Soot & Soot Losses

By: Fredric M. Blum

Combustion of any kind produces soot.  Soot is carbon in the form of tiny particles a fraction of the size of human hair.

Any hydrocarbon fuel — wood, oil, natural gas, propane, paraffin, plastic, rubber, etc. — consists of hydrogen and carbon.  Both will burn.  Hydrogen burns to H₂O, which is harmless water vapor, while carbon ideally burns to CO₂ (carbon dioxide) and CO (carbon monoxide), which are both noxious gases but usually dissipate readily.   However, if combustion is not complete, some carbon does not burn at all, creating soot.  Soot is an especially pervasive and persistent physical contaminant.

Because soot particles are so small, soot is able to waft widely from the combustion site.  Soot particles are so small that they readily infiltrate cracks, crevices and porous surfaces of all kinds, such as fabrics and carpets, even seemingly remote from the source site.  Soot can be electrostatically attracted to plastic.  Soot typically does not dissolve in ordinary cleaning liquids so ordinary washing may not remove soot. 

Perfect, soot-free combustion requires that the necessary amount of oxygen be perfectly mixed with the fuel.  In real life, these conditions rarely occur.  In accidental fires involving a variety of combustible substances, fuel-air intermixing is usually imperfect which leaves soot and unburned fuel among the byproducts.  If the burning substances include oil, paraffin or plastic, soot will often be oily which makes it all the more difficult to clean up.  Soot can be wiped off smooth, non-porous surfaces, but on other surfaces the options are to paint over the soot or discard the object.  Soot can also coat internal passages of heating system furnaces and boilers which can impede heat conduction and decrease heating system efficiency, thus increasing annual costs.

How is it possible to tell when combustion produces soot?  A rule of thumb is that perfect, sootless combustion creates blue flames and imperfect, sooty combustion creates yellow flames.  For instance, wood flames are yellow (or orange, which is essentially the same) which generates soot.  Natural gas flames at a stove or heating appliance burner are blue and do not generate soot, but if the burner flames are flecked with yellow, the yellow flecks produce soot which over time can result in soot damage unless exhaust is vented out of the premises.  Gas log fireplaces are deliberately designed to produce yellow flames to simulate wood flames, so gas log fireplaces always produce soot.  Therefore, unvented gas log fireplaces will cause soot damage in a house.  Candle flames are yellow and therefore generate soot but are small.  Portable kerosene heaters generate considerable soot because their flames are wick-fed like candles and so are completely yellow.  Scented candles have a scent oil mixed into the paraffin which tends to burn less completely than paraffin, so scented candles tend to generate more soot than plain candles.  Excessive use of scented candles can cause serious soot damage in a house, but even plain candles can cause soot damage over a long period of time.  For instance, continuous burning of multiple candles in medieval cathedrals causes the interiors to gradually soot-darken over centuries no matter how spacious and drafty they are.

The key to perfect, soot-free combustion is pre-mixing air into the fuel.  Air premixing is intended to assure that all fuel is adequately oxygenated to burn completely.  Insufficient air premixing will cause flames to be less than perfectly blue and generate soot.  The flame from a stove or gas log fireplace becomes more yellow if the burner’s air intake is obstructed by dust or foreign matter. 

Sooting losses are extremely costly because soot is so pervasive and difficult to remediate.  If a house is sooted, the entire house may be affected — every single surface and object — walls, ceilings, floors, carpets, furniture, cabinet and drawer interiors and contents, clothing, appliances, etc.  All must be cleaned or discarded.  Otherwise, soot will gradually re-elevate and re-pervade the premises. 

The feasibility of subrogation in soot losses depends on the source of the soot — a fire, an appliance malfunction, defective design or service of an appliance.  Examples of defectively designed appliances include unvented gas log fireplaces, portable kerosene heaters, and scented candles.  Liability is also possible if an appliance’s combustion air intake is difficult to clean by design or the maintenance instructions are deficient.  Whether you are the plaintiff or defendant, consider the source of the soot, how the product or equipment was used, and who was responsible for maintenance.