installed on plywood floor with no stand-off legs. Courtesy of Battalion Chief Kurt Hubele, Richland (WA) FD. 180 Chapter 6 Sources of Ignition excessive soot formation in the combustion chamber. This hot soot can then induce smol- dering ignition of nearby combustible surfaces. The “standing” pilot flame that was once standard on gas water heaters (and furnaces, ranges, and ovens) and offered a small but continuous and potentially competent ignition source, has largely been replaced by electric igniters. These igniters may be in the form of a high-voltage arc, an electric spark created by capacitive discharge or piezoelectric effect, or by a glow bar (electrically heated hot surface). The investigator should examine all gas appliances carefully before concluding their status as an ignition source.22 Although gas appliances do start fires, they are probably less often the cause of a fire than is generally assumed. Gas is the chief fuel in many places, and its intrinsic hazards tend to encourage an abnormal fear of it as a source of fire. Gas can serve as a convenient “probable cause” when the “poor” investigator finds no other that is provable; this approach is not recommended. Occasionally, gas is released as a result of a failure within the electrical system of the appliance and then ignited as it is released.23 The investigation of any fire involving fuel gases should include examination and documentation of the con- dition of the fuel supply, the ventilation available to the flame, electrical control system(s), and the system for removal of products of combustion (vents, flues, and chimneys). A common residential gas water heater (as diagrammed in Figure 6-5) produces a fire of 24,000 to 36,000 Btu/hr (6.66 to 10 Btu/s, 7 to 10.5 kW). A steel baffle is inserted in the central flue to slow the exhaust gases and give them more time to transfer heat to the water. There is typically a draft hood at the top of the heater to entrain room air into the exhaust gases and cool them. The vent is usually single-walled metal except where it passes through walls or ceilings, where it is at least double-walled, as shown in Figure 6-6. The flue-gas temperature at the top of the tank for steady-state operation of a 30,000 Btu/hr residential water heater is on the order of 300°C to 315°C (569°F to 601°F) with Vent stack (single-wall) Water in Draft hood w/standoffs Hot water outlet Typical Temperatures during Normal (Steady-State) Operation Center of flue (top of tank) With baffle 300ºC–315ºC (569ºF–600ºF) Without baffle 565ºC (1,050ºF) Baffle Exterior of vent connector 73ºC (164ºF) Top of 4-ft vent stack (gas temp.) 183ºC–205ºC Fiberglass (362ºF–400ºF) insulation Water tank Without baffle 400ºC (750ºF) Water 60ºC–80ºC (140ºF–175ºF) Top exterior of tank 36ºC (96ºF) Underside of front 63ºC (145ºF) Exhaust flue Drain Burners Service door Gas controls/thermostat FIGURE 6-5 Cross section of typical residential gas water heater and some typical temperatures developed. Some data courtesy Fire Findings. Additional data from tests by author (unpublished). Chapter 6 Sources of Ignition 181  FIGURE 6-6 Typical double-wall heater vents. Exterior—galvanized steel. Inner—steel or aluminum. Courtesy of John D. DeHaan. the baffle in place. If the baffle is left out, the gas temperature rises to 565°C (1,050°F). The draft hood allows entrainment of room air, reducing the gas temperature in the flue (with baffle) to around 200°C to 250°C (400°F to 450°F).24 On-Demand Heaters Energy conservation issues are prompting the installation of “on-demand” or “tankless” water heaters instead of traditional 30- to 50-gal reservoir units. These units can be gas or electrically fueled. They are typically wall-mounted metal boxes approximately 0.9 m (36 in.) high, 0.5 m (20 in.) wide, and 0.3 m (12 in.) deep. They may be concealed in clos- ets or service areas. They provide hot water only when they sense water flow. A gas-fueled burner provides 120,000–200,000 Btu/hr (31–57 kW) of heat to heat 4–8 gal of water per minute. Due to their very high heat outputs, on-demand units have to be properly installed and vented. Most gas units have external electrical connections for the ignition and control devices.25 Fire Cause Considerations When a gas appliance starts a fire, the fire pattern itself will often, if not usually, reveal the fact. Associated with the pattern showing the origin to be behind, around, or over the gas appliance there must invariably be some malfunction, improper installation, or other detectable fault or defect. Only the combination of factors can be considered as proof of origin from a gas appliance. The reason for the double requirement is relatively obvious. The arsonist often will set the fire under or close to a gas appliance to make it appear that the appliance was responsible for the fire. In such cases, liquid flammables are likely to be poured around the appliance and the fire started so that the origin seems to be obvi- ously associated with the appliance. An incorrect assessment can result if care is not exer- cised in the scene examination. In such an instance, such ignitable liquids are likely to penetrate below the appliance, and possibly below the floor, and produce the character- istic charring and damage in regions a gas fire could not reach. Natural gas fires are unlikely to burn much below their point of release, since the fuel is significantly lighter than the surrounding air. Fuel oil and the vapors from flammable liquids will settle down- ward. Propane released under pressure (i.e., from a delivery line or as a jet) mixes with air sufficiently that it will neither rise nor settle. Common sense to the contrary, people insist on storing flammable liquids in furnace or water heater closets. Leaks or spills from stored containers create vapors that are quickly drawn into the combustion chamber of the appliance (by the convection of either 182 Chapter 6 Sources of Ignition  FIGURE 6-7 New FVIR water heater with closed controls and fuel supply. Note flash suppressor mesh in air intake. Courtesy of Jim Albers, Santa Ana Fire Dept. (retired). the pilot flame or the main burner). If there is sufficient vapor mixed with the air, an explosion (usually followed by sustained fire of the liquid) will result. Recent changes in water heater design have been aimed at reducing the chances of ignition of flammable liquid vapors (called FVIR—flammable vapor ignition resistant— designs) (see Figure 6-7). For many years, building codes have required gas water heaters to be at least 18 in. (45.7 cm) above floor level if they are placed in a garage (where acci- dental releases of flammable liquids are likely). Testing has revealed that unless the liquid is poured near the heater and the room tightly sealed (to prevent losses by advective flow under doors), this elevation precaution will prevent ignition. If spillage of flammable liq- uids is a possibility even inside homes, to prevent ignition one design places a standard water heater into a steel bucket (or “weir”) with sides 18 in. high. Testing has revealed this design to be highly efficient in preventing ignitions.26 Another new design employs a flash suppressor screen on the combustion chamber and sealed service, electrical, and plumbing entry ports. This design will prevent any ignition (of vapors or fugitive gas) starting in the combustion chamber from propagating into the room.27 Insulation All approved gas appliances are surrounded by some type of insulation. In circulating heaters, this insulation consists merely of an outer wall separated from the wall of the fire- box, through which circulates the cool air that is to be heated. Such a wall will not become significantly hotter than the ambient air that circulates behind it if air circulation is maintained both within the heat exchanger and outside the appliance. Presence of com- bustible trash against such an appliance wall will not normally lead to ignition of the trash. The same cannot be said of the wall of the combustion chamber, which may well be hot enough to ignite material against it. However, something would come into direct contact with it only under very unusual circumstances. Shifting or even partial collapse of such air-space insulation may create hot spots on the exterior, as will settling of packed insulation. These hot spots then create a higher ignition Chapter 6 Sources of Ignition 183  risk. Improper clearances between vent stacks (flues) and wood framing can result in igni- tion. The absence of the draft hood or central baffle in a gas water heater will cause higher-than-design (normal) gas temperatures on the outside of the vent, which can trig- ger degradation and ignition of nearby wood. Backfire or Rollout A common claim that there has been a backfire from a furnace or other heating appli- ance carries somewhat more credibility than an ignition of combustibles against the heater. Nevertheless, a backfire is an extremely rare happening that can result only from gross maladjustment or malfunction within the appliance. To obtain such a “backfire,” it is necessary for the gas to accumulate in some quantity in the combustion chamber before it is ignited. Such accumulation can occur as a result of overpressure in the deliv- ery system, use of the wrong fuel delivery orifice, or blockage of the heat exchanger or exhaust vent.28 On ignition, a small explosion occurs and flames may blow out of the open front of the appliance. If there is a suitable combustible fuel in the path of this small amount of flame, a fire could result. The combination of factors is so unlikely, however, that any claim of this type must be very carefully scrutinized before being accepted. With a pilot light, it is difficult to accumulate sufficient gas for any signifi- cant backfire, although maladjustment of some type may be effective. In any event, so gross a defect is not difficult to recognize. In the absence of a pilot light, the gas will not ignite until it diffuses or spreads to some other source of ignition, which in all prob- ability will require enough gas to produce an explosion. In addition, escaped natural gas will often pass harmlessly up the vent pipe into a chimney. It is difficult to imagine any reasonably normal situation in which fires are caused by this type of situation. The common use of electric ignition systems on gas furnaces, stoves, and water heaters is replacing a continuous ignition source—the pilot light—with an intermittent source that does not function until demanded. This system could allow for the buildup of a volume of gas/air mixture with explosive potential instead of providing a continuous ignition source to burn off small quantities as they reach their lower flammable limit. With a fuel oil–fired furnace, the motor-driven pump can produce a sustained fire out- side the furnace housing if the exhaust or heat exchanger is blocked. The failure of a fuel gas pressure regulator (either in the appliance or external in the delivery system) can cause continuous massive “overfueling” that results in the exit of burner flames via rollout Exiting of appliance service doors or vent openings (rollout).” burner flames in an appliance due to mas- Direct Contact sive overfueling. Another manner in which a gas appliance can start a fire involves direct contact of a normal gas flame with flammable or combustible material. In general, this will occur only with open flames, such as gas range tops or broilers, although flammable materi- als can sometimes be drawn into a protected flame. The electric elements of stoves or broilers can also ignite solid fuels that come into direct contact, or the contents of skillets or pans. Plugged Vent It has been reported that if the vent of a gas water heater or furnace is plugged, the lack of a chimney can induce sufficient back pressure that the flames are forced out of the burner enclosure, possibly into contact with nearby combustibles.29 Examination of such devices should include careful examination of the exhaust duct for the remains of bird nests, dead leaves, construction debris, and the like. Blockage of the vent stack alone should cause the hot gases to vent from the draft hood if it is properly installed. The inves- tigator should also check to see that the lower flame shield (under the burner) has not been removed and that the service doors are in their proper locations. (Careless owners often neglect to replace the service doors after igniting the pilot light and should be inter- viewed to determine the last time the equipment was serviced or lighted.) 184 Chapter 6 Sources of Ignition Open Flames Open flames are most common on kitchen ranges, laboratory and industrial burners, and special types of heating appliances and are more hazardous as sources of fire. A common example is the ignition of cooking oils and greases in the kitchen, as well as towels, cloth- ing, curtains, paper, and the like near open flames. Fires started by stoves and ovens are the leading cause of residential fires.30 Autoignition temperatures of lard or vegetable cooking oils are quite high [over 350°C (662°F)], but enough boiling and splattering oils can splash onto the burner below to cause piloted ignition at lower temperatures. (The piloted ignition piloted ignition temperature is the minimum temperature at which a fuel will sustain a Ignition aided by the presence of a separate flame when exposed to a pilot source.) external ignition source Skillet fires can be very large (see Figure 10-40). Grease ignitions are especially com- such as a flame or mon as a cause of fires in restaurants, where the amount of cooking is great, and time spent electric arc. cleaning stove hoods may be minimal. Such fires may also spread upward in grease-coated piloted ignition exhaust ducts, where fans and vents carry the fire up with increasing magnitude. Most temperature The building codes require automatic fire protection of the cooking area and lower hood. Such minimum temperature systems can do little if the flame is established in the exhaust ducting above the hood. at which a fuel will The ignition temperatures and flash points of cooking oils are listed in Table 6-2. sustain a flame when Note that the fire points are well above the “normal” cooking oil maximum tempera- exposed to a pilot source. ture of 205°C (400°F).31 This suggests that unattended cooking is a major contributing factor. Tests observed by one of the authors of 500 ml (20 fl oz)of corn oil in a skillet on a gas stove revealed that heavy white smoke with a marked lachrymatory property was generated after ~10 minutes, and autoignition occurred about 25 minutes after the start of the test. The flames were between 0.5 and 0.7 m (19 and 28 in.) high and caused igni- tion of the cabinets above and to the nearest side of the stove.32 Note in Table 6-2 that the fire point, AIT, and hot-surface ignition temperatures of all cooking oils decrease with use. Small gas appliances that are not part of a permanent installation can be displaced or upset, with occasional drastic results. Laboratory burners are especially subject to being tipped over. They can start fires by playing the flame on top of a wooden table, for example, or against some other combustible material. Similar considerations may hold for small portable gas heaters sometimes used in homes and similar areas. These heaters may be permanently installed or temporary and may not meet any standard of safety or building codes. The investigator is referred to NFPA 54: National Fuel Gas Code or NFPA 58: Liquefied Petroleum Gas Code, which give detailed instructions for TABLE 6-2 Properties of Cooking Fats Measured at Fire Research Station HOT-SURFACE FAT FLASH POINT (°C) FIRE POINT (°C) AIT (°C) IGNITION TEMP. (°C) AFTER 8 AFTER 8 AFTER 8 AFTER 8 HEATING HEATING HEATING HEATING VIRGIN CYCLES VIRGIN CYCLES VIRGIN CYCLES VIRGIN CYCLES Corn oil 254 227 NA 321 309 283 526 542 Drippings (bacon) 254 241 NA 331 348 276 553 537 Hydrogenated cooking fat 260 210 NA 331 355 273 568 554 Lard 249 218 NA 326 355 282 541 568 Olive oil 234 218 NA 316 340 280 562 543 Peanut oil 260 243 347 335 342 280 552 535 F From Ignition Handbook by V. Babrauskas, © 2003, p. 886, Fire Science Publishers. Used by permission. Chapter 6 Sources of Ignition 185  the proper installation and operation of all types of gas-burning appliances. Fire Findings reported the air venting from a 60,000 Btu/hr (17 kW) kerosene-fueled salamander portable heater had a temperature of 639°C (1,182°F). Due to turbulent mixing, the tem- perature of the maximum hot-air stream dropped to 238°C (460°F) at 0.3 m (1 ft) and 148°C (298°F) at 0.6 m (2 ft) from the heater.33 Altered Gas Nozzle An unlikely, but possible, type of fire causation from a gas appliance involves some alter- ation in the gas nozzle in the venturi, possibly by partial or total unscrewing of the noz- zle or installation of the incorrect fuel nozzle. If this should happen, much larger than normal quantities of gas would reach the flame, which, in turn, might become much big- ger and spill from the top or front of the appliance. The authors have not witnessed such an accident, but it is conceivable, especially when tampering or ignorant efforts at repair or adjustment have occurred. Such fires would necessarily occur at the first use of the appliance after the opening was enlarged, which should again make their detection and proof of cause relatively simple. The service and repair history of the appliance may reveal recent problems or repairs. As discussed in Chapter 4, the venturi used must match the fuel gas and pressure regulator being used. Often, X-rays will reveal internal failures, blockages, or the open/closed position of fire-damaged valves and regulators. PORTABLE ELECTRIC APPLIANCES Heat-producing electric appliances can, of course, cause ignition of susceptible fuels even when operating normally. (Arcing, failures, and malfunctions will be discussed in Chapter 10.) Electric heaters, toasters, or toaster ovens with exposed Nichrome (resistance) heater wires offer an ignition source with surface temperatures on the order of 600°C (1150°F) or greater. Such hot surfaces are not able to ignite most flammable gases or vapors but can ignite most cellulosics (including foods) if they come into contact or near contact with them (see Figure 6-8). FIGURE 6-8 Toaster pastries ignited by repeated operation of toaster cause flames 30 cm (~12 in.) high sufficient to ignite nearby combustibles. Courtesy of Jamie Novak, Novak Investigations and St. Paul Fire Dept. 186 Chapter 6 Sources of Ignition  The heater elements in clothes dryers are especially susceptible to such contact. Dryer lint is one of the most readily ignitable solids in the average residence. It is usually a mix- ture of fine cellulosic, synthetic, and natural fibers (wool and hair). Accumulations of lint on heater grids or on adjacent surfaces of venting (both inside the dryer and out) are one of the most common “first fuels” in dryer fires. The radiant heat produced by most portable electric heaters is on the order of 10 kW/m2 or less at the wire guard. This may be enough to ignite cellulosics or char elas- tomeric and thermosetting materials if contact is prolonged. Cellulosic fuels that are brought into contact with the heater element itself are likely to be ignited quickly. Today, most consumer-type portable electric heaters have “tilt” switches to shut power off if the unit is knocked over, in addition to thermostats and TCOs. Investigators must carefully examine such heaters to determine what, if any, safety devices are present and if they have failed or been tampered with. KEROSENE HEATERS The fuel in kerosene heaters may be delivered to a constant flame by vertical transport up a wick that is partially inserted in a fuel reservoir, in the same way as in a kerosene lamp. A more effective means of fuel delivery is a barometric delivery system in which the fuel is delivered to a shallow horizontal reservoir from a vertically oriented removable tank. The flow of fuel is controlled by a barometric valve in the tank. A partial vacuum above the fuel in the tank prevents the fuel from flowing into the reservoir until the level drops below that preset by the valve. Modern heaters have a shutoff mechanism that extin- guishes the wick if the heater is tipped over or subjected to excessive vibration (move- ment). The main safety problem with kerosene heaters is that the capacity of the tank exceeds that of the reservoir, so that if there is a loss of the vacuum in the tank, the excess fuel can flood the reservoir and create a spill external to the heater, which then ignites with catastrophic result.34 This loss of vacuum can occur as the result of a leak in the tank or its associated parts, or as the result of fueling the heater with a fuel such as gasoline or camping fuel with a higher vapor pressure than that of kerosene. In these cases, the vapor then replaces the partial vacuum that holds back the flow of fuel, resulting in a flood of fuel that overwhelms the reservoir. Although a newer anti-flare-up device is available, most older heaters encountered pose a serious fire risk from this type of failure.35 Accidental spills during refilling can cause significant fires (especially if the burner is not extinguished). Any residual fuel left in such a heater should be collected and preserved for testing to establish its actual content. STOVES AND HEATERS A variety of liquid fuel portable heaters are also made for camping and boating use. These range from pressurized fuel delivery systems (Primus stoves) to denatured alcohol fuel burning on the surface of a noncombustible wick element. These heaters are subject to spillage of burning fuel if tilted or overturned. Although these heaters are not intended for use in enclosed spaces, they are often seen in tents and campers. Because such heaters may consist of a small circular burner pan and a perforated metal shield over it, they may not be recognized by an investigator as ignition sources (see Figure 6-9). OIL STORAGE External heating (fuel) oil storage tanks are common in both residential and commercial premises. Traditionally, these are welded steel tanks with rigid metal pipe connections. Those features, combined with the low vapor pressure (high flash point) of typical heat- ing fuels, mean they do not constitute a high risk of ignition (either accidental or intentional). However, when such fuels are absorbed on porous materials that can act as a wick, their low ignition temperatures (~205°C; 400°F) pose a risk. If plumbing connections are Chapter 6 Sources of Ignition 187 FIGURE 6-9 Alcohol- fueled units sold in camping stores can be used as tent heaters or cook stoves. Courtesy of John D. DeHaan. exposed to an external fire, they can fail quickly. Some jurisdictions allow fuel storage tanks to be made of high-density polyethylene (owing to their corrosion resistance and lower costs). Testing has shown that such tanks (in 300- to 400-gal sizes) can be caused to fail by external fires. If 5 L(1.5 gal) of gasoline is poured over the outside, or a mod- est cardboard/newspaper fire is ignited underneath the tank, a catastrophic failure and release of the tank contents can result. Placement of a cloth wick into the mouth of the tank also causes failure, since it continues to burn within the tank (due to the low vapor pressure) until the tank melts. The melted plastic of the tank can support a large pool fire of molten polyethylene, so even an empty tank poses a fire risk.36 Oil-fueled furnaces can cause fires by creating heavy soot if burners are maladjusted, or more commonly, leaks from fuel connections can support flames outside the combustion chamber, which then ignite nearby combustibles. ELECTRICITY Electricity represents a source of ignition in the form of arcs, electrically heated wires and heating elements, and even lightning. For a full discussion of all means of electrical fire causation, see Chapter 10. The Role of Hot and Burning Fragments in Igniting Fires Hot or burning particles, commonly termed sparks, are in a special category with regard to kindling of fires. As pointed out earlier in the chapter, the designation spark is an ambiguous one, since it can refer to incandescently hot metal from a mechanical or elec- trical source or to hot bits of debris from an established fire. Most fragments from an established fire are glowing, although they may at times be flaming. Bits of wood, paper, and other light organic fuels are most susceptible to the formation of glowing fragments, 188 Chapter 6 Sources of Ignition and larger pieces of paper may travel while still flaming. Bits of paper or tobacco may become detached during lighting of a cigarette or pipe. These particles naturally have the propensity to ignite other fuels with which they come in contact if they retain enough heat. It should be remembered that most such fragments will be carried upward by the draft from the fire that generates them, and air currents may carry them for significant distances to ignite other fuel. WINDBLOWN SPARKS The question frequently arises: How far can windblown sparks travel and still cause a sec- ondary fire? Although no precise answer is possible, some factors can be stated. The ambi- ent wind is highly important, because such sparks tend only to rise with the buoyant flow over the fire, drift a short distance, and fall nearby, unless propelled by wind. Another fac- tor is the type of material that is burning. Very small fragments and thin materials such as paper will normally be totally consumed before they return to some combustible surface. Fragments of wood, excelsior, or corrugated cardboard will burn considerably longer and travel farther before igniting something else. For the latter, distances of about 12 m (40 ft) have been observed. Tests by DeHaan demonstrated that burning wood chips and straw could be carried up to 15 m (50 ft) from their source in a 32-km/hr (20-mph) wind to ignite cloth, straw, and wood shavings targets.37 Light items will probably only rarely travel as much as 6 m (20 ft), except under significant wind conditions. Another factor is the height that they reach before being blown to one side. The higher they go, the more time they have to burn up totally or to cool off, and the longer their exposure to any air movement. In any event, distances in excess of 9 to 12 m (30 to 40 ft) are to be accepted only with the greatest caution. Documentation of the intensity and direction of wind at the time of the fire is critical to the evaluation of such mechanisms. Throughout this discussion of spark travel, the role of wind has repeatedly been referred to as modifying the fire spread. No investigation involving the distance or effectiveness of such spark ignition is complete without ascertaining from meteorological records, or at least from eyewitnesses, the wind conditions at the time in question. It is evident that violent winds will produce very different effects from mild ones. However, the difference is not that which might be supposed. A strong wind will blow a spark farther than a weak wind but will also speed the rate of combustion. With a given size of burning fragment, the wind effect does not vary greatly with different wind velocities. The most significant effect of the strong wind, by far, is its ability to blow large fragments of material, which will naturally burn longer than small ones. A wad of burning newspaper on the ground will not ordinar- ily travel along the ground for any significant distance if propelled only by a breeze. In a strong wind, however, it may roll for a considerable distance before it is consumed. The rela- tion of importance is the time necessary for it to burn as compared with the speed with which it is moving laterally with the wind. The consideration is far more relevant to mate- rials at ground level than to those that emerge from chimneys, because very large burning objects are rarely carried up a chimney to any significant height. In very large fires, how- ever, the updraft can loft large pieces of burning debris, boards, branches, cardboard, and rags high enough that they can be carried hundreds of yards by the prevailing wind, ignit- ing remote fires. This has been observed to be a major means of fire spread in large fires in both urban and wildland settings, as depicted in Figures 6-10a and b. In one recent major conflagration, burning wood fire brands (some as large as 2 4s) were lofted some 60 m (200 ft) above a multistory condo/retail complex under construc- tion and were blown more than 680 m (0.4 mi) to ignite wooden shake-shingle roofs of two apartment complexes (see Figure 6-10a). At the time, the winds were 2–27 km/hr (13–17 mph).38 See Figure 6-10b for an illustrative map. There are several common origins for hot fragments, and since these are the primary source of the fire that may result, they will be considered individually. Chapter 6 Sources of Ignition 189 FIGURE 6-10A Massive fire engulfing six-story residential/commercial FIGURE 6-10B Map showing trajectory of burning brands 0.7 km complex (wood framed). Courtesy of Don Perkins, Fire Cause Analysis. (0.4 mi) from fire scene to ignite roof of apartment complex 0.8 km (0.4 mi) away. © 2009 Google––Map data––© Tele Atlas FIREPLACES AND CHIMNEYS Fireplaces are blamed for igniting many fires, some of which they undoubtedly do. Nevertheless, the mere fact that a possible fire origin is close to a fireplace is not to be considered as positive indication that the fire was initiated by the fireplace. Discussed next are six ways in which a fireplace can be instrumental in initiating a fire. Emission of Sparks or Blazing Materials from the Front or Open Portion Such sparks or blazing materials may ignite nearby combustible floor coverings or furnishings. Serious fires from this cause are rare because the top of a wooden floor is very difficult to ignite to a sustained combustion. Scorch marks are not uncommon and are annoying, but wood floors do not usually represent the first fuel ignited. Light fuels, such as paper, may readily be ignited, so prefire actions and conditions may be critical. Some rugs or carpets are sufficiently flammable to be ignited under these circumstances, and furniture may occa- sionally be ignited. Most building codes require a noncombustible hearth extending well to the front and sides of a fireplace opening. Burning fragments of Christmas wrappings apparently ignited a large pile of crumpled wrappings in front of the fireplace in the fire shown in Figures 6-11a and b. Thus, there is a hazard from this source, but it is readily interpreted when encountered (when prefire activities are described). Intense Radiant Heat from a Fireplace Stove Filled with Inappropriate Fuels or Overfueled Sufficient radiant heat can be generated by intense fires in the firebox to create heat fluxes of 10–20 kW/m2 through the front opening. These can ignite ordinary combustibles in line of sight of the opening if they are within 1 m (3 ft). Inappropriate fuels include crum- pled masses of paper, Christmas tree branches, plastics, foams, cardboard boxes, and sim- ilar commodities. Some fuels are intended for fireplace use (e.g., compressed-sawdust artificial logs) but may cause overheating if overfueled. The manufacturer of artificial logs typically includes package instructions requiring that not more than one log be put on the fire at a time. Overheating of the firebox is also liable to cause ignition of wood materi- als located behind the fireplace or inside the wall. Emission of Sparks or Blazing Materials from a Chimney Chimney fires, which are the greatest danger factor, arise because soot, bird or rodent nests, creosote, dust, cobwebs, and a variety of combustible materials sometimes are allowed to 190 Chapter 6 Sources of Ignition FIGURE 6-11A Living room furniture burned when fragments of FIGURE 6-11B Heat and smoke layer enveloped the top of Christmas burning Christmas wrappings escaped from fireplace to ignite a pile tree but only scorched it (oxygen-deprived smoke layer). Courtesy of of wrappings on the floor. Courtesy of Derrick Fire Investigations. Derrick Fire Investigations. accumulate in the chimney, where they may be ignited by a spark or flame from the fire below and lead to a large blaze within the chimney itself. When this happens, large quanti- ties of burning fragments will be expelled from the chimney top and greatly increase the danger of igniting a roof or other adjacent combustible structure. It is clear that this possi- bility is the most dangerous from the standpoint of starting large fires. It is also clear that a large proportion of them will be roof fires, although blazing residues (firebrands) or large glowing sparks (embers) may travel some distance and ignite fires elsewhere than on a roof. One consideration of the effect of wind on chimney action is also relevant. Wind blowing across a chimney opening exerts what is known as the Bernoulli effect, that is, a lowering of the pressure at the chimney mouth. The result is a stronger draft up the chimney, which can increase the size of fragment that can move up to the top and be ejected into the stronger wind stream. This is an additional reason for the investigator to be very careful considering wind velocity when there is a possibility that spark movement started a fire. Emission of Sparks or Blazing Materials Resulting from Defects or Holes in a Chimney Such sparks can ignite timbers or other construction material adjacent to the chimney. Fires kindled by this method are equally rare but may be very seriou