I’m seeing too many “quick reminders” on firefighter training sites that tell you some variation of “The color of smoke tells you what is burning.” This is false. It has always been false. However well-intentioned the mythology has been when passed from one generation of firefighter to the next, it has never been true. In an ideal world, I would have a series of articles that start with the basics of The Art of Reading Smoke, and we would progress through the science and then into the smoke attributes (Volume, Velocity, Density, Color) as the classroom version of this instruction does. However, it seems the most common misunderstanding of smoke is the color aspect, so I feel obligated to address that portion first. The reality is, the color of smoke tells you the stage of heating of the fuel. We can make any object burn white, black, gray, and every shade in between in a single burn. We may also see brown. A smoke reader can rapidly identify, without ever having seen any flame:
1) Where the fire is.
2) How progressed the fire is.
3) What is going to happen next, and how much time they have until “next” happens.
A critical angle of answering those questions is analyzing the color of the smoke that is showing. Here is a quick breakdown of the colors, and what they mean, for your toolbox.
White – We will encounter two kinds of white smoke: Clean, pristine white that rapidly dissipates, and dirty ugly white that does not dissipate. The clean white smoke is simply steam. This is indicative of early stage heating, or a small, incipient stage fire. The fuels are just beginning to be heated, and the first product that they give off when they are heated will be their moisture content (water). The steam we encounter may be indicative of other things as well, besides early stage fires. These include an activated sprinkler system (red flag for a cold smoke fire), or that your interior crews have water on the fire, and we are seeing the resulting steam conversion.
The stuff that makes smoke a dirty white is ash. We will see ash from any flame. We will see lots of ash from a very hot fire. Ash is very light, fine, and silky smooth, so it will not be filtered out as it travels through and out of a structure within the smoke plume. So, lots of ash (dirty white smoke) = a very hot fire. When you arrive to find white smoke showing, be sure to differentiate between clean and dirty white. That can be the difference between an incipient stage and well-progressed fire. Align your tactics accordingly.
Black – Black smoke indicates late stage heating of a fuel. In other words, a well-progressed fire. There are a couple primary culprits that make smoke black – carbon and crude oil. In fact, carbon is generally the most prevalent solid (particulate) in smoke, and crude oil is the most prevalent liquid (aerosol) that we will encounter. Remember, though, that the carbon and crude oil are very heavy and very sticky. They will stick to whatever they touch (walls, furniture, FIREFIGHTERS), and they will gradually fall out of the plume due to the effects of gravity as the smoke travels. Just because we don’t see black smoke from the outside does not mean we don’t have a well-progressed fire inside. The black stuff may be getting filtered, either by distance or through resistance. So the presentation of a significant volume of dirty white smoke (high ash content) indicates a well-progressed fire whose smoke is also being filtered. Analyzing where the filtered smoke is coming from helps to narrow down where in the building the seat of the fire is.
Gray – Gray falls in between white and black. We will always have black smoke from flame tips. When a fuel is actively burning (combusting), this is late stage heating, and we will see black smoke as a result. As the smoke travels away from the fire seat, the black will begin to filter out as explained above. The grays that we see show us that some of the black has filtered out, but not all of it. When comparing smoke coming from different openings in the structure, make sure to consider the size/restriction of each opening. Larger openings mean less restriction, which means less filtering than smoke coming through the mortar in between bricks of a building.
Brown – Brown is the only exception that will tell you WHAT is burning. To be more precise, brown smoke tells you that unfinished, untreated wood is pyrolyzing due to heat. Pyrolyzation is the indirect heating of a fuel (read: it is being heated, but not yet actively combusting). Unfinished, untreated wood, rather than water content, will give off its moisture in the form of cellulosic organics. This is the liquid pulp that gives natural fibers (wood, wool, cotton, etc.) strength. As such, when we see brown smoke, we know that unfinished, untreated wood is losing its strength. As soon as you paint/stain/lacquer or otherwise treat wood, you are changing its chemical composition, and we will not see brown smoke as an early stage heating indicator. Where in a structure do we typically find unfinished, untreated wood? The structural members! Therefore, brown smoke should be your red flag that the structure itself is being heated and is experiencing degradation of its integrity. Consider the collapse potential and associated time-frame when attacking.
Please share this information. Understanding what smoke is, how it is impacted by the building/box it is coming from, how it affects fire, and firefighters, can be the difference between a successful aggressive attack and disaster.
ABOUT THE AUTHOR
Rob Backer is a 16-year veteran of the fire service and currently serves as a Captain for Thornton Fire Department in Thornton, Colorado. He is an instructor for Dave Dodson’s The Art of Reading Smoke and is available to teach nationally.