THE BURNING BUSH
BY WILLIAM J. RAFTERY
The run started out simple enough, as a run that probably occurs thousands of times every hour across the United States. “Dispatch: Full assignment dispatched for activation of a residential fire alarm system. Alarm time 0640.”
But this one was different and really left me wondering. In fact, it got me into a few weeks of research. On arrrival, we found a typical scene and began a typical fireground operation.
“IC to Dispatch: We have a 112-story residence, alarm sounding, nothing showing from the outside … investigating.” Ring the doorbell. No answer. Circle the building, peek in the windows, check the neighbors for a keyholder, look for an easy way in. No keyholder available, shades drawn, replacement windows are hard to open without causing damage, doors have deadbolts front and back.
We all know that 99 out of 100 of these alarm activations are false alarms for one reason or another. Is this number 100? Let`s look for some more clues: There`s a car in the garage and just a smidge of condensation on one window. Is somebody home? What caused the condensation?
Let`s make entry. The “easy in” is to pull the garage door key switch and short the connections. The door raises and in we go, only to find that the door between the garage and house has another deadbolt. Are we going to do more damage and find out that this is one of numbers 1 through 99? After a couple of pumps on the trusty hydraulic door opener, we are in.
“Captain to IC: We`ve got smoke.”
Moderate smoke condition … smells like wood. It`s number 100, all right. Good thing we played our hunches, but still the run is typical. Primary search, stretch in from the hydrant, preconnect to the door, find the source, let`s get ready to rumble.
“Captain to IC: Primary search complete and negative. We found the fire … under control … but you`ve gotta come see this!”
Up to now, you were probably thinking that the point of this article was going to be, “Watch out for number 100.” Wrong! This is number 101.
There it is … a lone potted plant in a four-inch pot, wrapped in decorative Mylar that the florist put on to make it attractive. It is sitting in the middle of the floor, nothing is within a foot of it, and it is emitting smoke like a huge cigar. It is now surrounded by five experienced firefighters who are on their hands and knees, scrutinizing it as if it were a jigsaw puzzle. The plant is dead, but the soil is intact, and the smoke is apparently coming from someplace on the inside of the potting soil. There are no signs of outside ignition, and nothing is visible on the plant such as a cigarette butt or a match ash. It has obviously ignited spontaneously.
The plant is carefully placed on a tray and brought outside for final extinguishment. It is placed on the driveway; once it is broken apart, it bursts into open flame. Spontaneous ignition is much like pornography. It might be a little hard to describe, but when you see it, you immediately recognize it.
The homeowners eventually arrive and are interviewed, and we try to make some sense out of the unusual occurrence. They are a middle-aged couple and do not smoke. They have no idea as to what kind of plant it was, because it was given to them a year or two ago. They added nothing to the plant except for water, and the plant eventually died. The dead plant sat on the tile floor of an unheated solarium for a month or so; it dried up. A couple of days ago, the homeowner moved it to a heated room and placed it on a carpeted floor, awaiting the next municipal trash pickup. The couple had left the premises about a half-hour before alarm time and noted nothing out of the ordinary.
Back at quarters, the story of the “burning bush” is passed from shift to shift. As we know, some of the great philosophers of the world sit around kitchen tables in America`s fire stations and discuss issues of world importance in any number of categories. High on our list of categories is, What if? What if things were different? What if that incident had turned into a working fire and destroyed much of the surroundings near the point of origin? What if all that was left at the point of origin was an area of low burning with a burn pattern on the floor? Would we have determined the right cause? Would we have said, “Hmmm,” and requested the Origin and Cause Team? What would the Origin and Cause Team have found?
We firefighters sometimes have a tendency to “What if” things to the point of being ludicrous, but faced with these questions, I also had to ask, “What if?” Had this fire progressed to a point of full-room involvement, I probably would have seen the low burn patterns with no apparent source of ignition and called for the Origin and Cause Team. It, in turn, would have seen the same patterns and probably have labeled the fire “suspicious,” thus casting suspicion on the innocent homeowners and placing their insurance claim in jeopardy. Had the Origin and Cause Team told me that it had determined the truly correct cause to be “the spontaneous ignition of a potted plant,” I wonder what I (and the whole arson investigation community) would have said. I have no answers for these questions. Remember, we are only talking about “What ifs.”
Considering the long history of mankind and the fire service, it would be unusual, indeed, if I had actually seen something that had never been seen before. I find it unbelievable that I, personally, might be the first fire officer to experience this phenomenon. Someone must have seen this before I did.
I began asking my peers for prior knowledge of any similar event. No luck. I tried researching “spontaneous combustion” via the common sources and found that most of the written material relates to large piles of susceptible materials such as coal, hay-stacks, vegetable oils, soybean bins, and so on. Other accounts of self-combustion occur in smaller piles of things like bleached rags–not really spontaneous combustion but exothermic chemical reactions.
My best source of information was the National Fire Academy Library, which gave me a very good list of documents and sources relating to the subject. My chief concern was how it happens, and I will now try to explain the phenomenon in terms that we all can understand.
Let`s go back to the fire triangle: heat, fuel, and oxygen. How basic can we get? Yet, if we can`t put the triangle together, we can`t have fire. As far as fuel goes, if we look closely at a potted plant, we find combustibles entrained within the soil. Along with the noncombustible dirt, we find things like root systems and peat moss. Normally, these roots and soil contents would be difficult to ignite because of their high moisture content; however, when dried sufficiently, they become pretty good tinder. Thus, the fuel is in the soil.
When we look at heat and oxygen, things become a little more complicated. There are several processes that must take place under the right conditions for “heat buildup” and “oxygen feed” to occur to support fire.
SPONTANEOUS COMBUSTION
Here is my hypothesis on the occurrences within this single plant that led to combustion.
Phase 1:
Oxygen. Even though potting soil appears to lack oxygen, a good potting soil is “aerated,” meaning the soil must be “loose” and contain enough oxygen to feed the plant. Tightly packed soil is like clay and will not support plant growth.
Heat. All plants contain bacteria. Bacteria respires. We normally associate the word “respire” with the human respiration process … “Breathe in, breathe out.” Respiration, however, is actually the process in which living things take in oxygen and, in turn, produce heat through molecular activity. All living things respire, but they all do not necessarily “breathe” as we generally know it.
Process. The metabolism of the bacteria produces heat and warms its surroundings. Scientists call this a thermopile (from the Greek–thermo meaning “heat” and pile meaning “a bunch”). We firefighters have experienced thermopiles many times, often during overhaul–we call them “hot spots.” With uninsulated surroundings, the heat escapes to the atmosphere and is dissipated unnoticed. Within the plant, however, the aerated soil acts much as a thermal bed blanket or the insulation of a winter jacket. The trapped air is warmed, and instead of escaping, the heat is held in around the close proximity of the source. The aerated soil and Mylar plant wrap surrounding the heat source are now keeping the heat contained at the source.
Phase 2:
Oxygen. Even though the soil surrounding the bacteria contains oxygen, this source of oxygen gradually becomes depleted, used up by the oxidation process. Without more oxygen, the reaction would die and never produce fire. Either an oxygen flow from the outside atmosphere or another source of oxygen is needed to support combustion. The oxygen source in this case comes from the breakdown of the fertilizers contained within the soil. Nitrates, which are contained within most fertilizers, release oxygen when they are heated. There`s our continuous source of oxygen.
Heat. The heat is still contained within the area of the bacterial source by the aerated soil and Mylar wrap.
Process. The bacteria will eventually die since they cannot survive in very high heat; however, before they die, they have produced enough heat over time to start the fertilizers to release oxygen and to pyrolize the combustibles in the surrounding soil. Remem-ber, back in basic fire chemistry, we were told about the “bulb too close to the paneling” or “the steam pipe too close to the joist”? This prolonged heat tends to lower the ignition temperatures of normally high-ignition-temperature substances. It is purported that the ignition temperature of wood can drop to as low as 1507F by this process. Our spontaneous process has now produced enough heat to lower the ignition temperature of the surrounding combustibles in the soil and also spawn a reaction that will feed new oxygen to the original reaction. The soil has now begun to release combustible gases, and even though the original heat-producing bacteria have died, there is something else to take their place. It has also provided its own self-generating source of oxygen and also generated its own source of fuel gases. The chain reaction of the fire tetrahedron is now in place. The reaction is now self-generating and commences to grow on itself.
The result? A plant, sitting in the middle of the floor, smoking like a huge cigar, with no outside source of ignition in the area.
Such an occurrence is not commonplace, nor does it occur over a short period of time. It is most likely that, in this case, the sun in the solarium dried the plant sufficiently to initiate the bacterial reaction. The NFPA Fire Protection Handbook cites fish oil, fish meal, fertilizers, and several plant oils as being susceptible to spontaneous heating under extremes of moisture conditions, and all of these items can be found within the soil of a common house plant. It is also possible that, although the spontaneous heating of the plant`s bacteria was occurring while the plant was in the solarium, the heat conductivity of the ceramic tile floor was sufficient to carry the heat off. It was not until the homeowner placed the plant in a heated room and on a carpeted floor that heat retention within the plant became sufficient to create a growing thermopile. Heat production then became more and more rapid as time progressed, and once it reached a certain point, the reaction became very rapid and unstoppable. Therefore, fire.
After much research, this is my determination of the cause of ignition in this particular incident. However, if you as an investigator were to tell me that spontaneous combustion caused the ignition of such a fire, I would still have some really serious doubts as to your findings. As they say in Missouri, “Show me.”
Thousands of years ago, the Old Test-ament tells of Moses encountering the burning bush. He was taken aback by such a phenomenon. The burning bush spoke to him and taught him an important lesson, and it told him to pass the lesson on to his people. I think I have learned quite a few things from my own encounter with a burning bush. Tom Brennan, former editor of Fire Engineering, has always lectured that one of the traits of a good fire officer is the ability to fill his head with a lot of “stuff” and be able to pull that “stuff” back out of his head on the fireground as circumstances warranted, often at a moment`s notice. I have learned quite a bit about my burning bush, and I wanted to pass this “stuff” on. n
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n WILLIAM J. RAFTERY is a deputy chief in Hartsdale in Westchester County, New York. He has more than 25 years of experience in the fire service and has a B.S. degree in fire science from Mercy College.
