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The time factors of any fire are often the most critical factors an investigator
had to establish and yet they are the most difficult. Establishing the fuel
load is often pretty straight-forward by direct examination of the remains,
interviews with owners, tenants, or fire-fighters, or review of inventories,
bills of lading or even surveillance tapes, the investigator can usually figure
out what fuels were there. Establishing possible ignition/heat sources may be
more of a problem. Barring the discovery of a broken Molotov cocktail, evidence
of a lightning strike or massive electrical or mechanical failure, sources of
heat may be small, subtle, or missing completely. To rule out various ignition
mechanisms, it will be critical to identify the first fuel ignited (even as a
series of hypothetical scenarios) and to establish time factors for each.1 We
normally think of "time" in a fire as: "At what time did the fire start?" But
in the absence of witnesses or surveillance videos we normally cannot answer
that question and we have to establish other factors related to time. These
factors may include: time to ignition, time to involve the first fuel ignited,
time to spread to other fuel packages, time to flashover, and time to
suppression. Each one of these "times" is important to the overall
reconstruction of the event and requires different information to be available
to the investigator, some from the scene and some from background knowledge.
Let's look at each of them.
The time to ignition is the time delay between contact of the ignition (heat)
source with the first fuel to be ignited and the moment combustion is
established. (Even here, we have to establish what we consider to be combustion
is it first open-flame or is self-sustaining glowing (smoldering) combustion
enough?) In its simplest form, time to ignition is dependent on (1) the nature
of the firstfuel ignited and (2), the rate at which heat is supplied to the
fuel. Some fuels ignite faster than others given the same heat source and the
faster the rate of heat transfer, the faster the fuel will heat up. The nature
of the fuel is a really broad conceptfrom its chemical and physical
properties to its physical state. Its chemical and physical properties include
its thermal conductivity (k), density (p) an its thermal capacity
(c) (often taken together as thermal inertia[kp c]), plus its
melting point, boiling point, and color. Its physical state, if it is a liquid
fuel, is whether it is in the vapor state at ambient starting conditions, and
if it is a solid fuel, whether it is finely divided (thermally thin like
crumpled sheets of paper, or thermally thick like a stack of phone books). It
would be nice to generalize and say that the larger the ignition source the
shorter the ignition time (and in fact, if all other conditions are the same, tig
is inversely proportional to q"2) but that is not true
in the general sense. A match (with a heat release rate HRR of 50
W) will ignite paper faster than a cigarette with its 5 W HRR. A very small
amount of energy released in an electric arc (even a low voltage one) will
ignite a fuel/air mixture if it is correctly mixed and in contact with the arc,
but it will not ignite most solid fuels. The important features of the ignition
source are: heat release rate, energy content (and therefore temperature), and
whether radiant, conductive, or convective transfer processes predominate. The
radiant heat output of a source goes up in proportion to its temperature raised
to the fourth power (T4) so the hotter it is (the larger T is), the
much higher the heat output is. The glowing coal of a puffing cigarette has a
temperature of around 1800°F but its HRR is only 5 W and its radiant and
convective transfer are so low that direct contact between the coal and the
fuel to be ignited is going to be necessary for ignition. The radiant heat
output from a fire is more significant than its convective heat output if the
fuel to be ignited is in front of the fire instead of directly over it.
For a solid fuel to be ignited by an external source, enough heat has to be
transferred to it so that its surface temperature reaches its ignition
temperature. This is where [kp c] comes in. If a fuel has a low thermal
conductivity (like urethane foam), heat losses to its interior are going to be
minimal, so the surface temperature goes up quickly, faster than if the same
amount of heat were applied to a fuel with a higher conductivity. This means
the fuel will ignite in a shorter time and flames will spread across it more
quickly. The same is true for a fuel with a low density or thermal capacity,
where a given amount of heat will cause a larger increase in temperature than
the same amount of heat would produce in a fuel with a high density or thermal
capacity. So to establish time to ignition we need to know: the nature and
condition of the first fuel ignited, the nature, HRR and temperature of
potential ignition sources, and their position and distance from the fuel. Most
fuels will reach smoldering combustion before (i.e., at a lower temperature)
than flaming, so time to ignition has to consider whether a glow or a flame is
needed to advance the fire. Another factor that must be considered is the
duration (life) of the ignition source. Ignition requires not only enough heat
in the source but enough contact and prolonged enough contact between
fuel and source, so that adequate energy is transferred. An electric arc
typically lasts only a fraction of a second, a burning match only a few
seconds, a burning piece of toast just 3 minutes, a cigarette about 15 minutes.
The question to be asked is: "Is this enough time to ignite the fuel
involved?"
Once the first self-sustaining combustion is established, a new time frame
becomes importanthow long does it take before the first fuel package is
fully involved, or at least involved enough to cause spread to other fuels.
This will depend first on whether smolder or flame is occurring. Obviously
combustion rates for smoldering are on the order of 0.1mm/second (0.25
in/min) where flame spread rates can be 0.01m/sec (0.4 in/sec) for solid fuels
and as high as 10m/second (40 ft/sec) for premixed vapor/air mixtures.
Variables include the chemical and physical characteristics of the fuel
involved (such as [kp c]), the physical state of the fuel, and the geometry,
shape and orientation of the components. There are too many variables to
discuss here but the issue is to realize that there will be a time delay,
ranging from a second to many hours between initiation of first combustion and
the time at which the fire is big enough to spread. An appreciation of time
factors has to be gathered from first-hand observations of test fires and also
from data on furniture tests such as those in James Quintiere's book and on the
BFRL.NIST web site. Such furniture tests show that it takes about three minutes
for a modern sofa to become completely involved from one point of ignition by
open flame, and about ten minutes to burn to completion. These times are
dependent on the size of the compartment, where heat reflected from nearby
walls or ceilings causes a heat feedback effect that increase fire growth
rates.
What of spread from the first fuel package to others in the vicinity? Here the
same concepts hold as described earlier, but on a larger scale. The question
is: "How big is the fire from the first fuel package (its HRR)?"
(Note that all fires at this stage have basically the same flame temperature
whether it is a pool of gasoline, a wooden desk, or a polyurethane foam sofa
burning.) The HRR determines how tall the flame plume is (from the Heskestad
relationship) and how intense the radiant heat flux is at various distances
(from the relationship: q" = Xr Q/ 4p d2 where Xr
is the radiant heat fraction, Q is the heat release of the fire and d is the
distance between the fire and the receiving surface). We also need to know the
position of other fuel packages in relation to the first fire. Much of the
initial heat energy of a fire is in the convective plume so fuels immediately
above the fire will be ignited soonest (heated by direct flame contact or
convective transfer). Fuels alongside the first fire will be heated
predominantly by radiant heat so they will not be involved until the fire is
quite large. In considering fire spread, two factors that get overlooked are
the possibility of "drop-down" ignition and the role of wall
coverings. When adjacent walls are painted plasterboard, there will not be much
contribution, but when the wall is easily ignited wood paneling, there can be
extremely rapid horizontal and vertical flame spread. Ignition of furnishings
can then be triggered by pieces of burning paneling falling or simply by
increased radiant heat. Separation between packages is very important. One rule
of thumb is that a 1 MW fire in one chair will not spread to another chair more
than 1 m (3.3 feet) away by direct radiant heating. A good example of this
would be a scene where one chair has burned completely away leaving adjacent
chairs scorched, but otherwise undamaged, since the first fire was simply too
low in HRR and too far away to bring about ignition. Fires can also spread by
ignition of carpets or other materials at floor level.2
Once again, times of spread to adjacent furnishings are dependent on the size
of the compartment, where heat reflected from nearby walls or ceilings causes a
heat feedback effect .
Time to flashover is finally being recognized as the critical factor it is in
many investigations and reconstructions. Flashover is the transition to the
point at which all of the fuels in the room are fully involved, heat fluxes and
temperatures are at a maximum throughout the room, the most intense fire is no
longer linked to location of fuel packages but instead is taking place where
the ventilation-driven mixing of air and pyrolysis gases are best. Such fires
are not survivable. Time to flashover is often the same as time to detection
since fires are detected in many cases only as they go to flashover, as windows
shatter, flames vent from doors and windows, and the shudder and rattle of the
turbulent combustion makes their presence known to people inside and outside
the building. The time required for a fire to progress to flashover is
controlled by the HRR of the fire, the size of the compartment, the nature of
walls and ceilings (as far as thermal losses) and the area and height of any
ventilation openings. The "average" (bedroom) size room requires a fire of,
say, 600800 kW to go to flashover. That can be created by a single over
stuffed foam chair fully involved after ten minutes, a PU foam mattress partly
involved after three minutes, or within seconds of half a gallon of gas being
poured on the floor and ignited. The sooner additional packages can be
involved, the sooner this trigger (critical) HRR can be reached, and the
shorter the time to flashover. This is often the role of an accelerant3
Applied in moderation, the HRR of a "pour" of a flammable liquid may not be
sufficient to trigger flashover but it serves as a big match. Lasting only a
minute or two, it manages to ignite enough of the furnishings in the room that
the critical HRR is achieved and flashover occurs sooner and more surely than
would occur if the fire were to grow from one item to the next by ordinary
routes of propagation.
It was once thought that if a fire in a room went to flashover in less than 15
minutes, it could not have been accidental in cause and had to have had help
(i.e., an accelerant) Unfortunately this is not true. For the reasons outlined
in a previous column, a fire of sufficient HRR to trigger flashover can occur
within a very few minutes if a single item of modern furniture is ignited by
open flame. Numerous experiments by this author and others have demonstrated
that an 8 x 8 x 8 compartment, with a moderate fuel load, can go to flashover
in less than 3 minutes from open flame ignition of modern furniture where
larger rooms (12 x 16 x 8) can be fully involved in less than five minutes if
the first fuel package is suitable. It should be noted that the time difference
between an accelerated fire in a room and a non-accelerated fire involving the
furnishings may be a matter of a very few minutes, and it is rare that
witnesses can help determine the actual time of ignition to that sort of
precision. The investigator has to be careful before deciding that a "fire was
so fast, it had to have been accelerated."
Assessment of all furnishings in a room (including carpets and wall finishes
and their potential heat release rates) must be carried out. A reconstruction
of the arrangement of the room (on paper if not in actual fact) is critical to
this assessment. This may be done by witness statements, physical remains, burn
indicators, or pre-fire photos or even videos.
Other times may be important to fire reconstructions: time to smoke detector or
sprinkler activation, time to evacuate, etc. But a major one is time to
suppression, defined as the time between ignition and the time at which the
fire is suppressed. This unfortunately is very hard to establish because it is
not reported in run reports or even radio communications transcripts, so it is
often an estimate from fire crews. (Only interviews with fire crews can reveal
what was burning on arrival and on entry (not often the same time), how the
fire was attacked, and how long it took to bring under control and extinguish
(at least in critical areas). There are those who think that "The fire did so
much damage, it had to have been arson." This ignores the time factor. Any fire
that burns uncontrolled long enough can result in tremendous damage, even
complete consumption, no matter how it was started. Despite the difficulty of
extracting this information, it really is critical to accurate reconstruction
and should be on the investigator's checklist for time really is the essence
(of thorough investigation).
1 Testing of hypotheses as the key to the scientific method as it is
applied to fire investigation will be discussed in a future column.
2 See previous column about the ignitability of new generation
synthetic carpets
3 A future column will address the role of accelerants.
Dr. John DeHaan has been a criminalist for some 32 years. He has worked at
county, State, and Federal forensic labs. He is a native of Chicago and his
Bachelor of Science degree in physics was from the University of Illinois at
Chicago. He has been involved with fire and explosion investigations for over
30 years, and has authored dozens of papers on fires, explosions, and their
investigation and analysis. He is probably best known as the author of the
textbook Kirk's Fire Investigation (now in its Fourth Edition). His doctorate
(in 1995), from the University of Strathclyde in Glasgow, Scotland, was on the
Reconstruction of Fires Involving Flammable Liquids.
He is a member of NFPA, and served on its 921 Technical Committee from
1991-1999. He is a member of the IAAI and serves on its Forensic Science
Committee. He holds a diploma in Fire Investigation from the Forensic Science
Society (United Kingdom) and one from the Institution of Fire Engineers (U.K.).
He is a Fellow of the American Board of Criminalistics in Fire Debris Analysis
and a member of the Institution of Fire Engineers. He retired from the
California Department of Justice in December 1998 and is now the president of
his own consulting firm, Fire-Ex Forensics, Inc., Based in Vallejo, California,
where he now serves as a consultant in fire and explosion cases all over the
U.S., Canada and overseas.
Other articles in the Our Changing World Series
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