There are three major categories of smoke. However, they all apply to a post-fire situation since that is where there is residue of several types of burned materials.
Oxygen-Starved (Wet Smoke) Residue
Residue from smouldering, cool or oxygen-starved fires is the most difficult to remedy. Due to it being slow burning, the smoke is not driven by strong convection currents from the heat of the fire. The incomplete combustion releases solvents, varnishes and other aerosols, which can soften, penetrate and stain finished surfaces. The resulting particles tend to smear easily and are somewhat larger than those produced by an oxygen-rich fire. Wet smoke residue moves slowly and has time to work its way into crevices and enclosed areas that normally would not be contaminated by faster burning fire.
Unfinished wood and fabrics may absorb these residues and then entrap them. The odour, which is produced, may be extremely pungent and the acid formed when combined with moisture may cause many surfaces to yellow. Damage in this category of smoke often requires special restoration action or replaced.
Oxygen-Rich (Dry Smoke) Residue
This is a fast burning, hot fire, which tends to carry few aerosols and deposit small particles, which do not smear or smudge easily. The residue is grey/black in colour and of a dry, powdery consistency. This soot is a result of burned wood, paper, cotton, wool, jute, cork, feathers, etc. Dry smoke does not stain surfaces as deeply as wet smoke, and the odour is more pungent. Smoke of this type is difficult to remove. However, the possibility of soot and odour penetration into porous surfaces is increased when excessive heat causes surfaces to expand. Hot smoke is pressurised and has energy or force behind it and is often referred to as ‘Driven Smoke’,
which is normally found on vertical surfaces. Another term you may hear is ‘Free Floating Smoke’ also sometimes referred to as ‘Cold Smoke’. This originates as pressurised smoke but has lost the energy and velocity behind it. Because free floating smoke is heavier than air, it normally will settle out on horizontal surfaces, which are the single biggest sources of smoke odour.
Plastic or Rubber Smoke Residues
These result from burned plastic, synthetic rubbers and other polymers, which tend to burn vigorously at low temperatures producing a large lightweight particulate. This residue smears or smudges easily and is usually curly and black in colour. A small quantity of plastic can produce a surprisingly large volume of smoke, which can travel long distances and leave deposits in heavy concentrations throughout the structure.
Chlorides are the products of combustion while the source material can contain chlorides (such as PVC). When broken down the chlorides bond with the atmospheric moisture which can result in Hydrochloric Acid HCL, which is extremely corrosive to metallic materials. High levels of chlorides (which are hygroscopic – absorbing moisture from the air above 60% RH) on vulnerable material surfaces require removal as soon as possible to prevent secondary damage.
Behaviour of smoke
There are four main factors that determine the behaviour of smoke:
The temperature of a fire is a major consideration in determining the amount of smoke penetration throughout the structure. The energy created by the fire produces heat and pressure. As the air temperature increases, pressure will also increase and the volume of air doubles. This enables the smoke to penetrate into minute cracks and crevices. The pressure also may cause impingement, splatter, when a substance hits a surface with sufficient velocity allowing it to remain upon that surface often referred to as ‘driven smoke’.
Temperature of the surroundings
Warmed or heated air rises and tends to migrate to colder surfaces, such as outside walls, closets, and windows. As the air cools, it drops following the contour of the wall. This initiates a convection current that continues to bring more warm air to the cooler surfaces to replace the air that has dropped due to the ‘temperature gradient’ or ‘temperature differential’. As a result of this action, we can expect smoke to migrate and consequently deposit residues more heavily to these cooler surfaces. For this reason, you will find heavier soot deposits or smoke residue on the top back or lined portion of a drape rather than on the front of the drape.
Ionisation and magnetism of smoke particles
Opposite electrical charges attract and identical electrical charges repel. Smoke residue may demonstrate a high degree of selectivity in where it deposits. Smoke clusters, just as metal filings do on a magnet and attract to metal surfaces (plumbing, metal coat hangers, etc.). Nail heads are invisible beneath the drywall but may be clearly outlined by smoke, since their magnetic attraction remains effective.
Pattern of airflow and space arrangement
Smoke travels around various obstructions following the air currents on its journey throughout a building. Each obstruction acts as a crude filter. As a result of this filtration, the amount of residue to be found decreases as the smoke travels further from its source.
Remember, fire is non-discriminatory. Any substance, which can burn, will burn! The more substances burn, the more penetration of the residue and the more complex the odour.
About the Author:
Omar Al Hadrami is the President of Smart Solutions Cleaning Services. He is an Emirati citizen and the first certified Arab in his field. Over the past 8 years, he has received training from reputed institutions from across the world including IICRC Las Vegas USA and the first IICRC certified firm in the Middle East. He is also considering a course on Asbestos Removal and Hazardous Waste Management from the UK. Al Hadrami evangelises disaster recovery cleaning and is passionate about imparting his knowledge in the trade of specialised cleaning techniques.